Conférences avec actes

Résumé: Adaptive optics imaging techniques are invaluable for cellular-level retina visualization. While AO Flood illumination ophthalmoscopes provide distortion-free, high-speed images, they lack contrast. On the other hand, AO scanning laser ophthalmoscopes offer highly contrasted images due to point by point illumination and spatial filtering but suffer from low pixel throughput and distortion artifacts. Our recent advancements, using a DMD integrated AO-FIO, show that we can illuminate and capture multiple spatially separated zones, achieving contrast close to the one of a confocal microscope. Our theoretical framework emphasizes that each zone must be smaller than 100 µm in both directions or smaller than 10 µm in only one direction to minimize the diffuse light component. Building upon these results, we developed a cutting-edge confocal rolling slit ophthalmoscope, able to achieve brightfield contrast similar to a confocal ophthalmoscope, along with phase contrast images. We utilize a classical sCMOS camera with a rolling shutter synchronized with the line source scanning of the field of view. The system makes use of all the incident photons that can be collected, whether singly, multiply scattered or absorbed. Easy digital switching between the darkfield and brightfield, as well as modification of the size and offset of the detection aperture, enhances the adaptability and versatility of this multimodal imaging system, allowing for fine-tuning of imaging modalities and comprehensive exploration of the retina.

Résumé: This paper presents the development of a novel communication model for reconfigurable intelligent surface (RIS)assisted wireless systems, utilizing a 2D formulation of electromagnetic (EM) fields based on Green's theorem. The proposed 2D model is both simple and accurate, adhering to the laws of electromagnetism for the generation, propagation, and scattering of EM fields. Specifically, we establish a direct relationship between the scattered field and the current and impedance of the scattering antenna, resulting in a tractable expression for the total field scattered by the RIS. Moreover, our model successfully explains and analyzes the various diffraction orders present in the scattered field. Through this analysis, we derive an analytical model that predicts and elucidates the different diffraction order components, including the central and replica components. The presented communication model offers valuable insights and predictions for RIS-assisted wireless systems, paving the way for improved system design and performance optimization.

Résumé: We propose to record "en face" Full Field OCT interferometric images of paintings on canvas using a cooled MCT camera working in the range 3-5.5 micrometers coupled to a glowbar broadband source that allows to reach shot noise limited detection. Penetration in various paints is spectacular at these large wavelengths: we measure a mean free path 5 to 10 times larger than at visible/near IR wavelengths. We find that the canvas is easily observed under the paint. This result, obtained without averaging, paves the way to a non-contact, non-destructive, more detailed analysis of paintings than existing IR techniques.

Résumé: The wave speed of pressure waves in water pipelines is sensitive to the reduction in pipe wall thickness and material strength, and it has been utilized as an indicator of water pipeline deterioration. These probing pressure waves are usually generated actively and is challenging to be incorporated into an automated senor network. This paper proposed a passive wave speed estimation method, which takes cross-correlation of the ambient noise in water pipeline networks measured by two synchronized pressure sensors to estimate the wave travel time. Field experiments were carried out in the operating water reticulation system at University of Canterbury campus for validation. In the experiments, pressure sensors were attached to fire hydrants to measure the ambient noise for 20 min. The experiment results indicate that pressure wave speed can be estimated using the proposed passive method, and the accuracy is at the same level compared with the conventional active method.

Résumé: State-of-the-art adaptive optics optical coherence tomography (AO-OCT) presents important limitations, such as small field-of-view (FOV) and require complex, cumbersome hardware, preventing wide clinical deployment. Here, we introduce adaptive-glasses full-field OCT (FFOCT), a compact retinal imaging system allying high resolution, high frame rate (300Hz) and 5° FOV (footprint: 50 cm × 30 cm).

Résumé: The electromagnetic modes of a sub-wavelength sized antenna can be excited by thermal fluctuations. This thermal radiation is extremely weak, requiring in general the fabrication of a huge number of identical antennas to be detectable. Here, we will first demonstrate experimentally emission spectrum measurement and superresolved mapping of a single nanoantenna, sub-λ in the infrared spectral domain, based on the detection of its thermal radiation despite an overwhelming background thermal radiation [1]. To achieve a background free detection of the thermal radiation from single or a few sub-λ sized resonators, we have developed an infrared spatial modulation spectroscopy (IR-SMS) technique using a lateral modulation of the sample heated at ~150 °C combined with lock-in detection of the infrared signal detected through a Fourier transform infrared spectrometer. We have applied it in combination with thermal radiation scanning tunnelling microscopy [2] to study the thermal radiation from single plasmonic metal-insulator-metal (MIM) antennas both in the near field and in the far field. Our studies performed on single MIMs have revealed the surprising result that when silica is used as insulating material, its strong dispersion in the mid-infrared domain is such that the fundamental spatial mode of the antenna can be thermally excited at various wavelengths. This causes multiple resonances in the thermal radiation spectrum to which correspond the same spatial distribution of near-field thermal radiation [1].

Résumé: MISO Time-Reversal (TR) communication marks a paradigm shift for ultrawideband (UWB) communications because the processing is mainly carried out by the transmitter, which is ideal for some applications. Thanks to its focusing property, TR is naturally dedicated to multiple access trans-mission (TRDMA). Previous research works have focused on the theoretical performance of TRDMA. In this paper, Multi-user MISO (MU-MISO) TRDMA transmission is evaluated experimentally for the first time to the best of our knowledge, and by simulations. An experimental setup is used to measure the TRDMA signals for different bandwidth in multi-user scenario. The experimental results are then used in a simulation to evaluate the bit error rate and the performance of the system.

Résumé: We present Optical Incoherence Tomography (OIT): a completely digital method to generate tomographic retinal cross-sections from en-face through-focus image stacks acquired by non-interferometric imaging systems, such as en-face adaptive optics (AO)-ophthalmoscopes. We show how to use OIT to guide focus position in cases where the user is “blind” focusing, such as autofluorescence imaging of the Retinal Pigment Epithelium (RPE). We also demonstrate that OIT can produce distinctive cross-sectional views of the retina using back-scattered, multiply-scattered or even fluorescent light, making it a complementary technique to OCT.

Résumé: © 2020 IEEE. Reconfigurable intelligent surfaces (RISs) are an emerging field of research in wireless communications. A fundamental component for analyzing and optimizing RIS-empowered wireless networks is the development of simple but sufficiently accurate models for the power scattered by an RIS. By leveraging the general scalar theory of diffraction and the Huygens-Fresnel principle, we introduce simple formulas for the electric field scattered by an RIS that is modeled as a sheet of electromagnetic material of negligible thickness. The proposed approach allows us to identify the conditions under which an RIS of finite size can be approximated as an anomalous mirror or as a scatterer. Numerical results are shown to confirm the proposed approach.

Résumé: © 2019 IEEE. Non-contact ultrasound applications are raising more and more interests. However, very few applications have been reported, for now in medical applications. In this work, compact airborne piezotransducers have been designed, built and characterized for a novel medical application: contactless corneal imaging. The emission transducer is cylindrical with a 5.5 mm diameter and the reception transducer is a 2D array of 4x16 elements. A specific emission-reception electronic is also designed for these transducers to drive their high electric impedances that are larger than kiloOhms. The measured parameters - directivity, sensitivity and frequency bandwidth - of both transducers respond well to the needs of corneal imaging application.

Résumé: © 2019 IEEE. Magnetic light and matter interactions are generally considered too weak to be detected, studied and applied technologically [1]. However, if one can increase the magnetic power density of light by several orders of magnitude, the coupling between magnetic light and matter could become of the same order of magnitude as the coupling with its electric counterpart. For that purpose, photonic nanoantennas have been proposed, and in particular dielectric nanostructures, to engineer strong local magnetic field and therefore increase the probability of magnetic interactions [2]. Unfortunately, dielectric designs suffer from physical limitations that confine the magnetic hot spot in the core of the material itself, preventing experimental and technological implementations. Here, we demonstrate that evolutionary algorithms [3] can overcome such limitations by designing new dielectric photonic nanoantennas, able to increase and extract the optical magnetic field from high refractive index materials. We also demonstrate that the magnetic power density in an evolutionary optimized dielectric nanostructure can be increased by a factor 5 compared to state-of-the-art dielectric nanoantennas [4]. In addition, we show that the fine details of the nanostructure are not critical in reaching these aforementioned features, as long as the general shape of the motif is maintained. This advocates for the feasibility of nanofabricating the optimized antennas experimentally and their subsequent application. By designing all-dielectric magnetic antennas that feature local magnetic hot-spots outside of high refractive index materials, this work highlights the potential of evolutionary methods to fill the gap between electric and magnetic light-matter interactions, opening up new possibilities in many research fields.

Résumé: © 2019 IEEE. Plasmonic nanoantennas featuring nanoscale gaps can exhibit strongly enhanced optical near-fields that have been extensively used in surface enhanced spectroscopy (Raman and Fluorescence) and in biosensing. However, deterministic nanostructures do not provide enough degrees of freedom to control optically these local field enhancements. By comparison, wavefront shaping techniques in disordered scattering media provide numerous degrees of freedom to control light focusing in space and time [1]. To associate local field enhancements and far-field wavefront control, we use disordered plasmonic surfaces close to the percolation threshold (see Fig. 1-a) that feature both hotspots [2] and delocalized plasmon modes. Disordered plasmonic surfaces can be controlled using a spatial light modulator [3].

Résumé: © 2019 IEEE. We propose using complex media as platforms to build any linear operator using wavefront shaping. We demonstrate that system composed of multimode fiber and spatial light modulator can act like any linear operator and can be reconfigured at will. We experimentally performed several 16×16-single-shot operations.

Résumé: © 2018 IEEE. 4D ultrafast imaging has been recently introduced to provide new anatomical, mechanical and functional information. It relies on 2-D matrix array to image the medium at high volume rate and quantify blood flow information. However, this approach requires high channel count electronics (NxN)to drive those probes. An alternative to reduce the density of elements and maintain a decent image at very high volume rate was shown using the Row and Column Addressed (RCA)approach with Orthogonal Plane Wave (OPW)compounding strategy. Furthermore, the RCA configuration presents the advantage of keeping a large surface by reducing the count of channel by a factor N/2. In this study, we designed and built a large 15 MHz 256 channels (128+128)probe dedicated to neurovascular imaging and connected to a 256 channels research electronics. Using OPW image strategy, 4D ultrafast Power Doppler of whole rat brain is performed in vivo with a 23dB dynamic range at 125 Hz on whole brain.

Résumé: © 2018 IEEE. We investigate here how passive imaging based on noise correlation can serve to retrieve the scattering strength in a reverberating elastic plate. In previous works, we demonstrated the possibility to detect and localize passively a single scatterer in a reverberating elastic plate combining a transducer array and beamforming imaging, when a diffuse flexural wave field is approximately generated. In particular, when the noise is non uniformly distributed over the plate surface, we can take benefit from reverberations to accelerate the convergence towards the Green's functions. A continuity is addressed here to passively characterize a single scatterer in a thin plate. Numerical simulations are conducted in the presence and absence of a defect within the plate. We calculate the differential correlation matrix obtained in these cases (i.e., with versus without the scatterer) as a measure of the temporal variations in the Green's function due to the defect. By back-propagating the differential matrix, an image is obtained with a maximum intensity on the defect location. We also develop a formula that relates this focal spot on the defect position to the scattering strength of the scatterer. The formula are numerically validated using simulations in an aluminum plate. We substantiate the reliability of this approach through a classical method for the measurement of the scattering cross-section based exclusively on direct arrivals. Finally, some defect localization images are shown where the amplitude represents the scattering cross-section of the defect at 5 kHz and 25 kHz.

Résumé: © 2018 IEEE. We propose the dual-tuned volume coil for magnetic resonance imaging at two nuclei. This coil is based on the combination of two independent metasurfaces that can be used simultaneously for fluorine and hydrogen 19F/1H imaging at 7 Tesla for small animals. Each metasurface is made of nonmagnetic metallic rods and structural capacities of etched metal strips. The geometry is chosen so that two modes supported by the two metasurfaces resonate at the required frequencies 282.6/300.1 MHz. The two metasurfaces can be tuned independently by mechanically adjusting the length of the two sets of wires. We have numerically studied the concept of the proposed dual-tuned coil.

Résumé: © 2018 IEEE. Negative refraction and flat lens focusing of elastic waves has been recently demonstrated experimentally, though restricted to periodic structures where dispersion relations are readily available for prediction and design of superlenses. This is not possible anymore when considering quasiperiodic structures. Here we demonstrate quasicrystal platonic superlensing of elastic waves in perforated metallic thin plates, opening new possibilities for applications of quasi-periodic structures in elastic waves.

Résumé: © 2018 IEEE. Thanks to their particularly efficient, low frequency Minnaert resonance, air bubbles are known to be excellent candidates for the realization of acoustic metamaterials. Here, we demonstrate that the introduction of pair-wise spatial correlations between the bubbles can result in double negativity. This can occur when the bubble pairs are arranged either in random or periodic configurations. Predictions for both types of structure will be presented and the influence of dissipation on the doubly negative behaviour discussed.

Résumé: © 2018 IEEE. A method for sound field reproduction with the suppression of exterior radiation is proposed, which makes it possible to synthesize a desired sound field in a reverberant environment without prior knowledge of the transfer functions of the multiple loudspeakers. The objective function used to achieve this is formulated as the weighted sum of the interior reproduction error and exterior radiation power. The optimal driving signals are derived by harmonic expansion of both the interior and exterior sound fields. In contrast to the empirical coefficient truncation in the state of the art, in the proposed method, an optimal weighting of the harmonic coefficients is derived analytically. Numerical simulation results indicated that high interior reproduction accuracy and exterior power suppression can be achieved by the proposed method compared with the mode-matching method using harmonic-order truncation owing to the optimal weighting.

Résumé: © 2018 Author(s). In this paper, we present two frequency-domain algorithms for 2D imaging with plane wave emissions, namely Stolt's migration and Lu's method. The theoretical background is first presented, followed by an analysis of the algorithm complexities. The frequency-domain methods are then compared to the time-domain plane wave imaging in a realistic inspection configuration where the array elements are not in contact with the specimen. Imaging defects located far away from the array aperture is assessed and computation times for the three methods are presented as a function of the number of pixels of the reconstructed image. We show that Lu's method provides a time gain of up to 33 compared to the time-domain algorithm, and demonstrate the limitations of Stolt's migration for defects far away from the aperture.

Résumé: © Springer International Publishing AG, part of Springer Nature 2018. Minimal path approaches for image analysis aim to extract curves minimizing an energy functional. The energy of a path corresponds to its weighted curve length according to a relevant metric function. In this study, we design a binary isotropic metric model with the use of a Hessian-based vascular enhancement filter in order to extract geometrical features from vascular networks. We introduce a constrained keypoint search method able to extract subpixel vessel centrelines, diameters and bifurcations. Experiments on retinal images demonstrated that the proposed framework achieves similar even better segmentation performances as compared with methods using more sophisticated metric designs.

Résumé: © 2017 IEEE. Sparse representations of sound fields have become popular in various acoustic inverse problems. The simplest models assume spatial sparsity, where a small number of sound sources are located in the near-field. However, the performance of these models deteriorates in the presence of strong reverberation. To properly treat the reverberant components, we introduce three types of reverberation models: a low-rank model, a sparse model in the plane-wave domain, and a combined low-rank+sparse model. We discuss corresponding decomposition algorithms based on ADMM convex optimization. Numerical simulations indicate that the decomposition accuracy is significantly improved by the additive model of low-rank and sparse plane wave models.

Résumé: © 2017 IEEE. Random lasing is reported in solid state PMMA-DCM doped 1D disordered organic microstructure with randomly distributed grooves along the length of dye doped polymer strip. Role of disorder which is provide by different thickness of dielectric layers separated by 100 nm grooves along the length of polymer strip is shown by the variation in emission spectra of random laser with local pump position. Introduction of long-lived solid state random laser will provide stable emission spectra that will be helpful in control of random laser.

Résumé: © 2017 IEEE. Evaluation of rheological behavior of soft tissues provides an important diagnosis tool. Nowadays, most of the available commercial ultrasound systems assume tissues as isotropic and purely elastic. Recently in order to fully characterize the rheological behavior of tissues, methods to estimate both shear wave speed (V) and attenuation (α) were developed for isotropic media [1, 2]. In acoustic radiation force elastography, generated shear waves are not plane and a diffraction correction is needed to estimate α [1] . Here a diffraction correction is studied through numerical simulation and experimentally to measure α for an homogeneous transverse isotropic, viscoelastic (TIV) medium.

Résumé: © 2017 IEEE. Brownout refers to dust cloud created by the rotor downwash of a helicopter. When it occurs, the visibility becomes limited, or even null. The pilot can be desorientated and accident may happen. No existing imaging systems can see through dust clouds, in real-time and with sufficient resolution. Using waves between 100GHz and 1THz seems to be a good solution to make a compact and suitable imaging system. After defining a brownout model, we establish theoretically the power balance of the involved sources of signal and noise. We compare a photometric detection system with one compounded of antennas.

Résumé: © 2017 IEEE. Fingerprint recognition has been extensively used in numerous civilian applications ranging from border control to everyday identity verification. The threats to current systems emerge from two facts that can be attributed to potential loss in accuracy due to damaged external fingerprints and attacks on the sensors by creation of an artefacts (e.g. silicone finger) simply by lifting the latent fingerprints. In the growing need for attack resistant biometric fingerprint recognition that can be operated without supervision, a new generation of sensors has been investigated, which can capture the subsurface fingerprint pattern. In this work, we explore a subsurface fingerprint imaging technique by employing a custom-built in-house Full-Field Optical Coherent Tomography (FF-OCT) sensor for capturing the subsurface fingerprint. Further, we evaluate a newly constructed database of 200 unique fingerprint samples collected in 2 different sessions with 6 layers of fingerprint images corresponding to 6 subsurface fingerprints. We also propose a framework based on quality metrics to fuse the subsurface fingerprint images to achieve a robust verification accuracy, which has resulted in Equal Error Rate (EER) of 0%. We also provide an extensive set of experiments to gauge the reliability of subsurface fingerprint recognition and deduce a set of important conclusions for the path forward in FFOCT subsurface fingerprint imaging.

Résumé: © The Authors, published by EDP Sciences, 2017. Discrete Element Method modeling is used to study the frequency spectrum of particle motion in dense 3D packings of glass beads with Hertz-Mindlin contacts. Frequency sweeps show a dependency of the resonant frequencies on the drive amplitude and confining stress on the system, showing material changes in the system. The amplitude dependency of the second thickness mode 3λ/4 as identified by the internal strain field scales as f ∝ σ 1/6 while the confining stress dependency scales as f ∝ σ 2/3 , as predicted by Hertzian theory.

Résumé: © 2017 IEEE. In this paper we present FRIDA - an algorithm for estimating directions of arrival of multiple wideband sound sources. FRIDA combines multi-band information coherently and achieves state-of-the-art resolution at extremely low signal-to-noise ratios. It works for arbitrary array layouts, but unlike the various steered response power and subspace methods, it does not require a grid search. FRIDA leverages recent advances in sampling signals with a finite rate of innovation. It is based on the insight that for any array layout, the entries of the spatial covariance matrix can be linearly transformed into a uniformly sampled sum of sinusoids.

Résumé: © Published under licence by IOP Publishing Ltd. We perform wave mixing and dynamic holography by exploiting gain saturation in diode pumped laser media like Nd-YVO4. Such crystals offer the possibility of performing adaptive gain interferometers with a sub ms response time, typically between 100- 200μs. This temporal response is of great interest for applications such as speckle interferometry and acousto-optic detection for tissue imaging in biophotonics. In this work we operate in new conditions of high sensitivity with a linear response due to a refractive index contribution in the gain media. It originates from the wavelength mismatch of about 0.1 nm between the incident Nd-YAG probe laser and laser transition in diode pumped Nd-YVO4 amplifier. This permits the recording of two types of volume holograms respectively a gain and a phase volume holograms. It is the index grating which provides a linear response to incident low phase modulation at frequencies higher than 10 kHz. Experimental results confirm this behavior for adaptive interferometry and speckle vibrometry. In addition, the fast response makes the two wave mixing interaction in Nd-YVO4 very interesting for the optical detection of large bandwith photoacoustic waves excited by a pulsed ns Q-Switch laser at 532 nm. Temporal photoacoustic signals and images of small objects behind a scattering media will be shown.

Résumé: © 2017 Author(s).Ultrasonic testing of jet engine titanium alloys is of high importance for the aircraft manufacturing industry. The quality of ultrasonic non-destructive testing is severely impacted by the titanium complex microstructure. These alloys have been extensively studied and single scattering models are now well known and implemented in ultrasonic propagation simulators. In addition, titanium billets and forged parts have been known to exhibit a highly anisotropic microstructure. We studied ultrasonic wave scattering in Ti17 forged disk, through statistical analysis of the backscattered noise generated by the microstructure. More specifically, we focused on the quantification of multiple scattering relative to single scattering in the backscattered wave. To that end, we used the full matrix capture acquisition with a linear transducer array. Two phenomena were used to quantify the proportion of single scattering with respect to multiple scattering. The first is the coherent backscattering effect, used as a binary indicator of multiple scattering. The second is a repurposed version of the multiple scattering filter, recently developed on random rod forest and applied on Inconel alloys. With these methods, significant level of multiple scattering was consistently measured in Ti17 forged disks, showing that ultrasonic testing could be enhanced by filtering the multiple scattering contribution.

Résumé: © 2016 IEEE.The recent introduction of functional ultrasound (fUS) based on ultrafast Doppler imaging for blood flow detection unveiled a gigantic field of applications in Neuroimaging. Its considerable sensitivity, temporal and spatial resolution enabled to image the neurovascular coupling in unprecedented situations such as olfactory stimulation or spatial representation in an awake animal. However, to date those applications have been restricted to small animal studies. Here we present for the first time fUS imaging in human. We were able to image a broad spectrum of cerebral activity in neonates, from quiet sleep to epileptic seizures, and we could even detect undescribed phenomenon under the form of slowly propagating vascular changes.

Résumé: © 2016 IEEE.Deep vein thrombosis can lead to pulmonary embolism when a part of the thrombus breaks causing pulmonary veins blockage. This complication is often fatal; therefore its prediction is of great interest. Knowing the viscoelastic properties of the blood clot can help both diagnosis and choice of treatment. In this work, the coagulation process was monitored using classical rheometry and supersonic shear imaging. Storage (G') and loss (G'') moduli were calculated from shear wave velocity and attenuation measurements. For the estimation of the attenuation coefficient it was necessary to consider the diffractive nature of the generated wave. A cylindrical propagation correction was applied in order to avoid overestimating the attenuation. The evolution of G′ and G″ during the coagulation process was in good agreement for both techniques. SSI proved to be useful to provide G′ and G″ maps of clots at different times during coagulation. This could eventually allow in vivo characterization of thrombi mechanical properties which have potential use in predicting thrombi breakage.

Résumé: © 2016 IEEE.A novel hybrid device is described herein for performing simultaneous Positron Emission Tomography (PET) and Ultrafast Ultrasound Imaging (UUI) in vivo. This study aims in qualifying this device for in-vivo studies. Two major issues are addressed: the co-registration of the images originating from each modality and the effect of placing a US probe inside a PET system on the PET images. Experiments on UUI/CT and PET phantoms were performed for the co-registration calibration of this prototype and for quantifying the effect of the US probe on the PET signal.

Résumé: © 2016 IEEE.In security applications, several methods are used for detection of hidden objects. Metal detectors and palpation are the most common methods. Non-contact acoustic waves imaging of elastic heterogeneities in superficial body tissues could be an interesting alternative. In this article a 3D airborne ultrasound vibrometer is developed to image with a high framerate the surface motion of the skin surface generated by the propagation of a low frequency surface acoustic wave (SAW). The phase velocity of the SAW is directly related to the elasticity of the superficial layer of the skin surface. The actual framerate of the vibrometer is 556Hz and the sensitivity displacement value is close to the micrometer. Experimental results using a biological tissue mimicking phantom show the ability of the vibrometer to detect a rigid inclusion below the phantom surface.

Résumé: © 2016 IEEE.In this study, we propose 4D Ultrafast vector flow imaging, through the use of 4D plane-wave ultrafast and cross-beam vector Doppler imaging in order to map in-vivo blood flows in 3D, provide volumes of complex vector flow fields at high frame rate, and quantify the volumetric flow rate accurately. 4D Ultrafast vector flow imaging was performed in large volumetric field of views at high volume rates (>4000 volumes/s) using a 1024-channel 4D ultrafast scanner and a 2D matrix-array probe. The accuracy and precision of the technique was evaluated in-vitro in an artery phantom and compared with an industrial flow meter. Volumetric flow rate errors of less than 5% were found when volumetric flow rates were less than 360 ml/min. In-vivo feasibility was evaluated in 2 human carotid arteries. 4-D blood flow velocity was assessed during one heartbeat and in a full volume and volumetric flow rates of 375 ± 57 ml/min and 275 ± 43 ml/min were estimated. Finally, vortices were imaged in 3D at the carotid artery bifurcation.

Résumé: © 2016 IEEE.The spatiotemporal study of the visual system of the rodent is a challenging area that conventional functional imaging modalities, despite fundamental achievements, still struggle to study nowadays. In the other hand Functional Ultrasound (fUS) offers an unprecedented combination of spatiotemporal resolution. By coupling fUS with an adapted visual stimulation device, we mapped the vision-evoked activity in the rodent brain. We then optimized the visual-stimulus toward the spatiotemporal study of the rodent visual pathway using fUS.

Résumé: Weintroduce an open microwave cavity that has a wall replaced by a sub-wavelength grating. Usually, sub-wavelength gratings show very low transmission. In our experiment, this phenomenon is compensated by the microwave cavity that finally allows all the energy to be transmitted. We study the far field emission of this system and show that coupling the cavity with a sub-wavelength grating gives rise to a zero order emission only at discrete angles and frequencies. We study the relations between angles of emissions and frequencies, the influence of geometric parameters such as the grating fill factor and the behavior of a chaotic cavity. We show that it allows us to make a configurable system that may have many applications in the fields of communications, detection and imaging, and may allow the study of open microwave cavities on a fundamental point of view. © 2013 IEEE.

Résumé: Interactions between waves can be turned into profit to break diffraction limits and invent new kinds of medical images. It consists in productively combining two very different waves -one to provide contrast, another to provide spatial resolution- in order to build a new kind of image. Contrary to multimodality medical imaging that remains the superposition of two different images limited by their respective contrast/resolution couples, MultiWave imaging overcomes this limitation by providing a unique image of the most interesting contrast with the most interesting resolution. MultiWave imaging can benefit from three different potential interactions among waves that will be described in this paper. © 2011 by Società Italiana di Fisica.

Résumé: © 2015 IEEE. Infrared near-field scanning optical microscopy (NSOM) has been used to investigate surface plasmon polaritons (SPPs) which are generated on an integrated device by means of a semi-conductor laser. Devices which realize a slit-doublet experiment with SPPs have been investigated. Using NSOM imaging, we demonstrate in situ generation of SPPs with quantum cascade lasers at λ=7.5 μm and with laser diodes at λ=1.3 μm. Using thermal radiation scanning optical microscopy (TRSTM), we measure the infrared thermal emission of a sample, which is scattered in the near-field by means of a tip. We have recorded images and Fourier transform infrared (FTIR) spectra of the near-field thermal emission of materials which support surface polaritons. We show that the TRSTM probes both the spatial and frequency dependence of the electromagnetic local density of states (EM-LDOS). The modes of SPPs confined on metallic patterns are observed with TRSTM imaging. At the surface of materials supporting surface phonon polaritons, the measured near-field thermal emission spectra are quasi-monochromatic, in striking contrast with blackbody-like far-field thermal emission. TRSTM combined with FTIR spectroscopy is a promising method for infrared spectroscopy with nanoscale resolution, i.e. well beyond the diffraction limit.

Résumé: © 2015 IEEE. Many virtual reality applications incorporate realistic room acoustic simulation to provide increased immersiveness and realism. Traditional geometric methods, although providing modeling accuracy, are usually impractical for use in interactive applications. At the same time, artificial reverberators, with feedback rendering structure, are widely used as a low-cost alternative. This paper presents the design of a geometric-based artificial reverberator inspired by the acoustic rendering equation (ARE) and the feedback delay networks (FDN). The simplified acoustic rendering equation, which models both specular and diffuse reflections, is incorporated with the FDN structure. Our reverberator, despite of modeling the diffuse and late reverberation, is also capable of simulating the early/specular reflections with accuracy. This novel work is among the very few works which are capable to simulate early reflections using feedback delay networks.

Résumé: © 2015 The Authors. The goal of Surface Wave Elastography (SE) techniques is to estimate the shear elasticity of the sample by measuring the surface wave speed. In SE the thickness of the sample is often assumed to be infinite, in this way, the surface wave speed is directly linked to the sample's shear elasticity. However for many applications this assumption is not true. In this work, we study experimentally the Rayleigh-Lamb modes in soft solids of finite thickness to explore the optimal conditions for SWE. Experiments were carried out in three tissue mimicking phantoms of different thicknesses (10 mm, 20 mm and 60 mm) and same shear elasticity. The surface waves were generated at the surface of the phantom using piston attached to a mechanical vibrator. The central frequency of the excitation was varied between 60 Hz to 160 Hz. One component of the displacement field generated by the piston was measured at the surface and in the bulk of the sample trough a standard speckle tracking technique using a 256 element, 7.5 MHz central frequency linear array and an ultrasound ultrafast electronics. Finally, by measuring the phase velocity at each excitation frequency, velocity dispersion curves were obtained for each phantom. The results show that instead of a Rayleigh wave, zero order symmetric (S0) and antisymmetric (A0) Lamb modes are excited with this type of source. Moreover, in this study we show that due to the near field effects of the source, which are appreciable only in soft solids at low frequencies, both Lamb modes are separable in time and space. We show that while the Ao mode dominates close the source, the S0 mode dominates far away.

Résumé: © 2015 The Authors. Actual implementation of an efficient SHM system is necessarily hampered by the constraints of power-consumption and intrusive- ness (weight, size, in-service integration) of sensors. In the field of ultrasound-based SHM, conventional methods rely on relatively powerful acoustic sources synchronized with the sensors, and exploit only the first propagated (ballistic) wavepackets. The aim of this paper is to present possible techniques to exploit the whole complexity of reverberation signals, in order to extract the maxi- mum information from limited hardware, software, or power resources. A first aspect is the extraction of statistical properties of the codas of multiply-reflected signals, which can be used to estimate structural properties from a small number of sensors. In this technique, the required signal processing is relatively light and synchronization between the acquisition channels is not necessary. A second aspect is concerned with the possibility of using ambient acoustic sources, naturally present for example in transportation applications, instead of artificial power-consuming ultrasound sources.

Résumé: © 2015 The Author(s) Published by the Royal Society. All rights reserved. We show that the classical homogenization is able to describe the dispersion relation of spoof plasmons in structured thick interfaces with periodic unit cell being at the subwavelength scale. This is because the interface in the real problem is replaced by a slab of an homogeneous birefringent medium, with an effective mass density tensor and an effective bulk modulus. Thus, explicit dispersion relation can be derived, corresponding to guided waves in the homogenized problem. Contrary to previous effective medium theories or retrieval methods, the homogenization gives effective parameters depending only on the properties of the material and on the geometry of the microstructure. Although resonances in the unit cell cannot be accounted for within this low-frequency homogenization, it is able to account for resonances occurring because of the thickness of the interface and thus, to capture the behaviour of the spoof plasmons. Beyond the case of simple grooves in a hard material, we inspect the influence of tilting the grooves and the influence of the material properties.

Résumé: We show that a Full-Field OCT imaging system could be directly coupled to a compact transmissive liquid crystal spatial light modulator to induce or correct aberrations. The metric is based on the FFOCT image quality. © 2015 OSA.

Résumé: We report on imaging through thick scattering media based on a matrix approach of wave propagation. We show how to overcome multiple scattering and demonstrate imaging of targets beyond several transport mean free paths. © 2015 OSA.

Résumé: A differential optical profilometry technique with picometre-range sensitivity is adapted to the non invasive measurement of the roughness inside hollow glass fibres by use of immersion objectives and index-matching liquid. © 2015 OSA.

Résumé: We report on the passive measurement of time-dependent Green's functions in optics with low-coherence interferometry. Inspired by previous studies in acoustics and seismology, we show how the correlations of a broadband and incoherent wave-field can directly yield the Green's functions between scatterers of a complex medium. © 2014 Optical Society of America.

Résumé: Acousto-optic imaging (AOI) is an emerging technique in the field of biomedical optics which combines the optical contrast allowed by diffuse optical tomography with the resolution of ultrasound (US) imaging. In this work we report the implementation, for that purpose, of a CMOS smart-pixels sensor dedicated to the real-time analysis of speckle patterns. We implemented a highly sensitive lock-in detection in each pixel in order to extract the tagged photons after an appropriate in-pixel post-processing. With this system we can acquire images in scattering samples with a spatial resolution in the 2mm range, with an integration time compatible with the dynamic of living biological tissue.

Résumé: © 2014 IEEE. Ultrasonography is proving to be an invaluable tool in brain surgery. Recently, new ultrasonic modalities called shear wave elastography (SWE) enabled living tissue assessment of stiffness. SWE is routinely used for breast or liver diseases, but brain data are missing. We aim to characterize elasticity of normal brain and brain tumors by using SWE and to study if there is a relationship between SWE and intracranial pressure (ICP). Improving quality of brain tumor resection and predict brain swelling are major concerns for neurosurgeons. A clinical study was undertaken, including normal brain and tumors data collected from intraoperative SWE. The aim is to improve shear wave imaging for brain tissue investigation by correlating in vivo stiffness data and histology. At the same time ICP was studied, first ex vivo and then during in vivo surgery, to observe brain swelling by using SWE. Shear waves were generated by using ultrasonic acoustic radiation force and imaged in real-time by an ultrafast ultrasound scanner up to 20 000 frames/s. This study demonstrates that there are significant differences in elasticity among the most common types of brain tumors. We show that SWE could help for diagnosis during tumor resection by distinguishing benign tumors from malignant tumors (AUROC: 0.77, p<10-4). Regarding pressure measurements, stiffness was found increasing with the pressure both ex vivo and in vivo. This study present great perspective for SWE to ensure grade differentiation and full tumor removal.

Résumé: © 2014 IEEE. Passive structural health monitoring (SHM) is an emerging technology. More than 10 years ago, it has been shown that transient response between two sensors can be passively estimated from cross-correlation of ambient noise. The work presented here is an experimental application of this approach dedicated to detect the occurrence of flaws on a thin aluminum plate. The detection sensitivity is directly related to the fidelity of the estimation of the transient response. Using a laser vibrometer, we show that it strongly depends on the number of uncorrelated noise sources. An artificial damage is detected from the difference between the cross-correlation matrices measured before and after defect appearance. To localize the defect, a beamforming array processing is applied on the matrix. The resolution can be as small as half a wavelength.

Résumé: Optoacoutic mesoscopy aims to bridge the gap between optoacoustic microscopy and optoacoustic tomography. We have developed a setup for optoacoustic mesoscopy where we use a high frequency, high numerical aperture spherically focused ultrasound transducer, with a wide bandwidth of 25-125 MHz. The excitation is performed using a diode laser capable of >500 μJ/pulse, 1.8ns pulse width, 1.4 kHz pulse repetition rate, at 515 nm. The system is capable to penetrate more than 5 mm with a resolution of 7 μm axially and 30 μm transversally. Using high-speed stages and scanning the transducer in a quasi-continuous mode, a field of view of 2×2 mm2 is scanned in less than 2 minutes. The system is suitable for imaging biological samples that have a diameter of 1-5 mm; zebrafish, drosophila melanogaster, and thin biological samples such as the mouse ear and mouse extremities. We have used our mesoscopic setup to generate 3- dimensional images of genetically modified drosophila fly, and drosophila pupae expressing GFP from the wings, high resolution images were generated in both cases, in the fly we can see the wings, the legs, the eyes, and the shape of the body. In the pupae the outline of the pupae, the spiracles at both ends and a strong signal corresponding to the location of the future wings are observed. © 2014 SPIE.

Résumé: Fingerprint-based Audio recognition system must address concurrent objectives. Indeed, fingerprints must be both robust to distortions and discriminative while their dimension must remain to allow fast comparison. This paper proposes to restate these objectives as a penalized sparse representation problem. On top of this dictionary-based approach, we propose a structured sparsity model in the form of a probabilistic distribution for the sparse support. A practical suboptimal greedy algorithm is then presented and evaluated on robustness and recognition tasks. We show that some existing methods can be seen as particular cases of this algorithm and that the general framework allows to reach other points of a Pareto-like continuum. © 2014 IEEE.

Résumé: © 2014 IEEE. We demonstrate how to measure accurately the transmission matrix of a complex medium. With this information, we show how to focus light, recover an image, and even perform efficient reconstruction of a sparse object.

Résumé: Optical forces can affect the motion of a Brownian particle. For example, optical tweezers use optical forces to trap a particle at a desirable position. Using more complex force fields it is possible to generate more complex configurations. For example, by using two optical traps placed next to each other, it is possible to obtain a bistable potential where a particle can jump between the two potentials with a characteristic time scale. In this proceeding, we discuss a simple finite difference algorithm that can be used to simulate the motion of a Brownian particle in a one-dimensional field of optical forces. © 2013 SPIE.

Résumé: Imaging intramyocardial vascular flows could strongly help to achieve better diagnostic of cardiovascular diseases but no standard imaging modality allows describing accurately myocardial blood flow dynamics with good spatial and temporal resolution. We recently introduced a novel Doppler imaging technique based on compounded plane waves transmitted at ultrafast frame rate. The high sensitivity and framerate of the Doppler technique enable imaging the intramyocardial blood flow and its dynamics. A special demodulation-filtering process achieved to compensate for the large tissue velocity of the myocardium and a signed power Doppler process gives the possibility to discriminate arterial and venous flows. Experiments were performed in vivo in N=5 open chest sheep using a conventional ultrasonic probe placed at the surface of the heart. Results show the capability of the technique to image intramyocardial vascular flows in normal physiological conditions with good spatial (200µm) and temporal resolution (10ms). The flow dynamics over the cardiac cycle was investigated and showed a phase opposition of flow waveform between arterial and venous flows. Finally, the main diagonal coronary artery was occluded and the vascular flows were found to completely disappear in the ischemic region.

Résumé: We present an innovative mirrorless optofluidic random laser where the optical cavity has been replaced by a random scattering structure.We achieve emission control at any desired wavelength by iteratively shaping the optical pump profile. © 2013 Optical Society of America.

Résumé: We experimentally investigate the dynamic of light transport in 3D disordered media with an engineered fractal-like heterogeneity. We observe fractal-like trajectories for light by measuring walk dimensions dw < 2, giving signatures of light superdiffusion. © 2013 Optical Society of America.

Résumé: Conventional ultrasound Doppler techniques are based on focused beam to insonify the medium which leads to a tradeoff between the field of view and the resolution: a limited amount of 15 temporal points for 2D imaging mode or only one line for pulse wave mode. Recently, the introduction of ultrafast plane wave imaging has enabling acquisitions of hundreds of temporal samples over a large field of view. This huge amount of data can be used in different ways than in conventional Doppler modes to extract information about the flow. First, using properties of Doppler spectrum and calibrated data, we retrieve the out of plane speed vector component in transverse flow acquisitions, improving a previous technique called spectral broadening. An experimental demonstration is performed in vivo on a carotid artery. Secondly, we study how ultrafast plane wave acquisitions allows to withdraw the geometric broadening on Doppler spectrums to visualize only the speed and its gradient. This new technique becomes highly sensitive to the type of the flow profile and turbulences. A comparison between this technique and conventional pulse wave Doppler is performed in vivo on the carotid artery.

Résumé: We investigate the nonlinear response of pulsed sound transmission in weakly compressed granular materials, composed of glass beads and sand. Amplitude and velocity of longitudinal waves are simultaneously measured. We observe that weakly compressed packings can both exhibit strengthening and weakening of the sound velocity and transmission amplitude when excited by high amplitude sound. These effects are due to the changes of the contacts between the particles and although the effective medium theory qualitatively describes these effects it fails to quantitatively account for them. © 2013 AIP Publishing LLC.

Résumé: Backward waves propagating on shell are guided modes with opposite phase and group velocities. For a shell in vacuum, backward modes are linked to zero group velocity modes and resonances which have been the object of recent studies. For a shell embedded in water, the group velocity does not vanish because of the leakage into the fluid. However, the group velocity of the backward mode has a minimum associated to a quasi-resonance. These phenomena are studied on air filled steel and zircaloy hollow cylinders, using a 3MHz linear array in pulse echo mode. The circumferential guided modes are generated and their radiation into water detected by the array. The modes are separated using the decomposition of the time reversal operator (TRO), each pair of counter-propagating modes being associated to 2 invariants of the TRO [Prada & al. J. Acoust. Soc. Am. 1998]. Two resonances are revealed by the eigenvalues of the TRO, one is associated with the first longitudinal thickness resonance and the other, very high, occurring at a slightly lower frequency, corresponds to the minimum of the group velocity of the backward mode. The back-propagations of the eigenvectors of the TRO provide the phase velocities of these modes. © 2013 Acoustical Society of America.

Résumé: In shallow water, active detection of a small moving target can be difficult because of strong echoes from large fixed obstacles. To cancel strong unwanted echoes, differences between successive acquisitions can be achieved, however they are very sensitive to fluctuations. A projection method combined with a fast acquisition technique is proposed as a robust alternative. An ultrasonic experiment is presented: a 64 transducers linear vertical array is used to detect a small target moving above a large obstacle in a waveguide. To reduce acquisition time, 8 groups of adjacent elements transmit linear frequency modulations with increasing delays in a single emission. The 8×64 array response matrix is then obtained by correlations and time windowing. The projection is achieved between two acquisitions obtained while the target is moving, in order to remove the obstacle's contribution. Namely, the second acquired matrix is projected on the space orthogonal to the 8 singular vectors of the first acquired matrix. Then, it is shown that the first singular vector of the projected matrix focuses on the second target's position. Comparisons are made with the decomposition of the time reversal operator in differential mode and conventional beamforming. © 2013 Acoustical Society of America.

Résumé: We present a new technique based on the self-interference of Supercritical Angle Fluorescence (SAF) emission in order to perform full-field cell membrane imaging. We show that our Point Spread Function (PSF) engineering technique allows us to obtain a 100 nm axial sectioning while conserving the original lateral resolution of the microscope. The images are acquired using an optical module that can be connected to any fluorescent microscope to simultaneously monitor in real time both the cell membrane and in-depth phenomena. © 2013 Copyright SPIE.

Résumé: Circumventing the limit imposed by diffraction is a major issue in the instrumental development to realize finer resolutions in biological samples. With STED microscopy, we exploit the molecular transitions of the fluorescent marker to image well below the Rayleigh criterion. Also in combination with STED, we propose to use an alternative technique for optically sectioning fluorescent emitters close to the water-glass interface by selectively filtering the supercritical emission at the pupil plane. We discuss the instrumental development of such a system and its combination with other imaging techniques. © 2013 Copyright SPIE.

Résumé: Propagation of ultrashort broadband pulses through a multiply scattering media result in complex spatio-temporal speckle pattern. Using spectral pulse shaping, we demonstrate the spatially localized temporal recompression of the output speckle to the Fourier-limit duration. © Owned by the authors, published by EDP Sciences, 2013.

Résumé: We report random laser emission in an innovative structure PDMS microfluidic channel filled with a dye solution. A systematic study on the origin of random lasing and its depedency on pump laser parameters is presented. © 2012 OSA.

Résumé: Propagation media containing resonant inclusions have been studied for over a century in acoustics, electromagnetism or solid state physics. There exist some in nature, such as dielectrics, which contain enormous amounts of atoms. To calculate those materials permittivities one considers that each atom "sees" the same electromagnetic field and calculates the average field that takes into account an incoming wave as well as the overall response of the ensemble of atoms [1]. This macroscopic view assumes that there is no variations of the electromagnetic field at the scale of the inter-atomic distance. © 2012 OSA.

Résumé: We demonstrate, based on spatial and frequency resolved wave front shaping of ultrasound with a nonlinear feedback signal, how to achieve optimal spatiotemporal focusing through a complex scattering medium. © 2012 OSA.

Résumé: Materials such as milk, paper, white paint and biological tissue scatter light. As a result, transmitted light intensity through these materials is a speckle pattern, having often a short persistence time. Recently, advances in optics to control light through disordered media have reported an increasing efficiency. Consequently, that allows us to foresee a real time sensor that achieve such task in an integrated way. Thereby, in this perspective, we propose a genetic algorithm implemented with pyramidal approach in a CMOS image sensor, which matches integrated data processing and short persistence time. Our algorithm have been simulated with a faithful model. Results show at least a gain of a factor 10 compared to the state of the art algorithms. © 2012 IEEE.

Résumé: This paper presents an informed source separation technique of monophonic mixtures. Although the vast majority of the separation methods are based on the time-frequency energy of each source, we introduce a new approach using solely phase information to perform the separation. The sources are iteratively reconstructed using an adaptation of the Multiple Input Spectrogram Inversion (MISI) algorithm from Gunawan and Sen. The proposed method is then tested against conventional MISI and Wiener filtering on monophonic signals and oracle conditions. Results show that at the cost of a larger computation time, our method outperforms both MISI and Wiener filtering in oracle conditions with much higher objective quality even with phase quantization.

Résumé: Monitoring the lesion formation induced by histotripsy has mainly relied on the quantitative change in backscatter intensity using ultrasound B-mode imaging. However, how the mechanical properties of the histotripsy-treated tissue region alter during the procedure is yet to be fully investigated. We thus proposed here to monitor such a therapeutic process based on shear modulus estimated by shear wave imaging (SWI). In the therapeutic procedure, a single-element piezo-composite focused transducer (Imasonic, Besançon, France) with a center frequency of 660 kHz, a focal length of 45 mm, and an fnumber of 1 was driven by a function generator (AFG 3101, Tektronix, Beaverton, OR) and a gated RF power amplifier (GA-2500A, RITEC Inc., USA) to generate ultrasound histotripsy pulses. Histotripsy pulses were delivered for 20 seconds and then followed by a 30-second pause and a rapid monitoring step. Such a treatment and monitoring scheme was repeated for 10 mins. Both the reference measurement and monitoring were realized by SWI, where plane shear waves were generated by an 8 MHz linear array probe connected to a prototype ultrasound scanner, and acquired at a frame rate of 10000 Hz. Shear modulus was estimated and mapped in 2D through a time-of-flight algorithm. Gelatin (8%)-agar (2%) phantoms and ex-vivo porcine liver samples were tested. Regions of interests (ROI's) of 2 mm-by-2 mm in both untreated and treated regions were selected to compute the contrast-to-noise ratio (CNR). In all three scenarios where different PD's and PRF's were implemented, during the first 100 seconds of the treatment, 50% decrease in the shear modulus within the histotripsy-targeted zone was already observed, and the CNR of the shear modulus increased by 18 dB. In contrast, the backscatter intensity began to reduce and the corresponding CNR was found to increase by 6 dB only after 120 seconds of treatment. The results demonstrated that SWI can map quantitatively the change of mechanical properties during histotripsy treatment. Moreover, the shear modulus estimated by SWI was a more sensitive indicator of the lesion formation than the backscatter intensity obtained from B-mode at the early stage of the histotripsy treatment. In-vitro experiments on liver samples have also been carried out. © 2012 American Institute of Physics.

Résumé: In this paper, we propose a new low complexity method of transient detection using an image edge detection approach. In this method, the time-frequency spectrum of an audio signal is considered as an image. Using appropriate mapping function for converting energy bins into pixels, audio transients correspond to rectilinear edges in the image. Then, the transient detection problem is equivalent to an edge detection problem. Inspired by standard image methods of edge detection, we derive a detection function specific to rectilinear edges that can be implemented with a very low complexity. Our method is evaluated in two practical audio coding applications, in replacement of the SBR transient detector in HEAAC+ V2 and in the stereo parametric tool of MPEG USAC.

Résumé: The goal of source separation algorithms is to recover the constituent sources, or audio objects, from their mixture. However, blind algorithms still do not yield estimates of sufficient quality for many practical uses. Informed Source Separation (ISS) is a solution to make separation robust when the audio objects are known during a so-called encoding stage. During that stage, a small amount of side information is computed and transmitted with the mixture. At a decoding stage, when the sources are no longer available, the mixture is processed using the side information to recover the audio objects, thus greatly improving the quality of the estimates at a cost of additional bitrate which depends on the size of the side information. In this study, we compare six methods from the state of the art in terms of quality versus bitrate, and show that a good separation performance can be attained at competitive bitrates. © 2012 EURASIP.

Résumé: We present an original framework for the detection of repeating objects in multimedia streams. This framework is designed so that it can work with any fingerprint model. A fingerprint is extracted for each incoming frame of the multimedia stream. The framework then manages this fingerprint so that if one similar frame comes later in the stream, it will be identified as a repetition. The framework has been tested with two distinct fingerprint models on simulated and real-world data. The results show that the framework performs well with both presented models and that it is suitable for industrial use-cases. © 2012 EURASIP.

Résumé: We consider the problem of calibrating a compressed sensing measurement system under the assumption that the decalibration consists in unknown gains on each measure. We focus on blind calibration, using measures performed on a few unknown (but sparse) signals. A naive formulation of this blind calibration problem, using ℓ 1 minimization, is reminiscent of blind source separation and dictionary learning, which are known to be highly non-convex and riddled with local minima. In the considered context, we show that in fact this formulation can be exactly expressed as a convex optimization problem, and can be solved using off-the-shelf algorithms. Numerical simulations demonstrate the effectiveness of the approach even for highly uncalibrated measures, when a sufficient number of (unknown, but sparse) calibrating signals is provided. We observe that the success/failure of the approach seems to obey sharp phase transitions. © 2012 IEEE.

Résumé: Taking advantage of the structures inherent in many sparse decompositions constitutes a promising research axis. In this paper, we address this problem from a Bayesian point of view. We exploit a Boltzmann machine, allowing to take a large variety of structures into account, and focus on the resolution of a joint maximum a posteriori problem. The proposed algorithm, called Structured Bayesian Orthogonal Matching Pursuit (SBOMP), is a structured extension of the Bayesian Orthogonal Matching Pursuit algorithm (BOMP) introduced in our previous work [1]. In numerical tests involving a recovery problem, SBOMP is shown to have good performance over a wide range of sparsity levels while keeping a reasonable computational complexity. © 2012 IEEE.

Résumé: In most parametric stereo audio coders, sets of spatial parameters are extracted from the audio channels in a time-frequency domain. In order to reduce the amount of data, the parameters plane is highly down-sampled, and transmitted together with a mono downmix. Then, in the decoding process, it is necessary to interpolate the upmix matrix computed from these parameters. Usually, this is done in the same way for each portion of signal, regardless of its nature. In this article, we propose a dynamic strategy of window splitting, estimation of the parameters and interpolation of the upmix matrix based on transient detection in the audio signal. Subjective tests show an improvement when applied to the new stereo parametric tool from MPEG USAC. © 2012 IEEE.

Résumé: Many proofs of concept studies have established the mechanical sensitivity of functionalized microcantilevers to a large spectrum of target molecules. However, moving to real-life applications also requires the monitored mechanical effect to be highly specific. On the other hand, describing the involved surface effects in the continuum mechanics framework is still challenging. Several attempts to overcome the "surface stress" failure to satisfy field equations tend to show such a description has to be non-local, so that at least one 'characteristic length' parameter has to be used. The consequence is twofold: first, suited modelings have to be developed to describe the surface effects at the cantilever scale; and second, the involved characteristic length is (thermodynamically) connected to the molecular mechanisms at the cantilever surface, and may therefore be a key parameter for the target molecules identification. This requires to experimentally access displacement fields induced by the molecular interactions under scrutiny. A set-up providing mechanical and chemical fields along the cantilever is described, and is implemented focusing on cases where the cantilever's surface reacts heterogeneously. The large amount of data obtained using full-field set-ups is redundant from the mechanical point-of-view, and this redundancy may be used to identify some of the key parameters describing the mechanical surface effects. © 2012 Published by Elsevier B.V.

Résumé: In this paper, we show how the concept of hybridization band gaps can be utilized to create antennas for MIMO applications. Those strongly decoupled antennas present at the same time a very small form factor and a very low correlation. To that aim, we first explain briefly the concept of hybridization between a resonator and the free space waves continuum. Then we expose the methodology we use to design multi-ports antennas based on that concept. We present numerical and experimental results of 2 ports MIMO antennas at 2.45 GHz, printed on a PCB, whose areas are smaller than 2.6*2.6 cm 2. The two ports display experimentally peak gains of a about 4 dB, efficiencies of 80%, a coupling lower than -30 dB and a correlation lower than 0.1. © 2012 IEEE.

Résumé: Optical Coherence Tomography (OCT) is an efficient technique for in-depth optical biopsy of biological tissues, relying on interferometric selection of ballistic photons. Full-Field Optical Coherence Tomography (FF-OCT) is an alternative approach to Fourier-domain OCT (spectral or swept-source), allowing parallel acquisition of en-face optical sections. Using medium numerical aperture objective, it is possible to reach an isotropic resolution of about 1x1x1 Am After stitching a grid of acquired images, FF-OCT gives access to the architecture of the tissue, for both macroscopic and microscopic structures, in a non-invasive process, which makes the technique particularly suitable for applications in pathology. Here we report a multimodal approach to FF-OCT, combining two Full-Field techniques for collecting a backscattered endogeneous OCT image and a fluorescence exogeneous image in parallel. Considering pathological diagnosis of cancer, visualization of cell nuclei is of paramount importance. OCT images, even for the highest resolution, usually fail to identify individual nuclei due to the nature of the optical contrast used. We have built a multimodal optical microscope based on the combination of FF-OCT and Structured Illumination Microscopy (SIM). We used x30 immersion objectives, with a numerical aperture of 1.05, allowing for sub-micron transverse resolution. Fluorescent staining of nuclei was obtained using specific fluorescent dyes such as acridine orange. We present multimodal images of healthy and pathological skin tissue at various scales. This instrumental development paves the way for improvements of standard pathology procedures, as a faster, non sacrificial, operator independent digital optical method compared to frozen sections. © 2012 SPIE.

Résumé: The first experimental evidence of speckle instability is realized by using a liquid crystal cell with a photosensitive wall. The light is transversally scattered in the liquid crystal cell, where a two-dimensional controlled disorder is imprinted through suitable illuminations of the photoconductive wall and the nonlinearity is obtained through optical reorientation of the liquid crystal molecules. Above a critical threshold of the input intensity, the speckle pattern starts spontaneously to oscillate with a characteristic frequency related to the response time of the nonlinear medium. Moreover, the oscillation threshold depends on the scattering mean free path, thus, confirming the crucial role played by disorder in inducing the instability. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Résumé: The goal of this paper is to interpolate Room Impulse Responses (RIRs) within a whole volume, from a few measurements. We here focus on the early reflections, that have the key property of being sparse in the time domain: this can be exploited in a framework of model-based Compressed Sensing. Starting from a set of RIRs randomly sampled in space by a 3D microphone array, we use a modified Matching Pursuit algorithm to estimate the position of a small set of virtual sources. Then, the reconstruction of the RIRs at interpolated positions is performed using a projection onto a basis of monopoles. This approach is validated both by numerical and experimental measurements using a 120-microphone 3D array. © 2011 IEEE.

Résumé: The cornea is mainly composed of type I collagen fibrils. The organization of those fibers ensures the cornea transparency and determines its biomechanical properties. Understanding the biomechanics of the cornea has become a crucial issue in ophthalmology to predict the cornea response to refractive surgery procedures and to avoid post-treatment complications. In this work, we proposed the Supersonic Shear Imaging (SSI) elastography as a method for the in vivo assessment of the cornea elastic anisotropy. The tissue shear modulus can be retrieved from the speed of a shear wave propagating in this tissue. In the SSI method, the transient shear wave is induced using the ultrasonic radiation force. The resulting shear wave propagates transversally to the ultrasound beam axis. The probe is then switched to an ultrafast imaging mode (30000 frames/sec) to follow the shear wave propagation and thus evaluate its local speed. We implemented SSI with a high-frequency rotating linear array (15 MHz, 128 elements) to estimate the elastic anisotropy of the cornea. We performed 3D scans in vivo and ex vivo on porcine eyes. Elasticity maps of the cornea surface were obtained. Porcine corneas exhibited a significantly higher shear wave speed along the horizontal meridian of the cornea than along the diagonal and vertical directions within a 50°-wide sector. These results are consistent with ex vivo X-ray diffraction measurements reported in the literature [1] that have shown that the collagen fibers are mainly oriented along one preferential direction in porcine corneas within a 45°-wide sector. © 2011 IEEE.

Résumé: The theory of the interaction of elastic waves with dislocations is reviewed, as is the extent to which it has been tested by experiment. There are two essential ingredients to the wave-dislocation interaction: one is that, when a wave hits a dislocation, the latter will respond by moving in some fashion. The other is that, when a dislocation moves, it generates ("radiates") elastic waves. For a linearly elastic solid continuum, both phenomena can be described by equations that are linear outside the dislocation core. One is a linear elastic wave equation with a right-hand-side term that is localized at the dislocation position. The other is a linear equation for the vibrations of a string (that is coincident with the dislocation), with an external loading provided by the wave. This provides the basic mechanism for the scattering of elastic waves by dislocations, and it can be worked out in considerable detail for pinned dislocation segments and prismatic dislocation loops in infinite media, as well as for the scattering of surface (Rayleigh) elastic waves by subsurface dislocation segments. The results for the scattering by a single dislocation can be used as input in a multiple scattering formalism to study the properties of a coherent wave propagating in a solid with many dislocations present. Expressions for the effective velocity of propagation, and for the disorder-induced (as distinct from the internal losses) attenuation can be found. They test successfully with Resonant Ultrasound Spectroscopy (RUS) experimental measurements. Open problems, possible further applications and current efforts are discussed. © 2012 Materials Research Society.

Résumé: Imaging intramyocardial vascular flows could strongly help to achieve better diagnostic of cardiovascular diseases but no standard imaging modalities allow describing accurately myocardial blood flow dynamics with good spatial and temporal resolution. We recently introduced a novel Doppler imaging technique based on compounded plane waves transmitted at ultrafast frame rate. The high sensitivity and framerate of the Doppler technique enable imaging the intramyocardial blood flow and its dynamics. A special demodulation-filtering process achieved to compensate for the large tissue velocity of the myocardium and a signed power Doppler process enabled the possibility to discriminate arterial and venous flows. Experiments were performed in vivo in N=5 open chest sheep using a conventional ultrasonic probe placed at the surface of the heart. Results show the capability of the technique to image intramyocardial vascular flows in normal physiological conditions with good spatial and temporal resolution. The flow dynamics over the cardiac cycle was investigated and showed a phase opposition of the velocity waveform between arterial and venous flows. Finally, the main diagonal coronary artery was occluded and the vascular flows were found to completely disappear in the ischemic region. © 2011 IEEE.

Résumé: In this paper we present the final results of a project devoted to show the abilities of Time Reversal processing for Through-The-Wall (TTW) detection of human targets. From a multistatic-multiview UWB radar system, experimental data from realistic TTW scenarios are collected and rearranged to form the multistatic data matrix K. The results of the singular value decomposition of the K matrix are used to build an efficient detector combining directions and arrival times. Images from different radar scenes (with canonic or human targets) are presented and discussed. © 2011 European Microwave Assoc.

Résumé: "Compressive sampling" (CS) is a new signal acquisition strategy that intends to reduce significantly the amount of recorded data by picking only a limited number of samples. CS theory asserts that one can reconstruct a given signal from a few randomly distributed samples if only the signal is sparse in a proper basis. CS ensures a minimum loss of information but requires, for the reconstruction of the signal, the use of dedicated sparsity-promoting algorithms. In this paper, CS is applied to the source localization problem using an array of randomly distributed microphones. In this case, the signal of interest is sparse in the spatial domain, i.e a few positions in space contain sources. We focus on the near-field beamforming where the array of sensors is sensitive to the sources directivity. The localization method is extended to complex sources and we attempt to identify them in terms of multipoles. Numerical simulations and experimental results prove this sparsity-promoting method to be powerful for source localization. However the identification step, quite successful on ideal data, is not sufficiently robust when applied to experimental data and need further investigations.

Résumé: Due to the simple properties of plane waves, 11011 lossy straight, pipes and their concatenation have been extensively used to compute acoustic transfer functions from bore profiles of wind instruments (input, impedance, transmittance, etc). This is also the case for real-time simulations: introducing travelling waves has led to the well-known digital waveguides formalism. Nevertheless, such discontinuous concatenations involve impulse responses composed of pulse trains of Dirac measures, which are structurally unrealistic for smooth bores. Similarly, continuous but. 11011 smooth approximations based on conical segments involve discontinuous pulse trains of damped exponentials. This invited paper presents an overview of results that, have been elaborated to weaken such artifacts and increase realism, while preserving most, of the worthwhile properties of straight, pipes. The key steps are based on the use of: (1) a refined ID wave equation (curvilinear horn equation) based on an isobar map rectification: (2) smooth (Cl-regular) junctions of constant-flared acoustic pipes: (3) a radiation model which is compatible with (1): (4) visco-thermal losses. It. allows to recover a standard matrix formalism to compute impedances and transmittances of smooth bore parts that, yield accurate results. It. still make definitions of travelling waves and digital waveguide-like structures possible for the simulation. Finally, by representing smooth bores by very few flared segments (compared to many straight, or conical pipes), such descriptions (with a few parameters) are an interesting alternative to optimize wind instrument, bores w.r.t.. some criteria (target, shape or impedance, harmonicity, etc).

Résumé: The ultrasonic characterization of bone biomechanical properties can improve bone fracture risk assessment with a non-invasive, non-ionizing and relatively inexpensive method. These methods appear to have significant potential and are currently underutilized because the physics of ultrasound propagation in bone is still poorly understood. Some phenomena experimentally observed in trabecular bone remain indeed unclear, such as the possible propagation of two longitudinal waves with different velocities. This study focuses on determining the conditions necessary to develop two waves on a simple model, and in particular on the influence of guided modes in their occurrence. © 2011 IEEE.

Résumé: Directly measuring the full set of acoustic impulse responses within a room would require an unreasonably large number of measurements. Considering that the acoustic wavefield is sparse in some dictionaries, Compressed Sensing allows the recovery of the full wavefield with a reduced set of measurements, but raises challenging computational and memory issues. Two practical algorithms are presented and compared: one that exploits the structured sparsity of the soundfield, with projections of the modes onto plane waves sharing the same wavenumber, and one that computes a sparse decomposition on a dictionary of independent plane waves with time/space variable separation. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Résumé: We study the phase of the electric field in the vicinity of two electric dipoles transversely coupled in two fundamental cases: a nanogap antenna illuminated from the far field and a directive antenna made of a single metallic particle coupled to an electric source dipole. We will link the scattering efficiency of the dimer (first case) and the directivity of the particle antenna (second case) to the relative phase of the two dipoles. The strong distance dependence of this phase term produces two unexpected effects: the bonding transverse mode can be the brightest mode of a nanogap antenna; and the emission directivity offered by a metallic particle can be tuned by varying the emitter/particle distance at a strongly sub-wavelength scale. © 2011 American Institute of Physics.

Résumé: Scattering-type near-field optical microscopy (sNSOM) allows one to map the propagation of purely evanescent waves such as surface plasmons polaritons (SPPs). We have recently used it to image SPPs generated electrically at the surface of a mid-infrared (MIR) quantum cascade laser (QCL) including all the building blocks for an integrated active plasmonic device [1]. This talk will report on sNSOM observations on our new generation of integrated active plasmonic devices operating in the MIR, in which SPPs are launched on a metallic waveguide at distances of several hundred of micrometers from the end facet of a QCL. © 2011 IEEE.

Résumé: The precision limit for the considered setups are depicted below. For TD and ITD fDOT, we note that (i) the accuracy of the depth estimation decreases with the depth of the fluorescent volume and (ii) this accuracy substatially depends on the fluorescence life-time . The longer the lifetime, the poorer the precision limit. Moreover, for realistic lifetime, the CW and the ITD setups in reflection geometry achieve comparable precision limits. © 2011 IEEE.

Résumé: Nanoantennas are the direct extension of conventional radio and microwave antennas to the visible frequency range and can be used to convert optical radiation into localized energy and resonantly enhance light scattering [1,2]. Here, we present a highly sensitive full-field imaging technique based on digital heterodyne holography [3] which allows the measurement of both amplitude and phase. This results in the 3D mapping of light scattered by plasmonic nanostructures at specific resonance wavelengths. Spectral and spatial properties of the field are compared to numerical simulations. © 2011 IEEE.

Résumé: This paper deals with sparse representations within a Bayesian framework. For a Bernoulli-Gaussian model, we here propose a method based on a mean-field approximation to estimate the support of the signal. In numerical tests involving a recovery problem, the resulting algorithm is shown to have good performance over a wide range of sparsity levels, compared to various state-of-the-art algorithms. © 2011 IEEE.

Résumé: We present a basic building block for the realization of integrated active plasmonic devices: a distributed-feedback semiconductor laser working at room temperature and 1.3m obtained with metal patterning on a thinned top cladding. © 2011 OSA.

Résumé: In this paper, a new class of audio representations is introduced, together with a corresponding fast decomposition algorithm. The main feature of these representations is that they are both sparse and approximately shift-invariant, which allows similarity search in a sparse domain. The common sparse support of detected similar patterns is then used to factorize their representations. The potential of this method for simultaneous structural analysis and compressing tasks is illustrated by preliminary experiments on simple musical data. © 2011 IEEE.

Résumé: Thulium-doped crystals are considered for programmable filtering. Such a filter can be acllieved by Spectral Hole Burning (SHB). One optically pumps the active ions into a long-lived shelving state, opening a narrow transparency window in the crystal absorption band. We investigate a new shelving scheme in Tm:YAG where the thulium ions are pumped into a ground state nuclear Zeeman sublevel, instead of their natural 10ms-lifetime metastable state. The shelving time is increased to several seconds, reducing the residual population in the transparency window by orders of magnitude. This should enhance the filter's dynamic range, which is essential in demanding filtering applications like ultrasound optical tomography. © 2011 SPIE.

Résumé: In this paper, we investigate the optimal ways to sample multichannel impulse responses, composed of a small number of exponentially damped sinusoids, under the constraint that the total number of samples is fixed - for instance with limited storage / computational power. We compute Cramér-Rao bounds for multichannel estimation of the parameters of a damped sinusoid. These bounds provide the length during which the signals should be measured to get the best results, roughly at 2 times the typical decay time of the sinusoid. Due to bandwidth constraints, the signals are best sampled irregularly, and variants of Matching Pursuit and MUSIC adapted to the irregular sampling and multichannel data are compared to the Cramér-Rao bounds. In practical situation, this method leads to savings in terms of memory, data throughput and computational complexity. © 2010 IEEE.

Résumé: The knowledge of the myocardial fiber architecture is deemed essential and yet to be nondestructively investigated for myocardial mechanics and its association with the progression of myocardial diseases. In this study, Supersonic Shear Wave Imaging (SSI) was performed for its first time to noninvasively assess the fiber orientation (i.e., anisotropy) in in vitro porcine (N=5) and in vivo open-chest ovine (N=1) hearts. The SSI technique combined with the coherent plane-wave compounding method was performed to achieve both high echocardiographic image quality and ultrafast frame rate (8000 fps). An 8 MHz linear array probe (pitch =0.2 mm) was mounted on a customized rotation device, which allowed 360 rotation at 5° increments. The probe was initially aligned (±90° inclination) with the longitudinal axis in the local cardiac coordinates. The variation of the myocardial fiber orientation across the wall was observed in both in vitro and in vivo cases. Myocardial fibers were oriented gradually from the left diagonal (upper left to bottom right) near the epicardium (100% wall thickness) to the right diagonal (upper right to bottom left) near the endocardium with the midwall region dominated by the circumferential fibers. This finding was in good agreement with the literature and histology and has demonstrated the feasibility of SSI in mapping myocardial anisotropy. © 2010 IEEE.

Résumé: Background: In order to focus ultrasound beams through aberrating layerssuch as fat or bones, adaptive focusing techniques have been proposed to improvethe focusing, mostly based on the backscattered echoes. We recently proposed anenergy-based technique with the sole requirement being knowledge of theacoustic intensity at the desired focus. Here, Magnetic Resonance- AcousticRadiation Force Imaging (MR-ARFI) is used to map the displacement induced by theradiation force of a focused ultrasound beam. As the maximum displacement isobtained with the best corrected beam, such a measurement can lead to aberrationcorrection. Material and methods: Proof of concept experiments were previouslyshown in a small animal MR at 7 T using a 64-elements linear phased arrayoperating at 6 MHz. Optimal refocusing was then obtained through numerical andphysical aberrating layers. This work is extended here in a clinical Philips 1.5T Achieva scanner. The HIFU beam is generated using a 512 elements US phasedarray (SuperSonic Imagine, France) dedicated to transcranial human experimentsand operating at 1 MHz. Experiments are conducted in phantom gels and ex vivobrain tissues through numerical phase aberrators. A motion-sensitized spin echosequence (TE 70 ms, TR 1200 ms, spatial resolution is 227 mm 3) isimplemented to measure displacements induced by the acoustic radiation force oftransmitted beams. Results: MR-ARFI allowed mapping the distribution of theradiation force at the focus of the array. After the recording of the MR phasesignals for different US emissions, the proposed adaptive focusing technique wasable to recover the spatial distribution of the phase aberrations. Totalacquisition time for 384 ultrasonic emission channels was 2 hours. Conclusion:Those first results in clinical MR at 1.5 T show that adaptive focusing of ahuman transcranial brain HIFU system can be achieved within reasonable timeunder MR guidance for aberrator layers as strong as human skull. Ongoing work isaiming at accelerating the acquisition in order to reach acceptable durationsfor in vivo protocols. © 2010 IEEE.

Résumé: Quantitative Increasing number of patients is affected by chronic kidney disease (CKD) world while with ensuing end-stage renal disease (ESRD). A change in global approach to CKD from costly treatment of ESRD to more aggressive primary and secondary prevention is imperative. This improvement needs: a better understanding of mechanisms underlying renal scarring, a development of specific therapies to slow the progression and a development of modern diagnostic tools to characterize these underlying processes. Number of patients with ESRD underestimates CKD has probably exceeded by a factor 50 this last ten years due to initiation factors as hypertension, diabetes, hyperlipidemia, obesity or smoking. In this work, the Supersonic Shear Imaging technique (SSI) was proved to be able to provide a quantitative follow up of kidney fibrosis on rats. Then, the technique was adapted on curved probes and proposed to map the in vivo viscoelastic properties of human renal transplants and compared with biopsy. The SSI technique is based on the radiation force induced by a conventional ultrasonic probe, to generate a planar shear wave deep into tissues. Then shear wave propagation throughout the medium is caught in real time thanks to an ultrafast ultrasound scanner (5000 frames/s). At last, a time of flight algorithm applied on these data allows to compute a map of the shear wave velocity of the kidney. A follow up study of 8 weeks on a model of glomerulosclerosis induced by L-Name on 50 rats has been conducted in order to investigate the accuracy of the technique. Quantitative maps of the kidney cortex were performed with an 8 MHz linear probe. Regarding human renal transplants, quantitative maps of the cortex elasticity were produced for each patient with a 2.5 MHz curved array. In the animal study the results show an increase of the elasticity of the cortex of the kidney in time with a Young's modulus ranging from 9 to 25 kPa. Results are compared to histological measurement as proteinuria and or quantification of fibrosis with trichrome. On human renal transplants, three groups were studied: a control group, subjects with an early or with a late unfunctional transplant. The results obtained using a curved ultrasonic probe show an 8 cm depth by 10 cm width map of the Young's modulus for the 49 patients. Elasticity varies between groups from 9 to 50 kPa. Results were compared to histology and showed a good agreement between fibrosis markers and Young's modulus. Here we have demonstrated the feasibility of inducing shear waves using the supersonic shear imaging technique in vivo in rats and human kidneys. The good reproducibility and accuracy of the shear elasticity are discussed. The elasticity was proved to be clearly related to kidney fibrosis. As fibrotic process is responsible for the development of most CKD, noninvasive identification and follow-up of this process could improve renal prognosis if adapted targeted therapies can be applied. © 2010 IEEE.

Résumé: We have previously proposed a novel method for measuring arterial stiffness using shear wave imaging (SWI). In this study, we evaluate the performance of this method on a healthy population (N30) and we compare it to local measurement of the arterial pulse wave velocity (PWV) achieved at the same arterial site using ultrafast imaging. Ultrafast imaging was used to track shear wave induced remotely by acoustic radiation force at the carotid site at a frame rate of 10000 images/second with a 8 MHz ultrasound probe. SWI was acquired with a repetition rate of 7 Hz in order to measure stiffness variation over the cardiac cycle. The axial-velocity field in the imaging plane was obtained using conventional Doppler algorithm. As shown in a previous study, acoustic radiation force applied normally to the arterial wall induces mainly a flexural guided mode (F0) which propagates between 100 to 1500 Hz. The dispersion curve of this mode is extracted from the 2D-FFT of the shear wave spatio-temporal velocity field within and along the arterial wall. The shear modulus is then estimated by a theoretical fit of the experimental dispersion curve. Ultrafast imaging was also used to measure the tissue velocity of the arterial wall at the same site at a frame rate of 1000 images/s during 1 second. PWV was estimated by tracking the pulse wave along the arterial wall based on spatio-temporal velocity field at early and end-systole. Each ultrasound measurement was repeated 3 times on each carotid to estimate the reproducibility of the technique. Lower reproducibility was found on systolic PWV measurements. Thanks to its higher frequency content, the PWV at end-systole was measured more accurately and was found to be more reliable, but was not visible in all volunteers. In contrast, SWI offered high reproducibility. Moreover, arterial stiffness was achieved 6 times per seconds contrary to one time per second for PWV, giving access to the arterial stiffness time variation during the cardiac cycle. © 2010 IEEE.

Résumé: In this paper is presented the DESAM Toolbox, a set of Matlab functions dedicated to the estimation of widely used spectral models for music signals. Although those models can be used in Music Information Retrieval (MIR) tasks, the core functions of the toolbox do not focus on any specific application. It is rather aimed at providing a range of state-of-the-art signal processing tools that decompose music files according to different signal models, giving rise to different "mid-level" representations. After motivating the need for such a toolbox, this paper offers an overview of the overall organization of the toolbox, and describes all available functionalities.

Résumé: Increasing interest is given to elasticity to monitor HIFU treatments, since elasticity changes during thermal ablation. Moreover, the shear modulus of in vitro soft tissue samples was proven to be linked to the thermal dose. Hence, the study aims to evaluate the link of in vivo elasticity changes, assessed by supersonic shear imaging, with temperature and thermal dose. Seven male rats (Sprague Dawley, 250 g) were anesthetized. The right leg was depilated and immersed in a thermo-regulated bath. Two optical temperature sensors were placed into the biceps femoris. The leg was warmed at a targeted temperature, from 38C to 50C, for a time corresponding to the theoretical threshold of necrosis in muscle (thermal dose 240 minutes at 43C). Every 40 seconds, shear plane waves were created in the biceps femoris, in a plane neighboring the thermocouples, using the Supersonic Shear Imaging method based on the radiation force. The shear wave propagation was acquired with 180 images at 10 000 frames/s. The changes of elasticity were assessed for the different temperature-time profiles in order to highlight its dependence with the thermal dose. The leg swelled when the temperature reached 39C. Then, contraction of the muscular fibers was observed on the Bmode images at 44-45C (thermal dose from 2 to 140 min), followed by a flow of fluid around the muscle. The effect of the thermal dose on the in vivo elasticity of the muscles was the same for all the rats. The shear modulus exponentially increased at a thermal dose was equal to 220 78 min. The cumulative thermal dose did not significantly change when the shear modulus reached 2.5 fold the initial value or more. Thus, this threshold value (220 min) was assumed to be the threshold of necrosis. This value is consistent with the necrosis of the skin and the theoretical threshold of necrosis of muscular tissues (240 min). This study gives promising prospects for the use of the elasticity to monitor thermal effects on tissues. © 2010 IEEE.

Résumé: A multi-modal ultrasound imaging study of cerebral ischemia was performed on a rat model. Ischemia was obtained by the transient 2h occlusion of a cerebral artery and the lesion induced was imaged 1, 2, 4 and 7 days after the stroke. With Bmode imaging, the lesion was found to be hyperechogenic compared to normal brain tissue. With Shear Wave Imaging, the elasticity of the brain was measured showing that the ischemic lesion is softer than normal brain tissue and that its elasticity decreases significantly over time. With Ultrafast Doppler, the perfusion in the brain was measured showing that the lesion is hyperperfused compared to normal brain tissue with a peak two days after the occlusion. Ultrafast Doppler was also proved able to monitor the occlusion. Future work will focused on applying this multi-modal study for ischemic lesions induced by apoxia on newborn babies through the fontanel. © 2010 IEEE.

Résumé: We investigate the benefits of evaluating Mel-frequency cepstral coefficients (MFCCs) over several time scales in the context of automatic musical instrument identification for signals that are monophonic but derived from real musical settings. We define several sets of features derived from MFCCs computed using multiple time resolutions, and compare their performance against other features that are computed using a single time resolution, such as MFCCs, and derivatives of MFCCs. We find that in each task - pairwise discrimination, and one vs. all classification - the features involving multiscale decompositions perform significantly better than features computed using a single timeresolution. © EURASIP, 2010.

Résumé: Early detection of biomechanical modifications in the arterial wall couldbe used as a predictor factor for various diseases, for example hypertension oratherosclerosis. In this work 1D transient elastography (TE) technique andSupersonic Shear Imaging (SSI) technique are used for the evaluation of theshear wave speed on a phantom consisting in 2.5 mm thickness viscoelastic layerand an arterial phantom embedded in gel. In the TE technique the polarization ofthe shear wave is parallel to its propagation and the true shear wave speed isretrieved. In that case the dispersion is mainly due to viscosity (Voigt'smodel). Regarding the SSI technique, the dispersion is due to the layerthickness being of the order of the shear wavelength: thus the shear wave isguided as a Lamb wave. In that case a model is needed in order to retrieve theshear wave speed from the dispersion curve. Finally through both techniquessimilar shear wave speed estimations are obtained. © 2010 IEEE.

Résumé: Quantitative measurements of cornea and skin biomechanical properties havemany applications in medicine. In ophthalmology, it could lead to a betterdiagnosis of pathologies or monitoring of treatments. In dermatology, it couldhelp the skin lesions removal monitoring. In the framework of Supersonic ShearImaging (SSI), these organs are characterized by their complex sub wavelengthgeometry (thin plate and thin capsule) that strongly influences the shear wavepropagation. In this work, a theoretical framework is proposed and validated inexperiments for the quantification of elastic modulus in these layered tissues.Shear wave dispersion induced by the guided propagation in such thin layers isestimated and fitted to the analytical dispersion curve derived from the leakyLamb Wave theory. SSI is refined and used in order to map in real time thetissues elasticity. This technique consists in generating a shear wave byultrasonic radiation force and imaging its propagation through the medium at ahigh frame rate (20 kHz). For infinite media the shear wave velocity is thenlinked to the Young's modulus. In cornea and skin layers, the high-frequencyshear wave (from 500 to 2000 Hz) is guided similarly to a Lamb wave, with platethickness (<1 mm) close to its wavelength. Experimental dispersion curveshave been confronted to numerical studies. First, finite differences simulationswere performed to obtain numerical dispersion curves in plates with exactlyknown thickness and elasticity. Besides, theoretical dispersion equations werederived by solving numerically the propagation equation. Dispersion curvesobtained in vitro on phantoms are found consistent with analytical calculations.Least mean squares fitting of curves enables to recover a quantitativeassessment of elasticity (standard deviation <10%). © 2010 IEEE.

Résumé: The Van Cittert-Zernike (VCZ) theorem describes the spatial covariance ofan incoherent source, such as the ultrasonic signal that is backscattered bytissue. A simulation tool that describes acoustic propagation, includingmultiple scattering and nonlinear propagation is used to characterize the effectof tissue structure on the spatial covariance. Simulated spatial covariancecurves are compared to in vivo data. It is shown that the non-random structureof tissue fundamentally modifies the spatial covariance function. Thereverberating echoes generated by the tissue layer add a decorrelated componentto the backscattered signal. It is shown that this decorrelation of the shortlag spatial covariance (SLSC) can provide a source of contrast to generateultrasound images at the fundamental and harmonic frequencies. Images ofanechoic lesions show that SLSC imaging has better contrast and boundarydefinition the B-mode images generated from the same echoes. © 2010 IEEE.

Résumé: This work deals with the physical modelling of acoustic pipes for real-time simulation, using the "Digital Waveguide Network" approach and the horn equation. With this approach, a piece of pipe is represented by a two-port system with a loop which involves two delays for wave propagation, and some subsystems without internal delay. A well-known form of this system is the "Kelly-Lochbaum" framework, which allows the reduction of the computation complexity. We focus this work on the simulation of pipes with a convex profile. But, using the "Kelly-Lochbaum" framework with the horn equation, two problems occur: first, even if the outputs are bound, some substates have their values which diverge; second, there is an infinite number of such substates. The system is then unstable and cannot be simulated as such. The solution of this problem is obtained with two steps. First, we show that there is a simple standard form compatible with the "Waveguide" approach, for which there is an infinite number of solutions which preserve the input/output relations. Second, we look for one solution which guarantees the stability of the system and which makes easier the approximation in order to get a low-cost simulation.

Résumé: We study the local density of states (LDOS) statistics near a dipole emitter embedded in a strongly scattering medium. We perform numerical simulations that emphasize the fact that LDOS fluctuations are strongly affected by the local environment of the emitter and is very sensitive to near-field interactions and correlation of disorder. © 2010 American Institute of Physics.

Résumé: A new dictionary selection approach for sparse coding, called parametric dictionary design, has recently been introduced. The aim is to choose a dictionary from a class of admissible dictionaries which can be presented parametrically. The designed dictionary satisfies a constraint, here the incoherence property, which can help conventional sparse coding methods to find sparser solutions in average. In this paper, an extra constraint will be applied on the parametric dictionaries to find a structured dictionary. Various structures can be imposed on dictionaries to promote a correlation between the atoms. We intentionally choose a structure to implement the dictionary using a set of filter banks. This indeed helps to implement the dictionary-signal multiplications more efficiently. The price we pay for the extra structure is that the designed dictionary is not as incoherent as unstructured parametric designed dictionaries. ©2010 IEEE.

Résumé: Optical nanoantennas are studied by full field heterodyne numerical holography in total internal reflection. After a spectroscopic characterization, the 3D scattering pattern of gold nanodisc chains was measured for wavelengths in and out of resonance. © 2010 Optical Society of America.

Résumé: We introduce a method to measure the transmission matrix of a complex medium. This matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. ©2010 IEEE.

Résumé: Shear Wave Imaging (SWI) is an ultrasound based technique for elasticity imaging that has been successfully tested on several organs in the framework of cancer diagnosis. In this work, the potential of this technique to map brain elasticity in vivo on trepanned small animals is investigated. From a SWI scan of the rat brain, 3D elasticity maps are reconstructed reaching a spatial resolution of 800 μm. The dynamic modulus of the brain tissues exhibits values in the 1 to 16 kPa range and is quantified for different anatomical regions. The propagation of shear waves is found to be anisotropic, which could be a consequence of fiber orientation. Finally, the interest of brain elasticity mapping for the monitoring of brain ischemia is investigated on a rat model. Focal cerebral ischemia is shown to induce a dramatic decrease of elasticity in the lesion. ©2010 IEEE.

Résumé: This paper describes an original microscopy imaging framework successfully employing Compressed Sensing for digital holography. Our approach combines a sparsity minimization algorithm to reconstruct the image and digital holography to perform quadrature-resolved random measurements of an optical field in a diffraction plane. Compressed Sensing is a recent theory establishing that near-exact recovery of an unknown sparse signal is possible from a small number of non-structured measurements. We demonstrate with practical experiments on holographic microscopy images of cerebral blood flow that our CS approach enables optimal reconstruction from a very limited number of measurements while being robust to high noise levels. © 2010 IEEE.

Résumé: Active plasmonic devices are much promising for optical devices and circuits at the nanoscale. We show that single nanoparticles coupled to metallic surfaces are good candidates for integrated components with nanometric dimensions. The localized plasmon of the nanoparticle launches propagating surface plasmons in the metallic thin film. Direct particle observation using leaky wave microscope geometry permits easy detection through the interference of the direct transmitted excitation light and the surface plasmon leaky mode. Investigations of the optical response of a nanoparticle deposited on metallic thin metal films reveals unexpectedly high transmission of light associated to contrast inversion in the images. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Résumé: Single biomolecule behaviour can reveal crucial information about processes not accessible by ensemble measurements. It thus represents a real biotechnological challenge. Common optical microscopy approaches require pico- to nano-molar concentrations in order to isolate an individual molecule in the observation volume. However, biologically relevant conditions often involve micromolar concentrations, which impose a drastic reduction of the conventional observation volume by at least three orders of magnitude. This confinement is also crucial for mapping sub-wavelength heterogeneities in cells, which play an important role in many biological processes. We propose an original approach, which couples Fluorescence Correlation Spectroscopy (FCS), a powerful tool to retrieve essential information on single molecular behaviour, and nano-fakir substrates with strong field enhancements and confinements at their surface. These electromagnetic singularities at nanometer scale, called "hotspots", are the result of the unique optical properties of surface plasmons. They provide an elegant means for studying single-molecule dynamics at high concentrations by reducing dramatically the excitation volume and enhancing the fluorophore signal by several orders of magnitude. The nano-fakir substrates used are obtained from etching optical fiber bundles followed by sputtering of a gold thin-film. It allows one to design reproducible arrays of nanotips. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Résumé: A novel MR-guided brain therapy device operating at 1 MHz has been designed and constructed. The system has been installed and tested in a clinical 1.5 T Philips Achieva MRI. Three dimensional time domain finite differences simulations were used to compute the propagation of the wave field through three human skulls. The simulated phase distortions were used as inputs for transcranial correction and the corresponding pressure fields were scanned in the focal plane. At half of the maximum power (10W/cm2 on the surface of the transducers), necroses were induced 2 cm deep in turkey breasts placed behind a human skull. In vitro experiments on human skulls show that simulations restore more than 85% of the pressure level through the skull bone when compared to a control correction performed with an implanted hydrophone. Finally, high power experiments are performed though the skull bone and a MR-Thermometry sequence is used to map the temperature rise in a brain phantom every 3 s in two orthogonal planes (focal plane and along the axis of the probe). © 2010 American Institute of Physics.

Résumé: Adaptive focusing of ultrasonic waves is performed under the guidance of a Magnetic Resonance (MR) system. The technique is based on the maximization of the ultrasonic wave intensity at a target point. The wave intensity is indirectly estimated from the local tissue motion induced at the chosen focus by the acoustic radiation force of the ultrasonic beam. A motion sensitive MR sequence is used to measure the resulting local tissue displacements. Based on the transmission of a set of spatially coded ultrasonic waves, a non iterative inversion process is used to estimate the phase aberrations induced by the propagation medium and to maximize the acoustical intensity at the target. Both programmable and physical aberrating layers introducing strong distortions (up to 2π radians) are recovered within acceptable errors (<0.8 rad). This non invasive technique is shown to accurately correct the phase aberrations in a phantom gel with negligible heat deposition and limited acquisition time. These refocusing performances demonstrate a major potential in the field of MR-Guided Ultrasound Therapy in particular for transcranial brain HIFU. © 2010 American Institute of Physics.

Résumé: Adaptive focusing of ultrasonic waves is performed under the guidance of a Magnetic Resonance (MR) system. The technique is based on the maximization of the ultrasonic wave intensity at a target point. The wave intensity is indirectly estimated from the local tissue motion induced at the chosen focus by the acoustic radiation force of the ultrasonic beam. A motion sensitive MR sequence is used to measure the resulting local tissue displacements. Based on the transmission of a set of spatially coded ultrasonic waves, a non iterative inversion process is used to estimate the phase aberrations induced by the propagation medium and to maximize the acoustical intensity at the target. Both programmable and physical aberrating layers introducing strong distortions (up to 2π radians) are recovered within acceptable errors (<0.8 rad). This non invasive technique is shown to accurately correct the phase aberrations in a phantom gel with negligible heat deposition and limited acquisition time. These refocusing performances demonstrate a major potential in the field of MR-Guided Ultrasound Therapy in particular for transcranial brain HIFU. ©2009 IEEE.

Résumé: In soft solids, such as biological tissue, the low value of shear elasticity allows the propagation of transverse waves with a high particle velocity when compared to the wave speed. With such a large Mach number shock waves can develop over a few wavelengths. We study the reflection of shocked shear waves traveling through a tissue-like medium, incident at a small angle on a free surface analytically and with numerical simulations. ©2009 IEEE.

Résumé: Supersonic Shear Imaging (SSI) is an ultrasound based technique for elasticity imaging that has been successfully tested on several organs in the framework of cancer diagnosis. In this work, the potential of this technique to map brain elasticity in vivo on trepanned small animals is investigated. From a SSI scan in the sagittal plane of the brain, a 3D elasticity map is reconstructed. A good correlation between the elasticity map and anatomy is observed. The values of dynamic modulus lie between 1 and 16 kPa and grey matter appears softer than white matter. The propagation of shear waves is found to be anisotropic in the cortex, which could be a consequence of the fiber orientation. Moreover, the good temporal resolution of this technique allows a dynamic estimation of brain elasticity within one single cardiac cycle. In the cortex, it is shown that the passage of the arterial pulse do not transiently modify elasticity. Next experiments will focus on applying this new technique in small animal models of neurological pathologies to estimate the effect of damaged tissues on elasticity. ©2009 IEEE.

Résumé: Although harmonic imaging is used extensively, the mechanisms for image quality improvement are still poorly understood. In the simplified case of a homogeneous medium the improvements in harmonic imaging can be linked to reductions in the main lobe width and the height of side lobes. It has been suggested, however, that the primary benefit of harmonic imaging is reduction of clutter noise and less sensitivity to phase aberration. This requires complex simulations or experiments that incorporate the effects of tissue heterogeneities, multiple scattering, and nonlinearity, which has imposed significant challenges in describing and quantifying the mechanisms of image quality improvement with harmonic imaging. ©2009 IEEE.

Résumé: The concept of synthetic imaging using circular wave is proposed to image with a very large field of view and at a very high frame rate (>1000 images/sec) heart motions with a conventional cardiac phased array probe. The goal of this study is to demonstrate in vivo the feasibility of this technique. Experiments are first performed in-vitro on ultrasound phantoms to optimize the trade-off between image quality and frame rate. An in vivo study is then performed on 10 sheep with a conventional phased array probe placed directly on the epicardium at different locations to obtain cine-loop of a complete heart cycle in the conventional imaging planes (long and short axis). After classical post processing of acquired cine-loop (wall tracking and tissue Doppler velocity estimation), the propagation of mechanical waves induced naturally during the heart cycle such as aortic and mitral valves closure can be observed. ©2009 IEEE.

Résumé: In the framework of HIFU transcranial brain therapy, it is mandatory to develop techniques capable of assessing the focusing quality and location before the treatment. Monitoring heat deposition in real time and verifying the extension of the treated area are also important steps. In this study, an imaging protocol is proposed to:1/ locate the US radiation force induced displacement in tissues and quantify the acoustic pressure at focus prior to HIFU; 2/ monitor the temperature rise during HIFU; and 3/ assess the changes in elasticity in the treated area. A 7T MRI scanner was equipped with a home-made stereotactic frame for rats and a US focused transducer working at 1.5MHz. Such a tool is key for the evaluation of the biological effects of HIFU on brain tissue and tumors. The proposed protocol was successfully tested on 12 rats with and without injected tumors. The accurate localization of the focal point prior to HIFU was demonstrated in vivo. Furthermore, the pressure estimation in situ allowed to accurately simulate the heat deposition at focus and to plan the treatment (electrical power, duration). The temperature measurements were in good accordance with the predicted curves. The elasticity maps showed significant changes after treatment in some cases. ©2009 IEEE.

Résumé: The targeting moiety (B-subunit) of the Shiga toxin (STxB) is a very potent ligand for the glycolipid Gb3, which is expressed in ovarian, colorectal and breast carcinomas. It is also present on endothelial cells of tumor neovascularization. This study demonstrates the use of STxB for targeting microbubbles onto cancerous cells. STxB-functionalized microbubbles and biotinylated controls were incubated with colorectal carcinoma HT29 in opticell plates. The culture plates were observed by fluorescence microscopy. The plates were then installed in a water-bath under a 8 MHz transducer array. STxB and control microbubbles were also injected in nude mice with subcutaneous breast tumors, before ultrasound imaging. FACS analysis demonstrated that STxB was stably associated with the microbubbles. Fluorescence microscopy showed that STxB-functionalized microbubbles adhered favorably to the Gb3 expressing cells, as compared to cells in which Gb3 expression was inhibited. Disruption ultrasonography of the culture plates showed a 12 dB difference in average backscatter intensity of the surface of Gb3 expressing cells, compared to Gb3-negative cells. An intensity difference of 18 dB was also observed between cells that were incubated with STxB-functionalized-microbubbles, as compared to unspecific microbubbles. The experiment in mice showed a significant increase in microbubble-signal intensity within gb3-positive tumors after being injected with STxB-microbubbles. These in vitro and in vivo experiments showed that STxB-functionalized microbubbles bind specifically to cells expressing the Gb3 glycolipid. The targeting moieties of toxins are a new group of ligands for microbubbles and have several advantages compared to antibodies and small peptides. ©2009 IEEE.

Résumé: Due to the non-linearity of Hertzian contacts, the speed of sound in granular matter increases with pressure. For a packing under gravity and in the presence of a free surface, bulk acoustic waves cannot propagate due to the inherent refraction toward the surface (the mirage effect). Thus, only modes corresponding to surface waves (Raleigh-Hertz modes) are able to propagate the acoustic signal. First, based on a non-linear elasticity model, we describe the main features associated to these surface waves. We show that under gravity, a granular packing is from the acoustic propagation point of view an index gradient waveguide that selects modes of two distinct families i.e. the sagittal and transverse waves localized in the vicinity of the free surface. A striking feature of these surface waves is the multi-modal propagation: for both transverse and sagittal waves, we show the existence of a infinite but discrete series of propagating modes. In each case, we determine the mode shape and and the corresponding dispersion relation. In the case of a finite size system, a geometric waveguide is superimposed to the index gradient wave guide. In this later case, the dispersion relations are modified by the appearance of a cut-off frequency that scales with depth. The second part is devoted to an experimental study of surface waves propagating in a granular packing confined in a long channel. This set-up allows to tune a monomodal emission by taking advantage of the geometric waveguide features combined with properly designed emitters. For both sagittal and transverses waves, we were able to isolate a single mode (the fundamental one) and to plot the dispersion relation. This measurements agree well with the Hertzian scaling law as predicted by meanfield models. Furthermore, it allows us to determine quantitatively relations on the elastic moduli. However, we observe that our data yield a shear modulus abnormally weak when compared to several meanfield predictions. © 2009 American Institute of Physics.

Résumé: We propose a general concept of adaptive focusing through complex media based on the estimation or measurement of the wave energy density at the desired focal spot. As it does not require the knowledge of phase information, this technique has many potential applications in acoustics and optics for light focusing through diffusive media. We present here the application of this technique to the problem of ultrasonic aberration correction for HIFU treatments. The estimation of wave energy density is based on the maximization of the ultrasound radiation force, using a multi-elements (64) array. A spatial coded excitation method is developed by using ad-hoc virtual transducers that include all the elements for each emission. The radiation force is maximized by optimizing the displacement of a small target at the focus. We measured the target displacement using ultrasound pulse echo on the same elements. A method using spatial coded excitation is developed in order to estimate the phase and amplitude aberration based on the target displacement. We validated this method using phase aberration up to 2π. The phase correction is achieved and the pressure field is measured using a needle hydrophone. The acoustic intensity at the focus is restored through very large aberrations. Basic experiments for brain HIFU treatment are presented. Optimal transcranial adaptive focusing is performed using a limited number of short ultrasonic radiation force pushes. © 2009 American Institute of Physics.

Résumé: Harmonic imaging has been shown to yield significant improvements in image quality over conventional ultrasound imaging. It has been proposed that harmonic imaging generates these improvements by the reduction in clutter from reverberation in the tissue layers underlying the transducer, a reduction in beam distortion from aberration, and a reduction in clutter due to suppressed sidelobes. There is little research indicating the exact sources of clutter and how they may relate to the improvements observed with in vivo harmonic imaging. We describe simulation and experimental studies in human bladders describing the sources and characteristics of clutter and discuss their relationship to the above proposed mechanisms. The results indicate that a large source of clutter is the product of reverberation in the abdominal layers. Experimental and simulated harmonic images indicate a 3-5 and 3-8 dB reduction in clutter over fundamental images, respectively, in the upper bladder cavity, lending support for the first mechanism described above. Scattering was also observed from off-axis sources in both the fundamental and harmonic images. Simulations of the fundamental point-spread-function (PSF) showed clutter magnitudes of -43 dB in the isochronous volume. Harmonic imaging marginally improved clutter magnitude to -47 dB in this same region. When aberration was removed from the simulation while keeping the impedance constant, the isochronous volume in the fundamental PSF marginally improved to -47 dB, while harmonic imaging improved this region to -58 dB, a reduction of 11 dB. This indicates that the image quality improvements seen with harmonic imaging are more dependent on the reduction in clutter from near-field layers than with reductions in clutter due to aberration. © 2009 SPIE.

Résumé: Because of their high surface to volume ratio, the mechanical behavior of micrometer sized structures is significantly more surface-driven than that of usual macroscopic objects. This property has been used to devise micromechanical sensors of environmental changes [1], with a main focus on biological sensors [2]. However, the development of reliable micromechanical sensors requires some basic modeling of the coupled phenomena under scrutiny [3], which is a challenging task, because of the difficulty to ensure the homogeneity of the chemical effects at the micrometer scale. An alternative route is to measure the chemical effect heterogeneity on the MEMS surface. This requires experimental data, complementary to the kinematic ones. We demonstrate herein that these complementary data may be obtained using a multiple wavelengths imaging set-up, which allows for the simultaneous kinematic and complementary fields measurements. These highly redundant data may then be used to experimentally build a mechanical modeling of the chemically induced mechanical loading. The whole process is then illustrated on micro-cantilevers subjected to electro- elastic coupling. © 2008 Society for Experimental Mechanics Inc.

Résumé: The concept of shear wave elasticity imaging is applied arteries. The goal of this study is to demonstrate the potential this technique for precisely and non-invasively quantifying the of the arterial wall. Experiments are performed in phantoms composed of elastic cylindrical shells that have elastic modulus in the range of human arteries stiffness. The of imaging the shear wave propagation in real-time measuring the frequency dispersion over a large bandwidth (100-1500 Hz) is demonstrated with good accuracy. Experimental curves are in good agreement with the theoretical computed for each phantom. Based on the dispersion, a simple method is developed to derive the elastic of the arterial wall. Finally, the feasibility of quantifying invasively the elastic modulus of the arterial wall is in vivo on healthy volunteers. ©2008 IEEE.

Résumé: ShearWaveTM Elastography (SWE) is a new real time ultrasound imaging mode that quantitatively measures local tissue elasticity in kPa. Based on the Supersonic Shear Imaging concept (developped at the Laboratoire Ondes et Acoustique, Paris), this new concept may appear as a promising tool to improve breast lesion characterization. In vitro experimental measurements have been performed to quantify SWE mode performances in terms of resolution, penetration and the ability to measure quantitatively elasticity. Results show that the SWE mode exhibits a millimetric resolution and quantifies properly tissue elasticity on a wide range of elastic contrasts (from 7 to 110 kPa). The real time capabilities and the robustness of the mode have been tested in clinical conditions, on breast lesions. 150 patients have been scanned with SWE mode in three different sites. Results show that SWE performs well on breast pathologies and presents a very good inter-site reproducibility. Finally, the quantitative elasticity value was analyzed as a function of pathology using FNA or core biopsy as the reference diagnostic method. © 2008 IEEE.

Résumé: Pulse-inversion sequences are sensitive to the nonlinear echoes from microbubbles allowing an improvement in the bloodto- tissue contrast. However, at larger mechanical indexes, this contrast is reduced by harmonics produced during nonlinear propagation. A method for tissue harmonics cancellation exploiting time-reversal is experimentally implemented using a 128-channel 12-bit emitter-receiver. The probe calibration is performed by acquiring the nonlinear echo of a wire in water. These distorted pulses are time-reversed, optimized and used for the pulse-inversion imaging of a tissue phantom. Compared to normal (straight) pulses, the time-reversed distorted pulses reduced the tissue signal in pulse-inversion by 11 dB. The second harmonics signal from microbubbles flowing in a wall-less vessel was unaffected by the correction. This technique can thus increase the blood-to-tissue contrast ratio while keeping the pressure and the number of pulses constant. ©2008 IEEE.

Résumé: The touch screen device is becoming more and more widespread because it is a very user friendly human/machine interface. In acoustic domains, several approaches are used to realize such a device. Triangulation or Rayleigh waves absorption are such classical methods. However, these approaches are limited because they need a large number of sensors and are only applicable to plates of constant thickness and homogeneous materials. To remedy these limitations, a new approach is proposed using only two sensors. In this approach, one sensor is used to excite the plate, either continuously or impulsively. The second sensor is used to detect the acoustic waves generated in the plate. When a human finger comes into contact with the plate, some acoustic wave characteristics change. These changes affect different frequencies and depend on the position of the contact point. Comparing these changes with pre-recorded values, it is possible to achieve a tactile touch device that only responds to specific touch locations.

Résumé: In this paper, we propose a numerical computation of the different waves generated when a spherical incident pulse is reflected and transmitted by a fluid/solid interface. In addition to the standard reflected and transmitted waves that propagate inside the volume, various surface waves can also be found in both propagation media. In the fluid, we can observe the longitudinal and transverse head waves, and the so-called leaky Rayleigh wave that generalizes the Rayleigh wave in a semi-infinite free solid medium to the case of an immersed interface. Similar effects can also be observed on the transmitted displacement field inside the solid material. We compare different numerical approaches, including semi-analytic methods (high-frequency approximation coupled to ray modeling approach) and implicit methods (finite elements and/or finite differences scheme), each method having its own advantages and inconvenients, and domains of validity. These different methods are used to evaluate the field reflected by the interface ; the transmitted displacement field is also analyzed from the same point of view.

Résumé: Although the rheology of soft tissue samples is subject to extensive studies, mainly via rheometer measurements, only a few papers discuss the mechanical behaviour of living biological tissues. This is mainly due to the lack of a reliable and accurate technique to quantitatively assess the stress-strain relationship in vivo. In this study, we show that MR-Elastography with its full 3D potential gives unique access to the frequency dependency of the complex shear modulus. In-vivo multi-frequent experiments were conducted in rat brain, fibrotic rat liver and human breast tissue. Additionally, a full physically-motivated model, in agreement with the causality principle, has been developed to explain the observed dispersion properties and finite element simulations were conducted to understand their microscopic origin. MR-Elastography can be efficiently used to study rheology in vivo. The frequency behaviour of the macroscopic viscoelastic parameters gives additional information about the microscopic structure of the material. The observed power-law leads to the hypothesis that an underlying self-similar network is responsible for it. Simulations show that the vascular network is a potential candidate. If proven, this could lead to a contrast provided on a macroscopic scale sensitive to changes on the level of the microscopic vascular architecture. Furthermore, at low frequency, the apparent loss modulus seems to originate from the multiple scattering at the micro level rather than from the intrinsic viscosity. © 2008 American Institute of Physics.

Résumé: Recently a nonlinear equation describing the plane shear wave propagation in isotropic quasi-incompressible media has been developed using a new expression of the strain energy density, as a function of the second, third and fourth order shear elastic constants (respectively μ, A, D) [1]. In such a case, the shear nonlinearity parameter βs depends only from these last coefficients. To date, no measurement of the parameter D have been carried out in soft solids. Using a set of two experiments, acoustoelasticity and finite amplitude shear waves, the shear elastic moduli up to the fourth order of soft solids are measured. Firstly, this theoretical background is applied to the acoustoelasticity theory, giving the variations of the shear wave speed as a function of the stress applied to the medium. From such variations, both linear (μ) and third order shear modulus (A) are deduced in agar-gelatin phantoms. Experimentally the radiation force induced by a focused ultrasound beam is used to generate quasi-plane linear shear waves within the medium. Then the shear wave propagation is imaged with an ultrafast ultrasound scanner. Secondly, in order to give rise to finite amplitude plane shear waves, the radiation force generation technique is replaced by a vibrating plate applied at the surface of the phantoms. The propagation is also imaged using the same ultrafast scanner. From the assessment of the third harmonic amplitude, the nonlinearity parameter βS is deduced. Finally, combining these results with the acoustoelasticity experiment, the fourth order modulus (D) is deduced. This set of experiments provides the characterization, up to the fourth order, of the nonlinear shear elastic moduli in quasi-incompressible soft media. Measurements of the A moduli reveal that while the behaviors of both soft solids are close from a linear point of view, the corresponding nonlinear moduli A are quite different. In a 5% agar-gelatin phantom, the fourth order elastic constant D is found to be 30±10kPa. © 2008 American Institute of Physics.

Résumé: The DORT method (French acronym for decomposition of the time reversal operator) is used to characterize small flaws of cylindrical geometry. Experiments performed with linear arrays of transducers in contact with an aluminum block with air-filled holes of different diameters demonstrates the possibility to estimate the size of a flaw by analyzing the singular value distribution and the singular vectors. These results are confirmed theoretically. © 2008 American Institute of Physics.

Résumé: We report the observation by scanning near-field optical microscopy of the evanescent electric field on the surface of a quantum cascade laser. The devices have been designed in order to let a consistent portion of its optical mode to leak out of the top surface. The laser wavelength in vacuum is ≈ 7.7 μm. ©OSA.

Résumé: The non invasive and in vivo assessment of the active and passive properties of the musculoskeletal system remains today a great challenge for the understanding of muscular diseases. In a previous paper [1], we showed that the transient contraction of a muscle fiber bundle triggered by electrostimulation can be followed in space and time by an ultrafast ultrasound system. On the one hand, this experiment is generalized to measure the three dimensional (3D) velocity fields allowing a much easier localization of the contracting fibers bundle. On the other hand, the supersonic shear imaging technique is applied to assess viscoelastic properties of the biceps brachü in different conditions. By combining in vivo imaging of both passive and active muscle activity, this study aim to provide new potential ultrasonic tools for muscle diseases diagnosis and monitoring. In order to image the contraction, which is a very quick phenomenon, an ultrasound scanner able to take up to 5000 frames/s was used. Tissue velocities were obtained from conventional speckle tracking techniques. The probe, positioned perpendicularly to the arm in a water tank, was moved with a linear motor. For each position, the imaging system was set to trigger an electrostimulation firing the contraction. A second experiment for the assessment of passive muscle elastic properties in different positions of the arm and contraction levels is then performed using the same probe in the supersonic shear imaging (SSI) mode. Generated by the radiation force induced by a focused ultrasound beam, shear waves propagate in the medium and are imaged by an ultrafast ultrasound scanner allowing the reconstruction of viscoelastic properties. The localization of the contracting fibers bundle and the resolution of its main temporal and spatial behavior are demonstrated. Paving the way to a clinical protocol on muscle diseases, viscoelastic parameters are measured in different conditions and reproducibility is discussed. These two complementary ultrasound techniques offer new perspectives for muscle diagnosis both as an active contractile tissue and as a passive tissue. © 2007 IEEE.

Résumé: Time-reversal is an efficient way to focus a wave back to its source and has been investigated in a wide range of situations including telecommunication, non-destructive testing, interactivity and medical applications. In this work we propose a one channel time-reversal experiment of shear waves in soft solids with application to elasticity assessment. From a physical point of view, the experiment presents some interesting features like asymmetry of the refocused field due to near field effects. From the point of view of the application, the proposed method has some advantages with respect to the existing ones: it is independent of boundary conditions, shape of the employed source and excitation function. Limitations could arise for high attenuating media where the system is not time-invariant. © 2007 IEEE.

Résumé: Bursts of focused ultrasound energy three orders of magnitude more intense than diagnostic ultrasound became during the last decade a noninvasive option for treating cancer from breast to prostate or uterine fibroid. However, many challenges remain to be addressed. First, the corrections of distortions induced on the ultrasonic therapy beam during its propagation through defocusing obstacles like skull bone or ribs remain today a technological performance that still need to be validated clinically. Secondly, the problem of motion artifacts particularly important for the treatment of abdominal parts becomes today an important research topic. Finally, the problem of the treatment monitoring is a wide subject of interest in the growing HIFU community. For all these issues, the potential of new ultrasonic therapy devices able to work both in Transmit and Receive modes will be emphasized. A review of the work under achievement at L.O.A. using this new generation of HIFU prototypes on the monitoring, motion correction and aberrations corrections will be presented. © 2007 IEEE.

Résumé: Near-field scanning optical microscopes (NSOMs) enable one to perform subwavelength optical imaging by scanning a nanosized probe in the near field at the surface of a specimen. NSOMs generally use a subwavelength aperture, a scattering tip, or a fluorescent nanoobject as local probes of the near-field. We review the basic principles of the different types of NSOMs. Illustrative examples are given to show how these probes can be used to perform optical mapping and characterization of materials with nanoscopic resolution. © 2007 American Institute of Physics.

Résumé: Bursts of focused ultrasound energy three orders of magnitude more intense than diagnostic ultrasound became during the last decade a noninvasive option for treating cancer from breast to prostate or uterin fibroid. However, many challenges remain to be addressed. First, the corrections of distortions induced on the ultrasonic therapy beam during its propagation through defocusing obstacles like skull bone or ribs remains today a technological performance that still needs to be validated clinically. Secondly, the problem of motion artifacts particularly important for the treatment of abdominal parts becomes today an important research topic. Finally, the problem of the treatment monitoring is a wide subject of interest in the growing HIFU community. For all these issues, the potential of new ultrasonic therapy devices able to work both in Transmit and Receive modes will be emphasized. A review of the work under achievement at L.O.A. using this new generation of HIFU prototypes on the monitoring, motion and aberrations corrections problems will be presented. © 2007 American Institute of Physics.

Résumé: We demonstrate the selective delivery of self-assembled luminescent semiconductor quantum dot (QD)-peptide bioconjugates into several eukaryotic cell lines. A 23-mer hetero-bifunctional peptide bearing a positively-charged oligoarginine domain and a terminal polyhistidine tract was synthesized and used to mediate the cellular internalization of the QD-bioconjugates. The polyhistidine tract allows the peptide to self-assemble onto the QD surface via metal-ion coordination while the oligoarginine domain mediates the specific uptake of the QD-bioconjugates via electrostatic interactions with cell surface receptors. In both HEK 293T/17 and COS-1 cells, this peptide-mediated delivery is concentration-dependent in terms of both the QD concentration and the peptide:QD ratio. Intracellularly, the QD signal is punctate in appearance and some, but not all, of the QDs are located within recycling endosomes as evidenced by their colocalization with transferrin. In both cell lines, the QD-bioconjugates elicit minimal cytotoxicity within the timeframe required for adequate cellular uptake. The specificity of this delivery strategy is demonstrated by performing a multicolor QD labeling, wherein the presence or absence of the peptide on the QD surface controls cellular uptake.

Résumé: We present a camera-based optical detection scheme designed to detect the transient motion created by the acoustic radiation force in elastic media. An optically diffusive tissue mimicking phantom was illuminated with coherent laser light, and a high speed camera (2 kHz frame rate) was used to acquire and cross-correlate consecutive speckle patterns. Time-resolved transient decorrelations of the optical speckle were measured as the results of localised motion induced in the medium by the radiation force and subsequent propagating shear waves. As opposed to classical acousto-optic techniques which are sensitive to vibrations induced by compressional waves at ultrasonic frequencies, the proposed technique is sensitive only to the low frequency transient motion induced in the medium by the radiation force. It therefore provides a way to assess both optical and shear mechanical properties.

Résumé: A method for tracking locally the 3D motion of biological tissues is developed and applied to the correction of motion during High Intensity Focused Ultrasound (HIFU) therapy. The motion estimation technique is based on an accurate ultrasonic speckle tracking method. A pulse-echo sequence is performed for a subset of the transducers of a phased array. For each of these sub-apertures, the displacement is estimated by computing the 1D cross-correlation of the backscattered signals acquired at two consecutive times. The local 3D motion vector is then computed using a inversion algorithm. This technique is experimentally validated in vivo on anesthetized pigs. The 3D motion of liver tissues is tracked in real-time. The technique is combined with HIFU sequences and a real-time feedback correction of the HIFU beam is achieved by adjusting the delays of each channel. The sonications "locked on target" are interleaved with very motion estimation sequences. © 2006 IEEE.

Résumé: Although numerous techniques are commonly used to study muscular or neuromuscular diseases, in vivo muscle contraction remains a difficult event to image in real time. On one hand, time accurate techniques such as for example, mechanomyography, electromyography, or acceleromyography; allow only the assessment of global parameters of musculo-tendinous complex. These estimated parameters are classically the force developed by a group of muscles fibers during contraction, the compound action potential or the acceleration of a muscle. On the other hand, mapping of local muscle structure parameters during contraction with a sub-millimetric resolution can be achieved by magnetic resonance imaging or ultrasound Doppler tissue imaging. However, these imaging modalities are not fast enough (less than hundreds of frames per second) to follow transient phenomena. Here, a new way of imaging the motion of an in vivo contracting muscle is proposed. The principle is to use an ultrafast ultrasound scanner to follow with a sub-millimetric resolution the axial motion of the muscle tissue in a two dimensional (2D) plane. This imaging technique designed at the "Laboratoire Ondes et Acoustique" for transient elastography, gives 2D radio frequency images at a few thousands Hertz (up to 5000 frames.s -1) and leads to both local and transient displacements of the muscle in vivo. Displacements as low as one micrometer are computed from the one dimensional (1D) cross-correlation between consecutive images. From these displacements, both local and transient informations on the muscle contraction are extracted such as the contraction time, the relaxation time or the mean velocity of the propagation of the contraction along a muscle fiber. During experiments the ultrafast scanner driving a 128 elements 8 MHz probe triggers a homemade electrostimulation device linked to a pair of electrodes placed on the motor point of the biceps brachii and at the elbow junction. In vivo studies of the contraction of a muscle are reported here by these first transient experiments achieved on the biceps brachii. The velocity distribution of the displacements induced in the tissue during contraction is measured as well as the main temporal features of the contraction. © 2006 IEEE.

Résumé: The heart is a mechanical pump that is electrically driven. We have previously shown that the contractility of the cardiac muscle can reliably be used in order to assess the extent of ischemia using myocardial elastography. Myoeardial elastograpby estimates displacement and strain during the natural contraction of the myocardium using signal processing techniques on echocardiograms in order to assess the change in mechanical properties as a result of disease. In this paper, we showed that elastographic techniques can be used to estimate and image both the mechanics and electromechanics of normal and pathological hearts in vivo. In order to image the mechanics throughout the entire cardiac cycle, the minimum frame rate was determined to be on the order of 150 fps in a long-axis view and 300 fps in a short-axis view. The incremental and cumulative displacement and strains were measured and imaged for the characterization of normal function and differentiation from infracted myocardium. In order to image the electromechanical function, the incremental displacement was imaged in consecutive cardiac cycles during end-systole in both dogs and humans. The contraction wave velocity in normal dogs was found to be twice higher than in normal humans and twice lower than in ischemic dogs. In conclusion, we have demonstrated that elastographic techniques can be used to detect and quantify the mechanics and electromechanics of the myocardium in vivo. Ongoing investigations entail assessment of myocardial elastography in characterizing and quantifying ischemia and infarction in vivo. © 2006 IEEE.

Résumé: We present results of ex vivo imaging of the mouse cornea following photorefractive keratectomy and in vivo imaging in the anterior segment of the rat eye using full-field optical coherence tomography. The instrument is based on the Linnik interferometer, illuminated by a white light source: a tungsten halogen lamp for ex vivo imaging and a fibered Xenon arc lamp for in vivo imaging. En face tomographic images are obtained in real-time without scanning by calculating the difference of two phase-opposed interferometric images recorded by a CCD or CMOS camera. Spatial resolution of ∼1 μm in both axial and lateral directions is achieved thanks to the short coherence length of the illumination source and the use of relatively high numerical aperture microscope objectives. A detection sensitivity of up to 90 dB is reached by means of pixel binning and image averaging. Photorefractive keratectomy was performed on mice and the excised eyes were examined under immersion 21 days after surgery. Rats were anesthetized and their anterior segments imaged under immersion. The high resolution of our instrument gives cellular-level resolution in the cornea, allowing visualization of individual stromal keratocytes and collagen fibers, and cells in the endothelium. The basal and Descemet's membranes are well defined. Quantitative measurement of scattering in each layer is possible. Penetration to the level of the lens surface is achieved. Acquisition of stacks of en face images permits three-dimensional navigation through the cornea. Development of image treatment algorithms to allow three-dimensional reconstruction is discussed. The full-field optical coherence tomography technique could be useful in monitoring corneal scattering following refractive surgery.

Résumé: Ultrasound focusing through complex media can be achieved using time-reversal techniques. These techniques make use of back-propagating ultrasonic waves generated by localized sources. Such sources generally consist of high acoustic contrasts echoing ultrasonic waves generated by an incident ultrasonic field, or directly by point-like transducers inserted at the desired focusing location. In this work, we experimentally investigate time-reversal of acoustic waves generated by photo-acoustic emission. A frequency-doubled Q-switched Nd:YAG laser was used to illuminate phantom with 5-ns laser pulses. A 128-element ultrasonic transducer array, with a center frequency of 1.5 MHz, was used to detect acoustic waves generated by optically absorbing targets suspended in water. A dedicated 32-channel electronics was used to time-reverse and re-emit the detected ultrasonic field. Gel spheres dyed with India ink (diameter approximately 1-2 mm) illuminated by the laser beam were used to generate the photo-acoustic waves. Time-reversal of the detected field was performed to focus ultrasound in the presence of highly defocusing media in front of the transducer array. We demonstrate how this allows correcting for the aberration in order to provide good quality images in the isoplanetic region surrounding the photo-acoustic source.

Résumé: Numerical simulation of wave propagation is performed through 31 volumes of trabecular bone. These volumes were reconstructed from high synchrotron microtomography experiments and were used as the input geometry in a three-dimensional (3D) finite-difference simulation tool developed in our laboratory. The simulation algorithm accounts for propagation in both the saturating fluid and bone but absorption is not taken into account. Numerical predictions are consistent with experimental observations in trabecular bones : linear frequency dependence of attenuation, quasi-linear increase of attenuation and speed of sound with the bone volume fraction, negative phase velocity dispersion in most of the specimens, propagation of fast and slow waves depending on the orientation of the trabecular network with respect to the direction of propagation of the ultrasound. Moreover, the predicted attenuation is in close agreement with the experimental data obtained for the same specimens. Coupling numerical simulation with real 3D bone microarchitecture provides a powerful tool to investigate the physics of ultrasound propagation in trabecular structures. © 2005 IEEE.

Résumé: Photothermal experiments on transparent film are extremely complex because of the optical modification such as reflective index and or expansion due to the heat source localised inside the layer. This paper presents a numerical model showing the interaction between reflectivity, thickness and thermal effects. © EDP Sciences.

Résumé: The decomposition of the time-reversal operator (DORT In French) is an active array detection technique. It requires the measurement of the array response matrix K(ω) and consists in the analysis of the eigenvalues and the eigenvectors of the time-reversal operator K*K which provides information on the presence and localization of scatterers in the medium. It was shown that the DORT method allows to separate and localize pointlike scatterers in a shallow water wave-guide [J. Acoust. Soc. Am. Mordant et al. (1998) and Folégot et al. (2003)]. Here, we extend the study to the detection and frequency characterization of two spherical targets. Small scale ultrasonic experiments are performed with a 3.9 MHz 24 elements transducer array and two spheres of 2 and 3 mm diameter in a 31 mm deep wave-guide. These scatterers correspond to 15 < ka < 25 leading to a non-isotropic scattered field. We have developed a theoretical model taking into account the wave-guide and the acoustic properties of the spheres and using the partial waves decomposition of the scattered field. We calculate the singular values of the array response matrix. This theoretical approach is in good agreement with the experimental results. This 1/325th scale ultrasonic experiment corresponds to a shallow water experiment with a 12 kHz Vertical Linear Array (VLA). © 2005 IEEE.

Résumé: Transcranial imaging of the brain is currently limited by the defocusing effect of the skull bone (absorption, diffusion and refraction of ultrasound A noninvasive brain imaging device is presented that takes into account the defocusing effect of the skull. This device is made of two identical "twin" linear arrays located on each side of the head. It is shown how to differentiate the respective influence of the two bone windows on the path of an ultrasonic wave going from one array to the other, and how to estimate at each frequency the attenuation and phase shifts locally induced by each of the bone windows. This information is then used to perform non invasive adaptive focusing through the skull. Compared to non corrected wavefronts used in the beamforming process of commercial scanners, the spatial shift of the focal point is cancelled, the width of the focal spot is reduced, and the sidelobes level is decreased up to 15dB. The technique has been used to acquire in vitro images of tissue phantoms behind a skull wall, exhibiting the image quality enhancement with respect to images obtained with cylindrical focusing. Further refinement should lead to an improvement of the image contrast greater than 20 dB compared to conventionnal scanners. © 2005 IEEE.

Résumé: We propose a simple configuration to obtain images of the diffusion of a quasi 1-dimensional "thermal wave" in homogeneous or heterogeneous media. One side of the sample is broadly heated by a modulated CO2 laser beam, while the alternative temperature distribution is imaged on a perpendicular facet using a CCD-based thermoreflectance microscope. We validate the principle of this technique on a simple aluminium-coated glass substrate, in which amplitude and phase images of the reflectivity variations are measured. From these measurements, we derive a diffusivity of 10-2 cm -2s-1, which is close to that of glass. This measurement geometry should prove useful for the characterization of complex media in which micro scale heat diffusion is difficult to measure and model. © EDP Sciences.

Résumé: Thanks to the Time Reversal theory, a technique of localization of an impact generated by a simple finger knock on plate-shaped solid objects has been developed. It is shown that the technique works with only one cheap accelerometer, and that adding sensors increases the contrast of the localization pattern but not the resolution. To better understand the phenomenon and to know exactly the nature of the created waves, a 2D elastic simulation is used, showing that in a very good approximation the A0 Lamb mode is the only propagating one. Moreover, at around 1 cm from the edges, even the non-propagating modes are negligible compared with the A0 mode. Furthermore, the stability of the technique to temperature changes is studied. Indeed, the TR theory predicts that the localization is effective only if the acoustic medium reciprocity has not been broken by any change in the medium including wave speed variation due to temperature change. To this end, a laser interferometer coupled to a low frequency demodulator measures the impulse responses created by a knock on a plate during the cooling. Given that there is only one propagating wave (A0), it is expected that temperature variations lead to a stretching of acoustic signatures that can be compensated for thanks to a simple contraction: this is observed experimentally. This shows the feasibility of the technique for outdoor Time Reversal interactive experiment. © 2005 IEEE.

Résumé: We present images of ocular tissues obtained using ultrahigh resolution full-field OCT. The experimental setup is based on the Linnik interferometer, illuminated by a tungsten halogen lamp. En face tomographic images are obtained in realtime without scanning by computing the difference of two phase-opposed interferometric images recorded by a highresolution CCD camera. A spatial resolution of 0.7 μm x 0.9 μm (axial × transverse) is achieved thanks to the short source coherence length and the use of high numerical aperture microscope objectives. A detection sensitivity of 90 dB is obtained by means of image averaging and pixel binning. Whole unfixed eyes and unstained tissue samples (cornea, lens, retina, choroid and sciera) of ex vivo rat, mouse, rabbit and porcine ocular tissues were examined. The unprecedented resolution of our instrument allows cellular-level resolution in the cornea and retina, and visualization of individual fibers in the lens. Transcomeal lens imaging was possible in all animals, and in albino animals, transscleral retinal imaging was achieved. We also introduce our rapid acquisition full-field optical coherence tomography system designed to accommodate in vivo ophthalmologic imaging. The variations on the original system technology include the introduction of a xenon arc lamp as source, and rapid image acquisition performed by a high-speed CMOS camera, reducing acquisition time to 5 ms per frame.

Résumé: We present results of studies in embryology and ophthalmology performed using our ultrahigh-resolution full-field OCT system. We also discuss recent developments to our ultrashort acquisition time full-field optical coherence tomography system designed to allow in vivo biological imaging. Preliminary results of high-speed imaging in biological samples are presented. The core of the experimental setup is the Linnik interferometer, illuminated by a white light source. En face tomographic images are obtained in real-time without scanning by computing the difference of two phase-opposed interferometric images recorded by high-resolution CCD cameras. An isotropic spatial resolution of ∼1 μm is achieved thanks to the short source coherence length and the use of high numerical aperture microscope objectives. A detection sensitivity of ∼90 dB is obtained by means of image averaging and pixel binning. In ophthalmology, reconstructed xz images from rat ocular tissue are presented, where cellular-level structures in the retina are revealed, demonstrating the unprecedented resolution of our instrument. Three-dimensional reconstructions of the mouse embryo allowing the study of the establishment of the anterior-posterior axis are shown. Finally we present the first results of embryonic imaging using the new rapid acquisition full-field OCT system, which offers an acquisition time of 10 μs per frame.

Résumé: Hidden corrosion in aircraft structures is difficult to detect at an early stage. Guided Lamb waves are sensitive to this damage and offer an important inspection potential compared to the point-by-point processes. To optimize the corrosion detection, it is important to understand the physical phenomenon governing the interaction between Lamb waves and corrosion. The research of the Lamb modes which are the most sensitive to corrosion is performed by calculating the mode's energy distribution across the thickness of the plate. Experimental studies correspond well with this criterion. © 2005 American Institute of Physics.

Résumé: A high power prototype dedicated to trans-skull therapy has been tested in vivo on 22 sheep. The array is made of 300 high power transducers working at 1MHz central frequency and is able to achieve 400 bars at focus in water during five seconds with a 50% percent duty cycle. In the first series of experiments, 10 sheep were treated and sacrificed immediately after treatment. A complete craniotomy was performed on half of the treated animal models in order to get a reference model. On the other half, minimally invasive surgery has been performed: a hydrophone was inserted at a given target location inside the brain through a craniotomy of a few mm2. A time reversal experiment was then conducted through the skull bone with the therapeutic array to treat the targeted point. Thanks to the high power technology of the prototype, trans-skull adaptive treatment could be achieved. In a second series of experiments, 12 animals were divided into three groups and sacrificed respectively one, two or three weeks after treatment. Finally, Magnetic Resonance Imaging and histological examination were performed to confirm tissue damage. © 2005 American Institute of Physics.

Résumé: Observation of transient shear-wave propagation in soft tissue is of great interest for the study of tissue viscoelastic properties. In previous work, we introduced a technique, called Supersonic Shear Imaging (SSI), able to generate transient shear waves using the acoustic radiation force and image their propagation in real time in soft tissues. Inversion methods were used to recover elasticity from the shear-wave propagation. In this work we present a precise and robust inversion algorithm taking into account not only elastic but also viscous properties of soft tissues. Based on a Voigt model, this algorithm is designed to provided quantitative and local estimation of soft tissue elasticity and viscosity. In a first part, the influence of viscosity on transient shear waves is modeled and analyzed using a 3D analytical formulation of the mechanical Greens function in a viscoelastic medium. It is shown that the spatial and temporal shape of experimental shear waves induced in soft tissues using SSI can only be accurately modeled by taking into account tissue shear viscosity. The respective influences of viscosity, elasticity or diffraction on the shear wave shape are carefully studied and discriminated. In a second part, taking advantage of the previous modeling, a local inverse problem permitting the recovery of shear elasticity and viscosity is presented and validated using the Greens function based simulation tool. The role of viscosity on the accuracy of the elasticity estimation is studied. The influence of out of plane shear propagation on the inversion algorithm is discussed. Finally, in media presenting shear viscoelasticity heterogeneities, finite differences simulations are used to study the spatial resolution of the algorithm and its sensitivity to the signal-to-noise ratio. Experiments on calibrated tissuemimicking phantoms presenting different viscoelastic properties are presented validating the simulation results. © 2004 IEEE.

Résumé: High resolution transcranial imaging of the brain requires adaptive focusing techniques in order to correct the defocusing effect of the skull bone (absorption, diffusion and refraction of ultrasounds). Here is presented a noninvasive brain imaging device made of two identical "twin" linear arrays located on each side of the skull. It is shown how to separate the respective influence of the two bone windows on the path of an ultrasonic wave going from one array to the other, and estimate at each frequency the attenuation and phase shift locally induced by each of the bone windows. This information is men used to correct, for a wide band signal, the wave fronts that have to be sent through the skull in order to obtain a good focusing inside the skull. Compared to uncorrected wave fronts, the spatial shift of the focal point is cancelled, the width of the focal spot is reduced, and sidelobes level is decreased up to 10dB. Simulated structural transcranial images of a brain model are presented to exhibit the improvement in image quality provided by this new noninvasive adaptive focusing method. © 2004 IEEE.

Résumé: An experimental set-up to measure the out-of-plane displacement field of surfaces in reflection microscopy is presented. It is derived from a Nomarski shear-interferometer. When the shear is greater than the length of the microcantilever, this interferometer, used with a sinusoidal phase modulation and four integrating buckets, allows one to obtain the displacement field of the observed surface, with a reproductibility in the 10 pm range. Identification techniques derived from the "Equilibrium Gap Method" developed recently to measure local mechanical properties and loading, the displacement field measured by the shear-interferometer may be used to determine simultaneously the "temperature" field and the local elastic properties of the cantilever. The feasibility of the in-situ (in water solution) full-field measurement of displacement fields in MEMS is proved, and preliminary results tends to show that mechanical effets induced by DNA hybridization are heterogeneous.

Résumé: The feasibility of performing 3D Dynamic Elastography using an ultrasound based imaging system was described. In addition to providing a low cost system, the ultrasound approach was found to enable the reduction of acquisition time by a factor 300. The visco-elastic maps obtained using this technique were anticipated to be more precise than MR-Elastography as the resolution could lead to a 1.5×1.5×1.5 mm voxel with ultrasound. The potential of this technique for 2D breast elastography for cancer diagnosis was also evaluated.

Résumé: We report on ultrahigh-resolution Optical Coherence Tomography (OCT) using white-light interference microscopy. The experimental setup is based on a Linnik interferometer illuminated by a tungsten halogen lamp. Tomographic images in the en face orientation are calculated by combination of images recorded by a silicon CCD camera. Axial resolution of 0.8 μm is achieved due to the short coherence length of the source and the compensation of dispersion mismatch in the interferometer arms. Transverse resolution of 1.6 μm is obtained by using relatively high numerical aperture microscope objectives. A nearly shot-noise limited detection sensitivity of 90 dB is achieved with 4s acquisition time. Images of the Xenopus Laevis tadpole are presented.

Résumé: Though HIFU may offer a practical alternative to conventional surgery for a range of abdominal tumors, patient motion (particularly respiratory motion) should be monitored and corrected. We propose here a new generation of multi-channel HIFU systems able to track and correct in real time 3D motion of tissues. The feasibility of a 3D real time motion tracking and feedback for the H.I.F.U. beam focal spot location is demonstrated. The process is based on an accurate ultrasonic speckle tracking method for following and compensating the 3D motion of the tumor. Experiments were conducted with an electronically focused phased array made of 200 elements. This unique system is used both for motion tracking and feedback electronic steering of the HIFU beam. The displacement estimation is based on the correlation of the RF signals at two different times. A pulse-echo sequence is performed for a set of transmit transducers of the phased array. The complete 3D motion estimation requires to use at least three sub-apertures on the large phased array. This motion tracking technique has been experimentally validated in phantoms moving as fast as 50 mm/s. The motion tracking sequences are interleaved with HIFU sequences at high frame rate. The HIFU sonication is interrupted every 100 ms in order to allow motion estimation performed in a few ms. This provides a real time feedback for the HIFU beam steering correction. A maximum duty cycle of about 80% was performed experimentally. HIFU experiments combined with 3D real time motion correction conducted in fresh tissues samples will be presented.

Résumé: Ex vivo experiments have been conducted through excised lamb rib cages, with bone, cartilage, muscle and skin. The ribs were placed between a hydrophone and a programmable sparse array made of 200 high power transducers working at 1 MHz central frequency. First, adaptive focusing through ribs has been experimentally studied at low power. Without any correction, the pressure fields in the focal plane were affected by both inhomogeneous attenuation and phase distortion and three main effects were observed: a mean 2 mm shift of the main lobe, a mean 1.25 mm spreading in the half width of the main lobe and up to 20 dB increase in the secondary lobe level. Three adaptive focusing techniques were tested with continuous waves: phase shift, time reversal and time reversal coupled with amplitude compensation. We show that time reversal is more appropriate, as it minimizes the heating of the ribs by naturally sonicating between the ribs. It is also shown that a non invasive technique could be implemented by using the imaging capabilities of the therapeutic array.

Résumé: Two-dimensional ultrasonic speckle correlation velocimetry (USV) is a new technique that allows to image moving scattering media, at a high frame-rate. In this paper we apply the technique to determine two-dimensional particle velocity profiles of multiphase flows. Experiments are realized with suspensions of Sonazoid' (medical contrast agent) and Magnetite (Fe3O4) in water. All measurements are performed in a vertical pipe with the flow moving downwards. The two-dimensional particle velocity profiles are then compared with a reference liquid volume flow velocity. As expected from theory, the heavier Magnetite particles have slightly higher velocity than the liquid whereas the contrast agent simply follows the liquid motion. The proposed technique can be used in combination with other techniques to measure the mass flow of the solid phase, in solid/liquid multiphase flow. This is generally more interesting than measuring the bulk mass or volume flow.

Résumé: Frame rate in ultrasound imaging can be increased by simultaneous transmission of multiple beams using coded waveforms. However, the achievable degree of orthogonality among coded waveforms is limited in ultrasound, and the image quality degrades unacceptably due to interbeam interference. In this paper, an alternative combined time-space coding approach is undertaken. In the new method all transducer elements are excited with short pulses and the high time-bandwidth (TB) product waveforms are generated acoustically. Each element transmits a short pulse spherical wave with a constant transmit delay from element to element, long enough to assure no pulse overlapping for all depths in the image. Frequency shift keying is used for "per element" coding. The received signals from a point scatterer are staggered pulse trains which are beamformed for all beam directions and further processed with a bank of matched filters (one for each beam direction). Filtering compresses the pulse train to a single pulse at the scatterer position with a number of spike axial sidelobes. Cancellation of the ambiguity spikes is done by applying additional phase modulation from one emission to the next and summing every two successive images. Simulation results presented for QLFM and Costas spatial encoding schemes show that the proposed method can yield images with range sidelobes down to -45 dB using only two emissions.

Résumé: Acousto-optic imaging in strongly light-scattering tissues seeks to reveal optical contrasts in these turbid media. Nevertheless, this technique happens to be also sensitive to their acoustic contrasts. We have built a new setup combining a dedicated echograph and an acousto-optic imager in a single apparatus. Thanks to this setup, we have studied ultrasound absorbent and light absorbent features embedded in several centimeter thick biological tissues, and we have compared for the first time the acoustic and acousto-optic signals recorded in the same configuration. We show that even though optical contrast is the ultimate goal of this technique, preliminary acoustic investigation of the tissue is necessary to interpret correctly acousto-optical signals.

Résumé: The symmetry-induced effects were studied on the band structure of wurzite III-V nitride-based quantum wells (QW). The symmetry of these QW was shown to be described by the layer group P3m1 and did not depend on the number of atomic monolayers constituting the QW. The modification of electron state symmetries and optical transition selection rules on inserting a QW was established.

Résumé: The influence of the thermal annealing on the optical properties of the porous silicon films was revealed by photoluminescence (PL) and spectroellipsometric measurements. As result of 200 deg;C annealing small changes of the dielectric functions could be understood by desorption process of some molecules from Si skeleton surface. Strong changes of PL and dielectric function spectra after the thermal annealing at high temperatures (up to 800°C) were explained by the change of the passivation from hydrogen to oxygen and then the beginning of the oxidation process. This oxidation process produces the disappearance of the PL slow component, an important enhancement of PL (2-3 orders of magnitude) and a shift of maximum position to higher energies, corresponding to the thinning of the nanocrystallites from the Si skeleton. ©2003 Copyright SPIE - The International Society for Optical Engineering.

Résumé: This paper contains two main sections. In the first one, we illustrate the technological interest of crystals of the sillenite family and we present our powerful methods of investigation in some details. These are essentially optical absorption and magnetic circular dichroism at liquid helium temperatures, down to 4000 cm-1. The latter is also used to detect electron paramagnetic resonance optically and to tag a given absorption component to a specific paramagnetic, intrinsic or extrinsic, defect. In the second part, we present a variety of experimental results and interpretations dealing with undoped crystals and with samples doped with transition ions of the iron (Mn,Cr,Fe,Co,Cu) group. Emphasis is put on the assignment of ligand field bands in the near infrared, since these are used to monitor the site, charge state, and amount of the dopants in a thermally bleached state or after various illuminations.

Résumé: Thermal wave methods of investigating solid, opaque materials in a nondestructive, noncontact way using the mirage effect, or optical beam deflection (OBD) detection method, have been shown to be sensitive to both surface and subsurface variations in the thermal properties of those materials. The technique of using thermal waves as an NDE method is useful for the detection of localized defects such as voids, cracks, and delaminations or debonding, for the determination of thermal properties such as thermal diffusivity and anisotropy, and for interrogating thin films/coatings, and surface studies. Corrosion is a problem which has only recently been investigated with thermal wave methods.