Articles 2011

Résumé: The objective of this study was to investigate in vivo the relationship between stiffness and thermal dose. For this purpose, shear wave elastography (SWE) - a novel ultrasound-based technique for real-time mapping of the stiffness of biological soft tissues - is performed in temperature-controlled experiments. Experiments were conducted on nine anesthetized rats. Their right leg was put in a thermo-regulated waterbath. The right leg of each animal was heated at one particular temperature between 38 °C and 48.5 °C for 15 min to 3 h. Shear waves were generated in the muscle using the acoustic radiation force induced by a linear ultrasonic probe. The shear wave propagation was imaged in real time by the probe using an ultrafast scanner prototype (10 000 frames s -1). The local tissue stiffness was derived from the shear wave speed. Two optical fiber sensors were inserted into the muscle to measure in situ the temperature. Stiffness was found to increase strongly during the experiments. When expressed as a function of the thermal dose, the stiffness curves were found to be the same for all experiments. A thermal dose threshold was found at 202 min for an eightfold stiffness increase. Finally, the time-temperature relationship was established for different stiffness ratios. The slope of the time-temperature relationship based on stiffness measurements was found identical to the one obtained for cell death in the seminal paper on the thermal dose by Sapareto and Dewey in 1984 (Int. J. Radiat. Oncol. Biol. Phys. 10 787-800). The present results highlight the stiffness increase as a good indicator of thermal necrosis. SWE imaging can be used in vivo for necrosis threshold determination in thermal therapy. © 2011 Institute of Physics and Engineering in Medicine.

Résumé: Using a Cramer-Rao analysis, we study the theoretical performances of a time and spatially resolved fDOT imaging system for jointly estimating the position and the concentration of a point-wide fluorescent volume in a diffusive sample. We show that the fluorescence lifetime is a critical parameter for the precision of the technique. A time resolved fDOT system that does not use spatial information is also considered. In certain cases, a simple steady-state configuration may be as efficient as this time resolved fDOT system. © 2011 Optical Society of America.

Résumé: We consider a purely quasi-incompressible elasticity model. We rigorously establish asymptotic expansions of near- and far-field measurements of the transient elastic wave induced by a small elastic anomaly. Our proof uses layer potential techniques for the modified Stokes system. Based on these formulae, we design asymptotic imaging methods leading to a quantitative estimation of elastic and geometrical parameters of the anomaly. © Copyright Royal Society of Edinburgh 2011.

Résumé: This work deals with the physical modelling of convex acoustic pipes for real-time simulation. The main purpose of this paper is the use of automatic control and signal processing to solve some stability problems for infinite dimensional systems involved in music acoustics. © 2011 Lavoisier, Paris.

Résumé: We investigate recursive nearest neighbor search in a sparse domain at the scale of audio signals. Essentially, to approximate the cosine distance between the signals we make pairwise comparisons between the elements of localized sparse models built from large and redundant multiscale dictionaries of timefrequency atoms. Theoretically, error bounds on these approximations provide efficient means for quickly reducing the search space to the nearest neighborhood of a given data; but we demonstrate here that the best bound defined thus far involving a probabilistic assumption does not provide a practical approach for comparing audio signals with respect to this distance measure. Our experiments show, however, that regardless of these non-discriminative bounds, we only need to make a few atom pair comparisons to reveal, e.g., the origin of an excerpted signal, or melodies with similar timefrequency structures. © 2011 Elsevier B.V. All rights reserved.

Résumé: We present a class of sonic meta-screens for manipulating air-borne acoustic waves at ultrasonic or audible frequencies. Our screens consist of periodic arrangements of air bubbles in water or possibly embedded in a soft elastic matrix. They can be used for soundproofing but also for exalting transmission at an air/water interface or even to achieve enhanced absorption. © 2011 American Institute of Physics.

Résumé: Recent studies have demonstrated the feasibility of transcostal high intensity focused ultrasound (HIFU) treatment in liver. However, two factors limit thermal necrosis of the liver through the ribs: the energy deposition at focus is decreased by the respiratory movement of the liver and the energy deposition on the skin is increased by the presence of highly absorbing bone structures. Ex vivo ablations were conducted to validate the feasibility of a transcostal real-time 3D movement tracking and correction mode. Experiments were conducted through a chest phantom made of three human ribs immersed in water and were placed in front of a 300 element array working at 1 MHz. A binarized apodization law introduced recently in order to spare the rib cage during treatment has been extended here with real-time electronic steering of the beam. Thermal simulations have been conducted to determine the steering limits. In vivo 3D-movement detection was performed on pigs using an ultrasonic sequence. The maximum error on the transcostal motion detection was measured to be 0.09 0.097 mm on the anterior-posterior axis. Finally, a complete sequence was developed combining real-time 3D transcostal movement correction and spiral trajectory of the HIFU beam, allowing the system to treat larger areas with optimized efficiency. Lesions as large as 1 cm in diameter have been produced at focus in excised liver, whereas no necroses could be obtained with the same emitted power without correcting the movement of the tissue sample. © 2011 Institute of Physics and Engineering in Medicine.

Résumé: We present an experimental study on gravity capillary wave turbulence in water. By using space-time resolved Fourier transform profilometry, the behavior of the wave energy density |η k,ω|2 in the 3D (k,ω) space is inspected for various forcing frequency bandwidths and forcing amplitudes. Depending on the bandwidth, the gravity spectral slope is found to be either forcing dependent, as classically observed in laboratory experiments, or forcing independent. In the latter case, the wave spectrum is consistent with the Zakharov-Filonenko cascade predicted within wave turbulence theory. © 2011 American Physical Society.

Résumé: Impulse responses of vibrating plates are classically measured on a fine spatial grid satisfying the ShannonNyquist spatial sampling criterion, and interpolated between measurement points. For homogeneous and isotropic plates, this study proposed a more efficient sampling and interpolation process, inspired by the recent paradigm of compressed sensing. Remarkably, this method can accommodate any star-convex shape and unspecified boundary conditions. Here, impulse responses are first decomposed as sums of damped sinusoids, using the Simultaneous Orthogonal Matching Pursuit algorithm. Finally, modes are interpolated using a plane wave decomposition. As a beneficial side effect, these algorithms can also be used to obtain the dispersion curve of the plate with a limited number of measurements. Experimental results are given for three different plates of different shapes and boundary conditions, and compared to classical Shannon interpolation. © 2011 Elsevier Ltd. All rights reserved.

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, Multiwave imaging provides a unique image of the most interesting contrast with the most interesting resolution. We will show in this paper how this general concept of multiwave imaging allow to perform high resolution and quantitative elasticity imaging of the human body. Here, the two waves are sonic shear waves and ultrasonic compressional waves. A comparison with static elastography (a single wave technique) will show the interest of multiwave imaging and various clinical examples will be presented to illustrate the efficiency of this approach. ©2011 Bentham Science Publishers.

Résumé: Local and non-contact measurements of the thickness of thin layers deposited on a thick plate have been performed by using zero group velocity (ZGV) Lamb modes. It was shown that the shift of the resonance frequency is proportional to the mass loading through a factor which depends on the mechanical properties of the layer and of the substrate. In the experiments, ZGV Lamb modes were generated by a Nd:YAG pulsed laser and the displacement normal to the plate surface was measured by an optical interferometer. Measurements performed at the same point that the generation on the non-coated face of the plate demonstrated that thin gold layers of a few hundred nanometers were detected through a 1.5-mm thick Duralumin plate. The shift of the resonance frequency (1.9 MHz) of the fundamental ZGV mode is proportional to the layer thickness: typically 10 kHz per m. Taking into account the influence of the temperature, a 240-nm gold layer was measured with a 4 uncertainty. This thickness has been verified on the coated face with an optical profiling system. © 2011 American Institute of Physics.

Résumé: In elastography, quantitative imaging of soft tissue elastic properties is provided by local shear wave speed estimation. Shear wave imaging in a homogeneous medium thicker than the shear wavelength is eased by a simple relationship between shear wave speed and local shear modulus. In thin layered organs, the shear wave is guided and thus undergoes dispersive effects. This case is encountered in medical applications such as elastography of skin layers, corneas, or arterial walls. In this work, we proposed and validated shear wave spectroscopy as a method for elastic modulus quantification in such layered tissues. Shear wave dispersion curves in thin layers were obtained by finite-difference simulations and numerical solving of the boundary conditions. In addition, an analytical approximation of the dispersion equation was derived from the leaky Lamb wave theory. In vitro dispersion curves obtained from phantoms were consistent with numerical studies (deviation <1.4%). The least-mean-squares fitting of the dispersion curves enables a quantitative and accurate (error <5% of the transverse speed) assessment of the elasticity. Dispersion curves were also found to be poorly influenced by shear viscosity. This phenomenon allows independent recovery of the shear modulus and the viscosity, using, respectively, the dispersion curve and the attenuation estimation along the propagation axis. © 2011 IEEE.

Résumé: Some of the research papers accepted and published in the 'Focused Section on Sensing Technologies for Biomechatronics' of the IEEE/ASME Transactions on mechatronics (T-Mec.) are discussed. Chapuis et al. used an ER fluid-based 'clutch' in combination with an ultrasound motor where the clutch was used to control the output force of the system. The paper, The Use of piezoceramics as electrical energy harvesters within instrumented knee implant during walking' from Almouahed et al. exploits the properties of piezoelectricity to build a sensor able to approximately assess the amount of electric power that can be produced within a diagnostic knee implant during normal walking without the need to be powered from an external source of energy. The paper 'A 1-D capacitive micromachined ultrasonic transducer imaging array fabricated with a silicon-nitride-based fusion process' from Logan et al. reports a 64-element array capacitive micromachined ultrasonic transducer.

Résumé: Pulses of light propagating through multiply scattering media undergo complex spatial and temporal distortions to form the familiar speckle pattern. There is much current interest in both the fundamental properties of speckles and the challenge of spatially and temporally refocusing behind scattering media. Here we report on the spatially and temporally resolved measurement of a speckle field produced by the propagation of an ultrafast optical pulse through a thick strongly scattering medium. By shaping the temporal profile of the pulse using a spectral phase filter, we demonstrate the spatially localized temporal recompression of the output speckle to the Fourier-limit duration, offering an optical analogue to time-reversal experiments in the acoustic regime. This approach shows that a multiply scattering medium can be put to profit for light manipulation at the femtosecond scale, and has a diverse range of potential applications that includes quantum control, biological imaging and photonics. © 2011 Macmillan Publishers Limited. All rights reserved.

Résumé: We demonstrate that the reflecting properties of a single particle nanoantenna can be extremely sensitive to its distance from a quantum emitter at frequencies lower than the plasmon resonance. The phenomenon is shown to arise from rapid phase variations of the emitter field at short distances associated with a phase of the antenna particle polarizability lower than π/4. © 2011 Optical Society of America.

Résumé: We introduce a full-field fluorescence imaging technique with axial confinement of about 100nm at the sample/ substrate interface. Contrary to standard surface imaging techniques, this confinement is obtained through emission filtering. This technique is based on supercritical emission selectivity. It can be implemented on any epifluorescence microscope with a commercial high numerical aperture objective and offers a real-time surface imaging capability. This technique is of particular interest for live cell membrane and adhesion studies. Using human embryonic kidney cells, we show that one can observe simultaneously the surface and in-depth cell phenomena. © 2011 Optical Society of America.

Résumé: In this paper, a novel approach to the problem of elasticity reconstruction is introduced. In this approach, the solution of the wave equation is expanded as a sum of waves travelling in different directions sharing a common wave number. In particular, the solutions for the scalar and vector potentials which are related to the dilatational and shear components of the displacement respectively are expanded as sums of travelling waves. This solution is then used as a model and fitted to the measured displacements. The value of the shear wave number which yields the best fit is then used to find the elasticity at each spatial point. The main advantage of this method over direct inversion methods is that, instead of taking the derivatives of noisy measurement data, the derivatives are taken on the analytical model. This improves the results of the inversion. The dilatational and shear components of the displacement can also be computed as a byproduct of the method, without taking any derivatives. Experimental results show the effectiveness of this technique in magnetic resonance elastography. Comparisons are made with other state-of-the-art techniques. © 2011 IEEE.

Résumé: We present functional ultrasound (fUS), a method for imaging transient changes in blood volume in the whole brain at better spatiotemporal resolution than with other functional brain imaging modalities. fUS uses plane-wave illumination at high frame rate and can measure blood volumes in smaller vessels than previous ultrasound methods. fUS identifies regions of brain activation and was used to image whisker-evoked cortical and thalamic responses and the propagation of epileptiform seizures in the rat brain. © 2011 Nature America, Inc. All rights reserved.

Résumé: The local resonances of a free isotropic elastic plate are investigated using laser ultrasonic techniques. Experimental results are interpreted in terms of zero group velocity Lamb modes and edge mode. At a distance from the edge larger than the plate thickness a sharp resonance is observed at the frequency where the group velocity of the first symmetrical Lamb mode vanishes. Close to the edge of the plate, the resonance due to the edge mode dominates. Both zero group velocity and edge resonances appear at the theoretically predicted frequencies. These frequencies do not vary with the distance from the edge of the plate and the transition between the two modes of vibration, at about the plate thickness, is abrupt. Using a laser excitation on the edge, the amplitude profile of the normal displacement at the edge resonance frequency was determined. © 2011 Acoustical Society of America.

Résumé: When a resonant laser sent on an optically thick cold atomic cloud is abruptly switched off, a coherent flash of light is emitted in the forward direction. This transient phenomenon is observed due to the highly resonant character of the atomic scatterers. We analyze quantitatively its temporal properties and show very good agreement with theoretical predictions. Based on complementary experiments, the phase of the coherent field is reconstructed without interferometric tools. © 2011 American Physical Society.

Résumé: Objectives: The goal of this study was to assess whether myocardial stiffness could be measured by shear wave imaging (SWI) and whether myocardial stiffness accurately quantified myocardial function. Background: SWI is a novel ultrasound-based technique for quantitative, local, and noninvasive mapping of soft tissue elastic properties. Methods: SWI was performed in Langendorff perfused isolated rat hearts (n = 6). Shear wave was generated and imaged in the left ventricular myocardium using a conventional ultrasonic probe connected to an ultrafast scanner (12,000 frames/s). The local myocardial stiffness was derived from shear wave velocity every 7.5 ms during 1 single cardiac cycle. Results: The average myocardial stiffness was 8.6 ± 0.7 kPa in systole and 1.7 ± 0.8 kPa in diastole. Myocardial stiffness was compared with isovolumic systolic pressure at rest and during administration of isoproterenol (10 -9, 10 -8, and 10 -7 mol/l, 5 min each). Systolic myocardial stiffness increased strongly up to 23.4 ± 3.4 kPa. Myocardial stiffness correlated strongly with isovolumic systolic pressure (r 2 = [0.94; 0.98], p < 0.0001). Conclusions: Myocardial stiffness can be measured in real time over the cardiac cycle using SWI, which allows quantification of stiffness variation between systole and diastole. Systolic myocardial stiffness provides a noninvasive index of myocardial contractility. © 2011 American College of Cardiology Foundation.

Résumé: We report on a numerical study of the optical properties of silver square nanospirals. The resonant modes of the nanospirals presented current distributions similar to those of U-shaped resonators. The resonance frequencies of the 'electric' modes were relatively insensitive to coupling, except for the shortest distance, whereas for the 'magnetic' modes they steadily increased with coupling. Aperiodicity in the strong coupling regime did not modify the strength and resonance frequency of all modes as compared to the case of periodic arrays. © 2010 Springer-Verlag.

Résumé: On a vertically vibrating fluid interface, a droplet can remain bouncing indefinitely. When approaching the Faraday instability onset, the droplet couples to the wave it generates and starts propagating horizontally. The resulting wave-particle association, called a walker, was shown previously to have remarkable dynamical properties, reminiscent of quantum behaviours. In the present article, the nature of a walker's wave field is investigated experimentally, numerically and theoretically. It is shown to result from the superposition of waves emitted by the droplet collisions with the interface. A single impact is studied experimentally and in a fluid mechanics theoretical approach. It is shown that each shock emits a radial travelling wave, leaving behind a localized mode of slowly decaying Faraday standing waves. As it moves, the walker keeps generating waves and the global structure of the wave field results from the linear superposition of the waves generated along the recent trajectory. For rectilinear trajectories, this results in a Fresnel interference pattern of the global wave field. Since the droplet moves due to its interaction with the distorted interface, this means that it is guided by a pilot wave that contains a path memory. Through this wave-mediated memory, the past as well as the environment determines the walker's present motion. © 2011 Cambridge University Press.

Résumé: Electric and magnetic hybridized plasmonic modes are obtained by stacking two T-shaped resonators. We show that head-to-toe configuration leads to inverse the hybridization. The frequency shift between the resonances is finely controlled by adjusting the gap between the two resonators. A negative refractive index close to -1 is numerically and experimentally demonstrated at 4:3 GHz for TE waves. This left-handed behavior is similar for parallel and normal TE incident wave vectors. The proposed double-T unit cell is well adapted for developing terahertz and IR metamaterials. © 2011 Optical Society of America.

Résumé: In this Letter, we study the Purcell effect in a 3D disordered dielectric medium through fluorescence decay rates of nanosized light sources. We report distributions of Purcell factor with non-Gaussian long-tailed statistics and an enhancement of up to 8 times the average value. We attribute this large enhancement to strong fluctuations of the local density of states induced by near-field scattering sustained by more than one particle. Our findings go beyond standard diagrammatic and single-scattering models and can be explained only by taking into account the full near-field interaction. © 2011 American Physical Society.

Résumé: We report a demonstration video-rate heterodyne holography in off-axis configuration. Reconstruction and display of a one megapixel hologram is achieved at 24 frames per second, with a graphics processing unit. Our claims are validated with real-time screening of steady-state vibration amplitudes in a wide-field, noncontact vibrometry experiment. © 2011 Optical Society of America.

Résumé: We propose a novel technique for measuring directly in real time, locally and non-invasively the pulse wave velocity (PWV) on peripheral arteries. Very high frame rate ultrasonic imaging (> 1000 frames/s) was achieved for tracking in 2D the propagation of transient vibrations along arterial wall. The arterial pulse waves are observed within a single cardiac cycle allowing the estimation of the pulse wave velocity with a good accuracy. In this study, this technique was validated for PWV evaluation on 25 healthy patients using conventional sonographic probes. The mean carotid PWV was found to be 5.5 ± 1.2 m/s (from 4.5 m/s to 7 m/s) with good intra and interobserver variability (inferior of 10% of the mean). These data suggest a good accuracy and reproducibility of the technique for real time PWV evaluation in vitro and on healthy volunteers. © 2011 Elsevier Masson SAS. All rights reserved.

Résumé: The phenomenon of Lamb waves focusing in anisotropic plates is theoretically and experimentally investigated. An analysis based on a far field approximation of the Green's function shows that Lamb waves focusing is analog to the phonon focusing effect. In highly anisotropic structures like composite plates the focusing of A 0 and S 0 mode is strong; the energy propagates preferentially in the fibre directions, which are minima of the slowness. This has to be taken into account when developing, for example, a transducer array for structural health monitoring systems based on Lamb waves in order to avoid dead zones. © Published under licence by IOP Publishing Ltd.

Résumé: Nanoantennas have the unique ability to affect the emission pattern of a dipole in free space. We present a technique based on full-field heterodyne holography for the mapping of the scattered field of plasmonic gold nanodisk chains in all three dimensions. A spectroscopic study allowed us to determine the resonant and nonresonant wavelengths at which we conducted a full characterization of the scattered field on a chosen nanodisk chain. © 2011 Optical Society of America.

Résumé: Plasmonic metamaterial with a negative refractive index is achieved in a periodical symmetric cut-wire pair without any structural asymmetry. The main control parameter is the lateral lattice size. The designed structure presents two discrete resonance modes due to hybridization. The electric resonance depends strongly on the structural parameter, while the magnetic resonance remains unchanged with this parameter. The hybridization scheme can be inverted by increasing the lateral lattice width. Theoretical and experimental results demonstrate a negative refraction index for the designed structure when the electric resonance frequency occurs below the magnetic one. This spatial arrangement presents many advantages since it is easier, repetitive and well-adapted for realizations in microwave and optical regimes. © 2011 American Institute of Physics.

Résumé: The present study demonstrates the targeting of ultrasound contrast agents to human xenograft tumors by exploiting the overexpression of the glycolipid Gb3 in neovasculature. To this end, microbubbles were functionalized with a natural Gb3 ligand, the B subunit of the Shiga toxin (STxB). The targeting of Gb3-expressing tumor cells by STxB microbubbles was first shown by flow cytometry and fluorescence microscopy. A significantly higher proportion of STxB microbubbles were associated with Gb3- expressing tumor cells compared to cells in which Gb3 expression was inhibited. Moreover, ultrasonic imaging of culture plates showed a 12 dB contrast enhancement in average backscattered acoustic intensity on the surface of Gb3-expressing cells compared to Gb3-negative cells. Also, a 18 dB contrast enhancement was found in favor of STxB microbubbles compared to unspecific microbubbles. Microbubble signal intensity in subcutaneous tumors in mice was more than twice as high after the injection of STxBfunctionalized microbubbles compared to the injection of unspecific microbubbles. These in vitro and in vivo experiments demonstrated that STxB-functionalized microbubbles bind specifically to cells expressing the Gb3 glycolipid. The cell-binding moieties of toxins thus appear as a new group of ligands for angiogenesis imaging with ultrasound. © 2011 Decker Publishing.

Résumé: Purpose: As an ultrasound wave propagates nonlinearly, energy is transferred to higher frequencies where it is more strongly attenuated. Compared to soft tissue, the skull has strongly heterogeneous material parameters. The authors characterize with experiments and establish a numerical method that can describe the effects of the skull on the nonlinear components of ultrasonic wave propagation for application to high intensity focused ultrasound (HIFU) therapy in the brain. The impact of nonlinear acoustic propagation on heat deposition and thermal dose delivery is quantified and compared to linear assumptions by coupling an acoustic simulation with a heating model for brain tissue. Methods: A degassed dessicated human skull was placed in a water tank and insonified at 1 MPa with 7 mm transducer from a custom array designed for HIFU treatment. Two dimensional scans were performed preceding and following propagation through the skull with a calibrated hydrophone. Data from the scan preceding the skull were used as an input to a three dimensional finite difference time domain (FDTD) simulation that calculates the effects of diffraction, density, attenuation with linear dependence on frequency via relaxation mechanisms, and second order nonlinearity. A measured representation of the skull was used to determine the skull's acoustic properties. The validated acoustic model was used to determine the loss due to nonlinear propagation and then coupled to a finite difference simulation of the bioheat equation for two focal configurations at 3 and 7.5 cm from the skull surface. Results: Prior to propagation through the skull, the second harmonic component was 19 dB lower than the fundamental, and the third harmonic component was 37 dB lower. Following the skull, the second harmonic component was 35 dB lower and the third harmonic was 55 dB lower. The simulation is in agreement with the measurements to within 0.5 dB across the considered frequency range and shows good agreement across the two dimensional scan. It is then shown that the volume of treated brain is at least twice as large when assuming nonlinear acoustics. Conclusions: The authors have established a three dimensional FDTD simulation that accurately models the effects of nonlinearity and attenuation for propagation through the skull. Experimental validation shows good agreement across a broad frequency range and spatial extent. The nonlinear thermal dose was over an order of magnitude larger at the focus than the linear thermal dose and the necrotic volume was larger by at least a factor of 2. These results have particular applications to treatment planning. © 2011 American Association of Physicists in Medicine.

Résumé: In order to characterize surface chemomechanical phenomena driving microelectromechanical systems behavior, we propose herein a method to simultaneously obtain a full kinematic field describing the surface displacement and a map of its chemical modification from optical measurements. Using a microscope, reflected intensity fields are recorded for two different illumination wavelengths. Decoupling the wavelength-independent and-dependent contributions to the measured relative intensity changes then yields the sought fields. This method is applied to the investigation of the electroelastic coupling, providing images of both the local surface electrical charge density and the device deformation field. © 2011 Optical Society of America.

Résumé: The clinical applicability of high-intensity focused ultrasound (HIFU) for noninvasive therapy is today hampered by the lack of robust and real-time monitoring of tissue damage during treatment. The goal of this study is to show that the estimation of local tissue elasticity from shear wave imaging (SWI) can lead to the 2-D mapping of temperature changes during HIFU treatments. This new concept of shear wave thermometry is experimentally implemented here using conventional ultrasonic imaging probes. HIFU treatment and monitoring were, respectively, performed using a confocal setup consisting of a 2.5-MHz single-element transducer focused at 30 mm on ex vivo samples and an 8-MHz ultrasound diagnostic probe. Thermocouple measurements and ultrasound-based thermometry were used as a gold standard technique and were combined with SWI on the same device. The SWI sequences consisted of 2 successive shear waves induced at different lateral positions. Each wave was created using 100-μs pushing beams at 3 depths. The shear wave propagation was acquired at 17 000 frames/s, from which the elasticity map was recovered. HIFU sonications were interleaved with fast imaging acquisitions, allowing a duty cycle of more than 90%. Elasticity and temperature mapping was achieved every 3 s, leading to realtime monitoring of the treatment. Tissue stiffness was found to decrease in the focal zone for temperatures up to 43°C. Ultrasound-based temperature estimation was highly correlated to stiffness variation maps (r2 = 0.91 to 0.97). A reversible calibration phase of the changes of elasticity with temperature can be made locally using sighting shots. This calibration process allows for the derivation of temperature maps from shear wave imaging. Compared with conventional ultrasound-based approaches, shear wave thermometry is found to be much more robust to motion artifacts. © 2006 IEEE.

Résumé: We study negative refraction and focusing of elastic waves in a simple mechanical system comprised of a freestanding plate with a step change in thickness. A point-focused and intensity-modulated laser source is used to excite backward-propagating Lamb waves on one side of the step, and the displacement field is probed using an optical interferometer. Conversion between forward- and backward-propagating modes at the interface leads to negative refraction, and we demonstrate for the first time the operation of a flat lens, similar to that predicted by Veselago in negative-index media, for guided elastic waves in isotropic media. We propose that guided elastic waves provide a convenient and powerful experimental test bed for the study of negative-index physics. © 2011 American Physical Society.

Résumé: We present an experimental study on the trapped modes occurring around a vertical surface-piercing circular cylinder of radius a placed symmetrically between the parallel walls of a long but finite water waveguide of width 2d. A wavemaker placed near the entrance of the waveguide is used to force an asymmetric perturbation into the guide, and the free-surface deformation field is measured using a global single-shot optical profilometric technique. In this configuration, several values of the aspect ratio a/d were explored for a range of driving frequencies below the waveguide's cutoff. Decomposition of the obtained fields in harmonics of the driving frequency allowed for the isolation of the linear contribution, which was subsequently separated according to the symmetries of the problem. For each of the aspect ratios considered, the spatial structure of the trapped mode was obtained and compared to the theoretical predictions given by a multipole expansion method. The waveguide'obstacle system was further characterized in terms of reflection and transmission coefficients, which led to the construction of resonance curves showing the presence of one or two trapped modes (depending on the value of a/d), a result that is consistent with the theoretical predictions available in the literature. The frequency dependency of the trapped modes with the geometrical parameter a/d was determined from these curves and successfully compared to the theoretical predictions available within the frame of linear wave theory. © 2011 Cambridge University Press.

Résumé: Waves scattered by a weakly inhomogeneous random medium contain a predominant single-scattering contribution as well as a multiple-scattering contribution which is usually neglected, especially for imaging purposes. A method based on random matrix theory is proposed to separate the single- and multiple-scattering contributions. The experimental setup uses an array of sources/receivers placed in front of the medium. The impulse responses between every couple of transducers are measured and form a matrix. Single-scattering contributions are shown to exhibit a deterministic coherence along the antidiagonals of the array response matrix, whatever the distribution of inhomogeneities. This property is taken advantage of to discriminate single- from multiple-scattered waves. This allows one to evaluate the absorption losses and the scattering losses separately, by comparing the multiple-scattering intensity with a radiative transfer model. Moreover, the relative contribution of multiple scattering in the backscattered wave can be estimated, which serves as a validity test for the Born approximation. Experimental results are presented with ultrasonic waves in the megahertz range, on a synthetic sample (agar-gelatine gel) as well as on breast tissues. Interestingly, the multiple-scattering contribution is found to be far from negligible in the breast around 4.3 MHz. © 2011 Acoustical Society of America.

Résumé: This Letter reports a demonstration of off-axis compressed holography in low-light level imaging conditions. An acquisition protocol relying on a single exposure of a randomly undersampled diffraction map of the optical field, recorded in the high heterodyne gain regime, is proposed. The image acquisition scheme is based on compressed sensing, a theory establishing that near-exact recovery of an unknown sparse signal is possible from a small number of nonstructured measurements. Image reconstruction is further enhanced by introducing an off-axis spatial support constraint to the image estimation algorithm. We report accurate experimental recovering of holographic images of a resolution target in low-light conditions with a frame exposure of 5 μs, scaling down measurements to 9% of random pixels within the array detector. © 2011 Optical Society of America.