Articles 2007

Résumé: A time-reversal mirror (TRM) is a plane apparatus that generates the time symmetric of a wave produced by an initial source. Here we look for the conditions to obtain subwavelength focusing when the initial source is in the near field of the TRM and the propagating medium is homogeneous and isotropic. Three variants of TRM are studied: TRM made of monopoles, dipoles, or both of them. The analysis is performed in terms of evanescent and propagative waves. We conclude that only the dipole-TRM leads to subwavelength focusing. © 2007 The American Physical Society.

Résumé: The assessment of viscoelastic properties of soft tissues is enjoying a growing interest in the field of medical imaging as pathologies are often correlated with a local change of stiffness. To date, advanced techniques in that field have been concentrating on the estimation of the second order elastic modulus (μ). In this paper, the nonlinear behavior of quasi-incompressible soft solids is investigated using the supersonic shear imaging technique based on the remote generation of polarized plane shear waves in tissues induced by the acoustic radiation force. Applying a theoretical approach of the strain energy in soft solid [Hamilton, J. Acoust. Soc. Am. 116, 41-44 (2004)], it is shown that the well-known acoustoelasticity experiment allowing the recovery of higher order elastic moduli can be greatly simplified. Experimentally, it requires measurements of the local speed of polarized plane shear waves in a statically and uniaxially stressed isotropic medium. These shear wave speed estimates are obtained by imaging the shear wave propagation in soft media with an ultrafast echographic scanner. In this situation, the uniaxial static stress induces anisotropy due to the nonlinear effects and results in a change of shear wave speed. Then the third order elastic modulus (A) is measured in agar-gelatin-based phantoms and polyvinyl alcohol based phantoms. © 2007 Acoustical Society of America.

Résumé: Structured illumination microscopy provides a simple and cheap mean to obtain optical sections of a sample. It can be implemented easily on a regular fluorescent microscope and is a scanning free alternative to confocal microscopy. We have analyzed theoretically the performances of the technique in terms of sectioning strength, resolution enhancement along the optical axis, and signal to background as a function of the objective used and the grid's characteristics (pitch and contrast). We show that under optimized conditions, the axial resolution can be improved by a factor of 1.5 in comparison with an epifluorescence microscope, and that optical cuts with a thickness of less than 400nm can be obtained with a 1.4 numerical aperture objective. We modified the original grid in-step modulation pattern and used a sinusoidal modulation for the grid displacement. Optical sections are computed by combining four images acquired during one modulation period. This algorithm is very stable even for modulations at high frequencies. The speed of the acquisition is thus only limited by the performance of the detector and the signal/background ratio of the sample. Finally, we compared our technique to commercial setups: a confocal microscope, a Spinning Disk Microscope and a Zeiss Apotome. © 2007 Optical Society of America.

Résumé: Bursts of focused ultrasound energy a billion times more intense than diagnostic ultrasound have become a non-invasive option for tumor ablation, from prostate cancer to uterine fibroid, during the last decade. Despite this progress, many issues still need to be addressed. First, for brain targeting, the correction of distortions induced by the skull remains today a technological achievement that still needs to be validated clinically. Secondly, the problem of motion artifacts for abdominal treatments becomes today an important research topic. For all these issues, the potential of new ultrasonic therapy devices able to work both in Transmit and Receive modes will be emphasized and clinical results on monkeys and pigs will be presented. © 2007. Éditions Françaises de Radiologie. Édité par Elsevier Masson SAS.

Résumé: This paper describes nonlinear shear wave experiments conducted in soft solids with transient elastography technique. The nonlinear solutions that theoretically account for plane and nonplane shear wave propagation are compared with experimental results. It is observed that the cubic nonlinearity implied in high amplitude transverse waves at f0 =100 Hz results in the generation of odd harmonics 3 f0, 5 f0. In the case of the nonlinear interaction between two transverse waves at frequencies f1 and f2, the resulting harmonics are fi ±2 fj (i,j=1,2). Experimental data are compared to numerical solutions of the modified Burgers equation, allowing an estimation of the nonlinear parameter relative to shear waves. The definition of this combination of elastic moduli (up to fourth order) can be obtained using an energy development adapted to soft solid. In the more complex situation of nonplane shear waves, the quadratic nonlinearity gives rise to more usual harmonics, at sum and difference frequencies, fi ± fj. All components of the field have to be taken into account. © 2007 Acoustical Society of America.

Résumé: A rigid 24-element source-receiver array in the 10-15 kHz frequency band, connected to a programmable electronic system, was deployed in the Bay of Brest during spring 2005. In this 10- to 18-m -deep environment, backscattered data from submerged targets were recorded. Successful detection and focusing experiments in very shallow water using the decomposition of the time reversal operator (DORT method) are shown. The ability of the DORT method to separate the echo of a target from reverberation as well as the echo from two different targets at 250 m is shown. An example of active focusing within the waveguide using the first invariant of the time reversal operator is presented, showing the enhanced focusing capability. Furthermore, the localization of the scatterers in the water column is obtained using a range-dependent acoustic model. © 2007 Acoustical Society of America.

Résumé: We present a polarization-sensitive full-field optical coherence tomography system that can produce highresolution images of the linear retardance and reflectivity properties of biological media. En face images can be delivered at a frame rate of 3.5 Hz by combination of interferometric images acquired by two CCD cameras in an interference microscope illuminated with a tungsten halogen lamp. Isotropic spatial resolution of ∼1.0 μm is achieved. The technique is demonstrated on ex vivo muscle tissues. © 2007 Optical Society of America.

Résumé: The scattering of a surface wave by a pinned edge dislocation in a semi-infinite, homogeneous, isotropic, three-dimensional elastic solid is investigated analytically and numerically. An incident wave excites the dislocation that responds by oscillating as a string endowed with mass, line tension, and damping. The oscillations of the stringlike dislocation generate secondary ("scattered") elastic waves that are the primary object of interest in this study. The back reaction of the re-emitted waves on the dislocation dynamics is neglected, but the wavelength of the radiation is allowed to be large, comparable, or small compared to the length of the dislocation. In view of recent experimental visualizations of these phenomena, we focus particularly on the field behavior at the free surface near the dislocation, and not just on the far field. For the same reason, it is the vertical component of displacement at the free surface that is studied in detail. An efficient numerical procedure for the computation of the appropriate components of the Green's function, using a Filon quadrature for the integration of rapidly oscillating functions, is developed. The numerics is validated with known analytical expressions. The secondary radiation generated by the response of the dislocation to the incident wave is also calculated numerically, and the results are also validated by comparing them with the analytical expressions that can be obtained when the radiation wavelength is very long compared to dislocation length. The interference pattern between incident wave and secondary wave that is generated at the free surface is studied in detail and found to depend strongly not only on wavelength and dislocation geometry (length and orientation) but also on dislocation depth, with the response of the dislocation being a particularly sensitive function of such depth. Results are compared with recent experiments of Shilo and Zolotoyabko [Phys. Rev. Lett. 91, 115506 (2003)] that report visualizations of the surface-wave-dislocation interaction using stroboscopic x-ray imaging. A satisfactory agreement is found. Dislocation velocities of a few percent of the speed of sound and viscosity coefficients of about 10-5 Pa s are inferred. © 2007 The American Physical Society.

Résumé: Using a sensitive optical interferometer, the low frequency displacement nonlinearly generated by an ultrasonic tone burst propagating in a liquid is studied. Close to the source, the low frequency displacement contains a quasi-static component, which is affected by diffraction effects farther from the transducer. The experimental setup provides quantitative results, which allow the determination of the nonlinearity parameter of the liquid with a good accuracy. Such measurements are carried out in water and ethanol. Finally, the pressure associated with the low frequency displacement is discussed. Introducing the temporal mean value of the displacement, as already done in lossless solids, the noncumulative part of this second order pressure is associated with the static part of the low frequency displacement. This interpretation leads to extend the definition of the Rayleigh radiation pressure usually introduced for a continuous plane wave radiated in a confined fluid. © 2007 Acoustical Society of America.

Résumé: To increase the sensitivity of Lamb waves to hidden corrosion in aircraft structures, a preliminary step is to understand the phenomena governing this interaction. A hybrid model combining a finite element approach and a modal decomposition method is used to investigate the interaction of Lamb modes with corrosion pits. The finite element mesh is used to describe the region surrounding the corrosion pits while the modal decomposition method permits to determine the waves reflected and transmitted by the damaged area. Simulations make easier the interpretation of some parts of the measured waveform corresponding to superposition of waves diffracted by the corroded area. Numerical results permit to extract significant information from the transmitted waveform and thus to optimize the signal processing for the detection of corrosion at an early stage. Now, we are able to detect corrosion pits down to 80-μm depth distributed randomly on a square centimeter of an aluminum plate. Moreover, thickness variations present on aircraft structures can be discriminated from a slightly corroded area. Finally, using this experimental setup, aircraft structures have been tested. © 2007 Elsevier B.V. All rights reserved.

Résumé: We present a group-theory analysis of the electron states in bulk and low-dimensional semiconductor structures under a uniform magnetic field. The analysis takes into account the gauge transformations under the symmetry operations, It is shown that the symmetry operations commute in dots, rods and tubes whatever is the orientation of the magnetic field whereas they commute in layers only when the field is in the plane. The commutations properties allow using conventional symmetry groups. In bulk materials, the confinement of electrons within rods whose axes are parallel to the field makes it possible to derive approximate wavefunction symmetries from the rod group involving the geometrical symmetry operations common to the field and the crystal but excluding the translations that are not parallel to the field. An approximation of the same kind can also be made for low-dimensional structures. Applications to bulk materials with the wurtzite or zinc blende structure as well as related nanostructures are presented. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Résumé: The authors present a camera-based optical detection scheme designed to detect the transient motion created by the acoustic radiation force in soft 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 decorrelation of the optical speckle was measured as the result of localized motion induced by the radiation force and the associated propagating shear waves. The proposed technique is sensitive only to the low frequency transient motion induced in the medium by the radiation force. © 2007 American Institute of Physics.

Résumé: A method is introduced to identify simultaneously elastic properties and loading fields from a measured displacement field. Since the mechanical behavior of micro-electro-mechanical systems (MEMS) is governed by surface effects, this type of identification tool is thought to be of major interest. However, increasing the number of parameters to retrieve affects the redundancy necessary for an accurate identification. A finite-element formulation of a distance between measured and statically admissible (SA) displacement fields is shown to be equivalent to a standard least-squares distance to kinematically admissible (KA) fields if the used modeling is suitable. Any deviation from this equivalence is then the signature of a modeling error. Balancing the distance to KA and SA displacement fields allows one to retrieve unknown modeling parameters. This method is detailed on heterogeneous Euler-Bernoulli beams submitted to an unknown loading field and applied to experimental displacement fields of micro-cantilevers obtained with an electrostatic set-up. An elastic property field and a parameterized loading field are then identified, and the quality of the identification is assessed. © 2006 Elsevier Ltd. All rights reserved.

Résumé: We describe a novel method for localizing a fluorescent inclusion in a homogeneous turbid medium through the use of time-resolved techniques. Based on the calculation of the mean time of the fluorescence curves, the method does not require a priori knowledge of either the fluorescence lifetime or the mean time of the instrument response function since it adopts a differential processing approach. Theoretical expressions were validated and experiments for assessing the accuracy of localization were carried out on liquid optical phantoms with a small fluorescent inclusion. The illumination and detection optical fibers were immersed in the medium to achieve infinite medium geometry as required by the model used. The experimental setup consisted of a time-correlated single-photon counting system. Submillimeter accuracy was achieved for the localization of the inclusion. © 2007 Optical Society of America.

Résumé: Despite recent progress, breast cancer remains today a major public health problem as it represents the main morbidity incidence for woman with 42,000 new cases and 11,600 deaths per year in France. X-Ray mammography which is the "gold standard" exam for breast screening relies on an excellent sensitivity (nevertheless its quality is varying with respect to breast density). However, its specificity for malignancy diagnosis remains moderate leading to many useless interventions for lesions proven a posteriori to be benign by histology. The other imaging techniques such as echography and magnetic resonance imaging (MRI) also possess their own limits. Echography is strongly operator-dependent. Dynamic MRI with injection of contrast agents has a high sensitivity for breast cancer detection (>90%) but suffers from a moderate specificity (50 to 80% according to the type of cancer). In parallel, although it is strongly subjective, the act of palpation remains today a major act in the workflow of breast screening. Since Egyptian ancient ages, the physicians practise the act of palpating body parts in order to determine tissues stiffness and a hardly deformed mass within an organ is often related to the presence of an abnormal lesion. Palpation is not only useful for screening and diagnosis as the surgeon also uses it during interventions to be effectively guided towards the pathological area. Recently, new techniques based on ultrasound or magnetic resonance imaging finally made it possible to map organs elasticity in a quantitative way. These "elastography" techniques could play soon an important role in medical imaging. © 2007 Elsevier Masson SAS. All rights reserved.

Résumé: Bursts of focused ultrasound energy a thousand times more intense than diagnostic ultrasound have become a non-invasive option for treating cancer, from breast to prostate or uterine fibroid, during the last decade. Despite this progress, many issues still need to be addressed. First, the distortions caused by defocusing obstacles, such as the skull or ribs, on the ultrasonic therapeutic beam are still being investigated. Multi-element transducer technology must be used in order to achieve such transcranial or transcostal adaptive focusing. Second, the problem of motion artifacts, a key component in the treatment of abdominal lesions, has been shown significantly to influence the efficacy and treatment time. Though many methods have been proposed for the detection of organ motion, little work has been done to develop a comprehensive solution including motion tracking and feedback correction in real time. This paper is a review of the work achieved by authors in transcranial high-intensity focused ultrasound (HIFU), transcostal HIFU and motion compensated HIFU. For these three issues, the optimal solution can be reached using the same technology of multi-element transducers devices able to work both in transmit and receive modes.

Résumé: This paper presents a combined finite element and modal decomposition method to study the interaction of Lamb waves with damaged area. The finite element mesh is used to describe the region around the defects. On the contrary to other hybrid models already developed, the interaction between Lamb waves and defects is computed in the temporal domain. Then, the modal decomposition method permits to determine the wave reflected and transmitted by the damaged area. Modal analysis allows also identifying the mode conversions induced by the defects. These numerical results agree with previous finite element results concerning the interaction of Lamb modes with a notch. Experiments, carried out with gauged defects on an aluminum plate, are also compared to numerical predictions to validate the simulation. Compared to classical techniques of simulation, this new method allows us to investigate the interaction of Lamb modes generated at high frequency-thickness product with micro-defects as corrosion pitting. © 2006 Elsevier B.V. All rights reserved.

Résumé: We present an approach for subwavelength focusing of microwaves using both a time-reversal mirror placed in the far field and a random distribution of scatterers placed in the near field of the focusing point. The far-field time-reversal mirror is used to build the time-reversed wave field, which interacts with the random medium to regenerate not only the propagating waves but also the evanescent waves required to refocus below the diffraction limit. Focal spots as small as one-thirtieth of a wavelength are described. We present one example of an application to telecommunications, which shows enhancement of the information transmission rate by a factor of 3.

Résumé: The thermal conductivity of a two-phase material, based on a glass matrix containing mm sized spherical alumina inclusions, has been studied as a function of the alumina phase volume fraction. The glass matrix and the alumina phase were chosen with almost identical coefficients of thermal expansion to ensure good thermal contact at the interface between the two phases. The thermal conductivity of the alumina phase was determined by local measurements on the inclusions using the mirage technique. For the glass phase and the two-phase samples, the thermal conductivity values were evaluated with the laser flash technique and compared to predictions by analytical models. The Maxwell-Eucken model gives a close agreement to these experimental values for alumina volume fractions up to 55%. In fact, we show that for large mm sized alumina inclusions, Hasselman's correction for the interface thermal resistance is not necessary. © 2006 Elsevier Ltd. All rights reserved.

Résumé: Localizing an impact generated by a simple finger knock on plate-shaped solid objects is made possible in an acoustic time reversal experiment. It is shown that the technique works with a single accelerometer. To better understand the phenomenon and to know exactly the nature of the created waves, a two-dimensional (2-D) elastic simulation is used, showing that in a very good approximation the Ao Lamb mode is the only propagating one. However, it is shown that, within one wavelength distance from the edges, evanescent waves must be taken into account. As a first consequence, the ability to distinguish two neighboring impacts improves when the plate thickness decreases and the frequency increases. As a second consequence, it is expected theoretically that temperature variations lead to a stretching or a contraction of acoustic signatures. The experimental demonstration used a heterodyne interferometer to measure the impulse responses created by a knock on a plate during the cooling. A simple algorithm is shown to perfectly compensate for temperature impacts, which demonstrates the feasibility of the technique for outdoor time reversal interactive experiments. © 2007 IEEE.