Timereversal waves and super resolution. Fink, M. Journal of Physics: Conference Series 124 (2008).
Résumé: Timereversal mirrors (TRMs) refocus an incident acoustic field to the position of the original source regardless of the complexity of the propagation medium. TRM's have now been implemented in a variety of physical scenarios from MHz ultrasonics with order centimeter aperture size to hundreds/thousands of Hz in ocean acoustics with order hundred meter aperture size. Common to this broad range of scales is a remarkable robustness exemplified by observations at all scales that the more complex the medium between the probe source and the TRM, the sharper the focus. The relation between the medium complexity and the size of the focal spot is studied in this paper. It is certainly the most exciting property of TRM compared to standard focusing devices. A TRM acts as an antenna that uses complex environments to appears wider than it is, resulting for a broadband pulse in a refocusing quality that does not depend of the TRM aperture. In this paper, we investigate the timereversal approach in various media of increasing complexity We will also demonstrated that timereversal focusing opens completely new approaches to superresolution. We will show that in medium made of random distribution of subwavelength scatterers, a timereversed wave field interacts with the random medium to regenerate not only the propagating but also the evanescent waves required to refocus below the diffraction limit. Finally, we will discuss the link existing between timereversal approaches and new imaging methods recently developed where Green's functions of complex media can be extracted from diffusive noise by crosscorrelating the recordings of a diffuse random wave field. © 2008 IOP Publishing Ltd.
MotsClés: Aperture sizes; Broadband pulse; Complex environments; Complex media; Diffraction limits; Evanescent wave; Focal spot; Focusing device; Imaging method; Medium complexity; Ocean acoustics; Physical scenario; Probe source; Random distribution; Random medium; Random waves; Subwavelength; Super resolution; Timereversal; Timereversal focusing; Timereversed; Wavefields; Acoustic fields; Diffraction; Inverse problems; Differential equations


The Minnaert bubble: An acoustic approach. Devaud, M., T. Hocquet, J.  C. Bacri, and V. Leroy. European Journal of Physics 29, no. 6 (2008): 1263–1285.
Résumé: We propose an ab initio introduction to the wellknown Minnaert pulsating bubble at graduate level. After a brief recall of the standard stuff, we begin with a detailed discussion of the radial movements of an air bubble in water. This discussion is managed from an acoustic point of view, and using the Lagrangian rather than the Eulerian variables. In unbounded water, the airwater system has a continuum of eigenmodes, some of them correspond to regular FabryPérot resonances. A singular resonance, the lowest one, is shown to coincide with that of Minnaert. In bounded water, the eigenmodes spectrum is discrete, with a finite fundamental frequency. A spectacular quasilocking of the latter occurs if it happens to exceed the Minnaert frequency, which provides an unforeseen onebubble alternative version of the famous 'hot chocolate effect'. In the (low) frequency domain in which sound propagation inside the bubble reduces to a simple 'breathing' (i.e. inflation/deflation), the light air bubble can be 'dressed' by the outer water pressure forces, and is turned into the heavy Minnaert bubble. Thanks to this unexpected renormalization process, we demonstrate that the Minnaert bubble definitely behaves like a true harmonic oscillator of the springbob type, but with a damping term and a forcing term in apparent disagreement with those commonly admitted in the literature. Finally, we underline the double role played by the water. In order to tell the water motion associated with water compressibility (i.e. the sound) from the simple incompressible accompaniment of the bubble breathing, we introduce a new picture analogous to the electromagnetic radiative picture in Coulomb gauge, which naturally leads us to split the water displacement in an instantaneous and a retarded part. The Minnaert renormalized mass of the dressed bubble is then automatically recovered. © 2008 IOP Publishing Ltd.
MotsClés: Acoustics; Oscillators (electronic); Resonance; Abinitio; Air bubbles; Airwater systems; Coulomb gauges; Eigenmodes; Electromagnetic radiative; Eulerian; Forcing terms; Frequency domains; Fundamental frequencies; Graduate levels; Harmonic oscillators; Lagrangian; Pulsating bubbles; Radial movements; Renormalization; Sound propagation; Water compressibilities; Water displacements; Water motions; Water pressures; Heavy water


Electrical impedance tomography by elastic deformation. Ammari, H., E. Bonnetier, Y. Capdeboscq, M. Tanter, and M. Fink. SIAM Journal on Applied Mathematics 68, no. 6 (2008): 1557–1573.
Résumé: This paper presents a new algorithm for conductivity imaging. Our idea is to extract more information about the conductivity distribution from data that have been enriched by coupling impedance electrical measurements to localized elastic perturbations. Using asymptotics of the fields in the presence of small volume inclusions, we relate the pointwise values of the energy density to the measured data through a nonlinear PDE. Our algorithm is based on this PDE and takes full advantage of the enriched data. We give numerical examples that illustrate the performance and the accuracy of our approach. © 2008 Society for Industrial and Applied Mathematics.
MotsClés: 0Laplacian; Asymptotic formula; Elastic perturbation; Electrical impedance tomography; Reconstruction; Substitution algorithm; 0Laplacian; Asymptotic formula; Asymptotics; Conductivity distributions; Conductivity imaging; Coupling impedances; Elastic perturbations; Electrical impedance tomography; Electrical measurement; Energy density; Measured data; Non linear PDE; Numerical example; Algorithms; Diagnostic radiography; Electric impedance measurement; Electric impedance tomography; Perturbation techniques; Electric impedance


Polarization conversion with a photonic crystal slab. Laroche, M., F. Marquier, C. Vandenbem, and J.  J. Greffet. Journal of the European Optical Society 3 (2008).
Résumé: We show that a photonic crystal slab can enable efficient polarization conversion. Two mechanisms are identified. The first mechanism relies on the anisotropy of the bulk properties of the metamaterial and is mediated by interferences. The second mechanism is due to the resonant excitation of leaky surface waves at the interface of the photonic crystal. The latter is analogous to the polarization conversion by excitation of surface plasmons on a metallic grating. This is another example of the possibility of mimicking plasmonics with photonic crystals.
MotsClés: Photonic crystals; Polarization; Surface waves


Analysis of the depth resolution limit of luminescence diffuse optical imaging. Boffety, M., M. Allain, A. Sentenac, M. Massonneau, and R. Carminati. Optics Letters 33, no. 20 (2008): 2290–2292.
Résumé: We introduce a methodology to determine quantitatively the depth resolution limit in luminescence diffuse optical imaging. The approach is based on a CramerRao statistical analysis, a noise model, and calculations of photon transport in tissues. We illustrate the method in the case of luminescence imaging in a brainskull model, showing its potential applications in molecular imaging on small animals. © 2008 Optical Society of America.
MotsClés: Light emission; Luminescence; Optical image storage; Optical variables measurement; Depth resolutions; Diffuse optical imaging; Luminescence imaging; Molecular imaging; Noise models; Photon transports; Potential applications; Skull models; Small animals; Statistical analysis; Light sources


Spectroscopic ultrahighresolution fullfield optical coherence microscopy. Dubois, A., J. Moreau, and C. Boccara. Optics Express 16, no. 21 (2008): 17082–17091.
Résumé: We have developed a fullfield optical coherence microscopy system providing intensitybased tomographic images and spectroscopic information with ultrahigh spatial resolution. Local measurements of the backscattered light spectrum center of mass are achieved through shorttime Fourier analysis of a stack of en face interferometric images acquired with a Linniktype microscope. Using a halogen lamp as an illumination source enables us to achieve spectroscopic imaging over a wavelength range from 600 to 900 nm with a spatial resolution of ∼ 1 μm. Absorption measurements of a colored gel are reported as a validation of the technique. Enhancement of tissue imaging contrast is demonstrated by imaging a Xenopus Laevis (African frog) tadpole ex vivo. © 2008 Optical Society of America.
MotsClés: Absorption; Coherent light; Colloids; Fourier analysis; Gelation; Image enhancement; Optical microscopy; Absorption measurements; Backscattered light; Center of masses; Exvivo; Halogen lamps; Illumination sources; Imaging; Interferometric images; Local measurements; Optical coherence microscopies; Spatial resolution.; Spatial resolutions; Spectroscopic imaging; Spectroscopic informations; Tissue imaging; Tomographic images; Ultrahigh; Wavelength ranges; Xenopus laevis; Absorption spectroscopy; article; computer aided design; computer simulation; equipment; equipment design; image enhancement; instrumentation; light; microscopy; optical coherence tomography; radiation scattering; sensitivity and specificity; spectroscopy; theoretical model; Computer Simulation; ComputerAided Design; Equipment Design; Equipment Failure Analysis; Image Enhancement; Light; Microscopy; Models, Theoretical; Scattering, Radiation; Sensitivity and Specificity; Spectrum Analysis; Tomography, Optical Coherence


Suppression of tissue harmonics for pulseinversion contrast imaging using time reversal. Couture, O., J.  F. Aubry, G. Montaldo, M. Tanter, and M. Fink. Physics in Medicine and Biology 53, no. 19 (2008): 5469–5480.
Résumé: Pulseinversion (PI) sequences are sensitive to the nonlinear echoes from microbubbles allowing an improvement in the bloodtotissue contrast. However, at larger mechanical indices, this contrast is reduced by harmonics produced during nonlinear propagation. A method for tissue harmonics cancellation exploiting time reversal is experimentally implemented using a 128channel 12bit emitter receiver. The probe calibration is performed by acquiring the nonlinear echo of a wire in water. These distorted pulses are timereversed, optimized and used for the PI imaging of a tissue phantom. Compared to normal (straight) pulses, the timereversed distorted pulses reduced the tissue signal in PI by 11 dB. The second harmonic signals from microbubbles flowing in a wallless vessel were unaffected by the correction. This technique can thus increase the bloodtotissue contrast ratio while keeping the pressure and the number of pulses constant. © 2008 Institute of Physics and Engineering in Medicine.
MotsClés: Blood; Harmonic analysis; Nuclear physics; Quantum theory; Contrast imaging; Contrast ratio; Mechanical indexes; Microbubbles; Nonlinear echoes; Nonlinear propagations; Probe calibration; Second harmonic signals; Time reversal; Timereversed; Tissue phantoms; Tissue; article; calibration; contrast radiography; image quality; imaging system; mathematical model; microbubble; priority journal; pulse inversion; Artifacts; Calibration; Contrast Media; Copper; Microbubbles; Phantoms, Imaging; Time Factors


Assessment of the mechanical properties of the musculoskeletal system using 2D and 3D very high frame rate ultrasound. Deffieux, T., J.  L. Gennisson, M. Tanter, and M. Fink. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 55, no. 10 (2008): 2177–2190.
Résumé: One of the great challenges for understanding muscular diseases is to assess noninvasively the active and passive mechanical properties of the musculoskeletal system. In this paper we report the use of ultrafast ultrasound imaging to explore with a submillimeter resolution the behavior of the contracting tissues in vivo (biceps brachii). To image the contraction, which is a very brief phenomenon (<100 ms), a recently designed ultrasound scanner prototype able to take up to 6000 frames/s was used. A very high frame rate from 1000 to 2500 frames/s was used to image the cross section plane of the muscle (transverse to fibers) enabling us to catch in real time the muscle contraction during a transient electrostimulation. Tissue velocities were obtained from radio frequencybased speckle tracking techniques and their profiles are discussed with respect to electrostimulation intensities and pulse repetition frequencies for different volunteers. Threedimensional (3D) very high frame rate movies were also acquired by repeating the experiment for different acquisition planes while triggering the imaging system with the electrostimulation device. The reconstructed 3D velocity field allows the full localization of the contracting fibers bundle. This ultrasound technique, referred to as echo mechanomyography, offers new perspectives for in vivo and in situ noninvasive muscle diagnosis of an active contractile tissue. © 2008 IEEE.
MotsClés: Acoustic waves; Biomechanics; Fibers; Mechanical properties; Muscle; Musculoskeletal system; Optoelectronic devices; Shrinkage; Three dimensional; Ultrasonic applications; Ultrasonics; Biceps brachii; Cross section planes; Electrostimulation; High frame rates; Insitu; Invivo; Mechanomyography; Muscle contractions; Noninvasive; Pulse Repetition frequencies; Radio frequencies; Real times; Speckle tracking techniques; Submillimeter resolutions; Tissue velocities; Ultrafast; Ultrasound imaging; Ultrasound scanners; Velocity fields; Ultrasonic imaging; algorithm; article; biomechanics; computer assisted diagnosis; echography; elastography; equipment; equipment design; human; instrumentation; mechanical stress; methodology; muscle contraction; physiology; skeletal muscle; Algorithms; Biomechanics; Elasticity Imaging Techniques; Equipment Design; Equipment Failure Analysis; Humans; Image Interpretation, ComputerAssisted; Muscle Contraction; Muscle, Skeletal; Stress, Mechanical


Symmetry and optical properties of wurtzite nanostructures with the c axis in the layer plane. Tronc, P., and P. Vennéguès. Physics of the Solid State 50, no. 10 (2008): 1803–1807.
Résumé: In wurtzitebased quantum wells and superlattices with the c axis parallel to the layer plane, this plane is parallel either to a symmetry plane of the wurtzite lattice (type I structures, the 〈11 2 0〉growth direction) or to a glide plane containing the c axis (type II structures, the 〈10 1 0〉 growth direction). In both cases, the space symmetry of the structure depends on the parity of the number of monolayers within the slab(s). The point symmetry is C 2vexcept for the type II structures with odd monolayer number(s). The latter structures have the σ vpoint symmetry and can have a builtin electric field. Quite different selection rules, depending on the structure symmetry, govern electron optical transitions and exciton radiative recombination. © 2008 Pleiades Publishing, Ltd.


Iterative highresolution wavenumber inversion applied to broadband acoustic data. Philippe, F. D., P. Roux, and D. Cassereau. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 55, no. 10 (2008): 2306–2311.
Résumé: This paper presents the results of an iterative highresolution wavenumber inversion algorithm applied to acoustic waveguides in which wave propagation is described by propagating modes. Following work on the first description of the inversion algorithm, a generalization to broadband signals is performed and experimental data are investigated. The algorithm leads to dispersion curves for propagating modes in the case of an ultrasonic waveguide and an elastic plate. Results are compared with spatialtemporal Fourier transforms. © 2008 IEEE.
MotsClés: Acoustic datum; Acoustic waveguides; Broadband signals; Dispersion curves; Elastic plates; Experimental datum; Highresolution; Inversion algorithms; Propagating modes; Ultrasonic; Wavenumber; Acoustics; Boolean functions; Particle size analysis; Telecommunication systems; Waveguides; Fourier transforms


Fourthorder shear elastic constant assessment in quasiincompressible soft solids. Ŕnier, M., J.  L. Gennisson, C. Barrìre, D. Royer, and M. Fink. Applied Physics Letters 93, no. 10 (2008).
Résumé: In isotropic quasiincompressible media, an expression of the elastic energy density has been developed as a function of the second, third, and fourthorder elastic constants (respectively μ, A, D). Thus the shear nonlinearity parameter ΒS depends only on these coefficients. In this letter ΒS is measured using finite amplitude plane shear waves in agargelatin based phantoms. Combining the results with recently published measurements of μ and A on the same phantoms, the fourthorder shear elastic constant D is found to be of the order of 10 kPa and thus of the same order of magnitude as μ and A. © 2008 American Institute of Physics.
MotsClés: Incompressible flow; Elastic energy density; Nonlinearity parameter; Soft solids; Elastic constants


Controlling the fluorescence lifetime of a single emitter on the nanoscale using a plasmonic superlens. FroufePérez, L. S., and R. Carminati. Physical Review B – Condensed Matter and Materials Physics 78, no. 12 (2008).
Résumé: Coupling a single dipole emitter to a metallic nanoparticle through the optical modes of a planar superlens made of lefthanded material can lead to substantial modifications of its spontaneous decay rate. We provide a quantitative study based on exact numerical simulation and show that such a scheme could allow the detection, the localization, and the control of the emitter dynamics with nanometerscale sensitivity, as well as the determination of its transition dipole orientation. © 2008 The American Physical Society.


Quantitative Assessment of Breast Lesion Viscoelasticity: Initial Clinical Results Using Supersonic Shear Imaging. Tanter, M., J. Bercoff, A. Athanasiou, T. Deffieux, J.  L. Gennisson, G. Montaldo, M. Muller, A. Tardivon, and M. Fink. Ultrasound in Medicine and Biology 34, no. 9 (2008): 1373–1386.
Résumé: Abtract. This paper presents an initial clinical evaluation of in vivo elastography for breast lesion imaging using the concept of supersonic shear imaging. This technique is based on the combination of a radiation force induced in tissue by an ultrasonic beam and an ultrafast imaging sequence capable of catching in real time the propagation of the resulting shear waves. The local shear wave velocity is recovered using a timeofflight technique and enables the 2D mapping of shear elasticity. This imaging modality is implemented on a conventional linear probe driven by a dedicated ultrafast echographic device. Consequently, it can be performed during a standard echographic examination. The clinical investigation was performed on 15 patients, which corresponded to 15 lesions (4 cases BIRADS 3, 7 cases BIRADS 4 and 4 cases BIRADS 5). The ability of the supersonic shear imaging technique to provide a quantitative and local estimation of the shear modulus of abnormalities with a millimetric resolution is illustrated on several malignant (invasive ductal and lobular carcinoma) and benign cases (fibrocystic changes and viscous cysts). In the investigated cases, malignant lesions were found to be significantly different from benign solid lesions with respect to their elasticity values. Cystic lesions have shown no shear wave propagate at all in the lesion (because shear waves do not propage in liquid). These preliminary clinical results directly demonstrate the clinical feasibility of this new elastography technique in providing quantitative assessment of relative stiffness of breast tissues. This technique of evaluating tissue elasticity gives valuable information that is complementary to the Bmode morphologic information. More extensive studies are necessary to validate the assumption that this new mode potentially helps the physician in both falsepositive and falsenegative rejection. (Email: ). © 2008 World Federation for Ultrasound in Medicine & Biology.
MotsClés: Acoustic radiation force; Breast; Cancer; Elastography; Shear wave imaging; Ultrasound; Acoustic radiation force; BIRADS; Breast; Breast lesions; Breast tissues; Cancer; Clinical evaluation; Clinical feasibility; Clinical investigation; Elasticity values; Elastography; Imaging modalities; Invivo; Linear probing; Local estimation; Malignant lesions; Quantitative assessments; Radiation force; Real time; Relative stiffness; Shear elasticity; Shear modulus; Shear wave imaging; Shear wave velocities; Supersonic shear imaging; Tissue elasticity; Ultrafast; Ultrafast imaging; Ultrasonic beams; Ultrasound; Bismuth plating; Elastic waves; Elasticity; Imaging techniques; Medical imaging; Shear waves; Standards; Ultrasonic applications; Ultrasonic testing; Ultrasonic imaging; adult; aged; article; breast cancer; clinical article; clinical assessment; clinical evaluation; controlled study; elastography; female; human; human tissue; imaging system; intermethod comparison; priority journal; shear rate; ultrasound; viscoelasticity; Adult; Aged; Breast Cyst; Breast Diseases; Breast Neoplasms; Elasticity; Elasticity Imaging Techniques; Female; Humans; Image Processing, ComputerAssisted; Middle Aged; Sensitivity and Specificity; Stress, Mechanical; Ultrasonography, Mammary; Viscosity; Young Adult


Multiple scattering by random configurations of circular cylinders: Weak scattering without closure assumptions. Martin, P. A., and A. Maurel. Wave Motion 45, no. 78 (2008): 865–880.
Résumé: Acoustic scattering by random collections of identical circular cylinders is considered. Each cylinder is penetrable, with a soundspeed that is close to that in the exterior: the scattering is said to be “weak”. Two classes of methods are used. The first is usually associated with the names of Foldy and Lax. Such methods require a “closure assumption”, in addition to the governing equations. The second class is based on iterative approximations to integral equations of LippmannSchwinger type. Such methods do not use a closure approximation. Our main result is that both approaches lead to exactly the same formulas for the effective wavenumber, correct to secondorder in scattering strength and secondorder in filling fraction. Approximations for the average wavefield are also derived and compared. © 2008 Elsevier B.V. All rights reserved.
MotsClés: Acoustic waves; Closure assumptions; LippmannSchwinger equation; Multiple scattering; Random media; Circular cylinders; Cylinders (shapes); Integral equations; Scattering; Acoustic Scattering; Acoustic waves; Closure approximation; Closure assumptions; Filling fractions; Governing equations; Iterative approximations; LippmannSchwinger equation; Multiple scattering; Random configurations; Random media; Scattering strength; Second orders; Wave numbers; Acoustic wave scattering; acoustic wave; cylinder; numerical method; theoretical study; wave field; wave scattering


Effective propagation in a perturbed periodic structure. Maurel, A., and V. Pagneux. Physical Review B – Condensed Matter and Materials Physics 78, no. 5 (2008).
Résumé: In a recent paper inspected the effective parameters of a cluster containing an ensemble of scatterers with a periodic or a weakly disordered arrangement. A small amount of disorder is shown to have a small influence on the characteristics of the acoustic wave propagation with respect to the periodic case. In this Brief Report, we inspect further the effect of a deviation in the scatterer distribution from the periodic distribution. The quasicrystalline approximation is shown to be an efficient tool to quantify this effect. An analytical formula for the effective wave number is obtained in onedimensional acoustic medium and is compared with the Berryman result in the lowfrequency limit. Direct numerical calculations show a good agreement with the analytical predictions. © 2008 The American Physical Society.


Characterization of an elastic target in a shallow water waveguide by decomposition of the timereversal operator. Philippe, F. D., C. Prada, J. De Rosny, D. Clorennec, J.  G. Minonzio, and M. Fink. Journal of the Acoustical Society of America 124, no. 2 (2008): 779–787.
Résumé: This paper reports the results of an investigation into extracting of the backscattered frequency signature of a target in a waveguide. Retrieving the target signature is difficult because it is blurred by waveguide reflections and modal interference. It is shown that the decomposition of the timereversal operator method provides a solution to this problem. Using a modal theory, this paper shows that the first singular value associated with a target is proportional to the backscattering form function. It is linked to the waveguide geometry through a factor that weakly depends on frequency as long as the target is far from the boundaries. Using the same approach, the second singular value is shown to be proportional to the second derivative of the angular form function which is a relevant parameter for target identification. Within this framework the coupling between two targets is considered. Small scale experimental studies are performed in the 3.5 MHz frequency range for 3 mm spheres in a 28 mm deep and 570 mm long waveguide and confirm the theoretical results. © 2008 Acoustical Society of America.
MotsClés: Waveguides; Experimental studies; Form function; Frequency ranging; Modal interference; Second derivatives; Shallow water waveguides; Singular values; Small scale; Target identification; Target signatures; Timereversal operator; Waveguide geometry; Targets; water; article; decomposition; elasticity; frequency analysis; geometry; priority journal; theoretical study; waveform; Acoustics; Elasticity; Models, Theoretical; Motion; Sound; Sound Spectrography; Steel; Surface Properties; Time Factors; Water


Multivariate reconstruction of functional networks from cortical sources dynamics in MEG/EEG. Dossevi, A., D. Cosmelli, L. Garnero, and H. Ammari. IEEE Transactions on Biomedical Engineering 55, no. 8 (2008): 2074–2086.
Résumé: In this paper, we present a simple method to find networks of timecorrelated brain sources, using a singular value decomposition (SVD) analysis of the source matrix estimated after any linear distributed inverse problem in magnetoencephalography (MEG) and electroencephalography (EEG). Despite the high dimension of the source space, our method allows for the rapid computation of the source matrix. In order to do this, we use the linear relationship between sensors and sources, and show that the SVD can be calculated through a simple and fast computation. We show that this method allows the estimation of one or several global networks of correlated sources without calculating a coupling coefficient between all pairs of sources. A series of simulations studies were performed to estimate the efficiency of the method. In order to illustrate the validity of this approach in experimental conditions, we used real MEG data from a visual stimulation task on one test subject and estimated, in different time windows of interest, functional networks of correlated sources. © 2006 IEEE.
MotsClés: Correlation; Electroencephalography (EEG); Functional networks; Magnetoencephalography (MEG); Multivariate analysis; Singular value decomposition (SVD); Eigenvalues and eigenfunctions; Electroencephalography; Estimation; Inverse problems; Magnetoencephalography; Matrix algebra; Brain sources; Correlated sources; Coupling coefficients; Different time windows; Experimental conditions; Fast computations; Functional networks; Global networks; High dimensions; Linear relationships; Magnetoencephalography (MEG); MEG/EEG; Simple methods; Singularvaluedecomposition (SVD); Source space; Visual stimulation task; Singular value decomposition; article; artificial neural network; biosensor; correlation analysis; data analysis; electroencephalogram; magnetoencephalography; mathematical analysis; multivariate analysis; task performance; validity; visual stimulation; Algorithms; Brain Mapping; Cerebral Cortex; Diagnosis, ComputerAssisted; Electroencephalography; Evoked Potentials, Visual; Humans; Magnetoencephalography; Multivariate Analysis; Nerve Net


In vivo brain viscoelastic properties measured by magnetic resonance elastography. Green, M. A., L. E. Bilston, and R. Sinkus. NMR in Biomedicine 21, no. 7 (2008): 755–764.
Résumé: Magnetic resonance elastography (MRE) is a noninvasive imaging technique used to visualise and quantify mechanical properties of tissue, providing information beyond what can be currently achieved with standard MR sequences and could, for instance, provide new insight into pathological processes in the brain. This study uses the MRE technique at 3 T to extract the complex shear modulus for in vivo brain tissue utilizing a full threedimensional approach to reconstruction, removing contributions of the dilatational wave by application of the curl operator. A calibrated phantom is used to benchmark the MRE measurements, and in vivo results are presented for healthy volunteers. The results provide data for in vivo brain storage modulus (G′), finding grey matter (3.1 kPa) to be significantly stiffer than white matter (2.7 kPa). The first in vivo loss modulus (G″) measurements show no significant difference between grey matter (2.5 kPa) and white matter (2.5 kPa). Copyright © 2008 John Wiley & Sons, Ltd.
MotsClés: Brain; Elastography; MRI; Viscoelastic properties; Atoms; Benchmarking; Biomechanics; Chlorine compounds; Elastic moduli; Image segmentation; Magnetic fields; Magnetic resonance; Mechanical properties; Standards; Three dimensional; Brain; Brain tissues; Complex shear modulus; Curl operators; Elastography; Grey matter; Invivo; Loss modulus; Magnetic resonance elastography; MRI; Noninvasive imaging; Pathological processes; Storage modulus; Viscoelastic properties; White matter; Resonance; article; brain tissue; calibration; controlled study; elastography; gray matter; human; human experiment; image reconstruction; in vivo study; neuroimaging; normal human; nuclear magnetic resonance imaging; operator; phantom; priority journal; quality control; three dimensional imaging; viscoelasticity; volunteer; white matter; young modulus; Adult; Brain; Calibration; Computer Simulation; Elasticity; Elasticity Imaging Techniques; Humans; Male; Middle Aged; Phantoms, Imaging; Rotation; Viscosity


Local vibration of an elastic plate and zerogroup velocity Lamb modes. Prada, C., D. Clorennec, and D. Royer. Journal of the Acoustical Society of America 124, no. 1 (2008): 203–212.
Résumé: Elastic plates or cylinders can support guided modes with zero group velocity (ZGV) at a nonzero value of the wave number. Using laserbased ultrasonic techniques, we experimentally investigate some fascinating properties of these ZGV modes: resonance and ringing effects, backward wave propagation, interference between backward and forward modes. Then, the conditions required for the existence of ZGV Lamb modes in isotropic plates are discussed. It is shown that these modes appear in a range of Poisson's ratio about the value for which the cutoff frequency curves of modes belonging to the same family intercept, i.e., for a bulk wave velocity ratio equal to a rational number. An interpretation of this phenomenon in terms of a strong repulsion between a pair of modes having a different parity in the vicinity of the cutoff frequencies is given. Experiments performed with materials of various Poisson's ratio demonstrate that the resonance spectrum of an unloaded elastic plate, locally excited by a laser pulse, is dominated by the ZGV Lamb modes. © 2008 Acoustical Society of America.
MotsClés: Alumina; Cutoff frequency; Lasers; Light velocity; Plates (structural components); Poisson distribution; Poisson equation; Pulsed laser applications; Pulsed laser deposition; Resonance; Ultrasonic testing; Acoustical Society of America (ASA); Backward wave propagation; Elastic plates; Guided modes; Isotropic plates; Lamb modes; Locally excited (LOC); Nonzero values; Rational numbers; Resonance spectrum of; Ultrasonic techniques; Wave numbers; Wave velocity (IGC:D7); Zerogroup velocity; Poisson ratio; article; dispersion; phenomenology; Poisson distribution; priority journal; spike wave; velocity; vibration


Characterization of muscle belly elastic properties during passive stretching using transient elastography. Nordez, A., J. L. Gennisson, P. Casari, S. Catheline, and C. Cornu. Journal of Biomechanics 41, no. 10 (2008): 2305–2311.
Résumé: Passive muscle stretching can be used in vivo to assess the viscoelastic properties of the entire musculoarticular complex, but does not allow the specific determination of the muscle or tendon viscoelasticity. In this respect, the local muscle hardness (LMH) of the gastrocnemius medialis (GM) belly was measured during a passive ankle stretching of 10 subjects using transient elastography. A Biodex isokinetic dynamometer was used to stretch ankle plantar flexors, to measure ankle angle, and the passive torque developed by the ankle joint in resistance to the stretch. Results show that the LMH increased during the stretching protocol, with an averaged ratio between maximal LMH and minimal LMH of 2.62±0.46. Furthermore, LMHpassive torque relationships were nicely fitted using a linear model with mean correlation coefficients (R 2) of 0.828±0.099. A good reproducibility was found for the maximal passive torque (ICC = 0.976, SEM = 2.9Nm, CV = 5.5%) and the yintercept of the LMHpassive torque relationship (ICC = 0.893, SEM = 105 Pa, CV = 7.8%). However, the reproducibility was low for the slope of this relationship (ICC = 0.631, SEM = 10.35m2, CV = 60.4%). The yintercept of the LMHpassive torque relationship was not significantly changed after 10 min of static stretching. This result confirms the finding of a previous study indicating that changes in passive torque following static stretching could be explained by an acute increase in muscle length without any changes in musculoarticular intrinsic mechanical properties. © 2008 Elsevier Ltd. All rights reserved.
MotsClés: Muscle stiffness; Passive torque; Static stretching; Transient elastography; Correlation methods; Elasticity; Mathematical models; Mechanical properties; Network protocols; Secondary emission; Tendons; Torque; Ankle angles; Ankle joints; Correlation coefficients; Elastic properties; Invivo; Isokinetic dynamometers; Linear models; Muscle hardnesses; Muscle stiffness; Passive torque; Reproducibility; Static stretching; Transient elastography; Viscoelasticity; Viscoelastic properties; Muscle; abdominal wall musculature; adult; article; dynamometer; elastography; gastrocnemius muscle; human; human experiment; human tissue; in vivo study; local muscle hardness; male; mathematical computing; muscle contractility; muscle length; muscle stretching; normal human; priority journal; reproducibility; torque; viscoelasticity; algorithm; biomechanics; elasticity; electromyography; kinesiotherapy; methodology; muscle contraction; musculoskeletal function; pathology; physiology; skeletal muscle; stretching exercise; Adult; Algorithms; Biomechanics; Elasticity; Elasticity Imaging Techniques; Electromyography; Exercise Therapy; Humans; Male; Muscle Contraction; Muscle Stretching Exercises; Muscle, Skeletal; Musculoskeletal Physiology; Reproducibility of Results; Torque


Eigenvalue distributions of correlated multichannel transfer matrices in strongly scattering systems. Sprik, R., A. Tourin, J. De Rosny, and M. Fink. Physical Review B – Condensed Matter and Materials Physics 78, no. 1 (2008).
Résumé: We experimentally study the effects of correlations in the propagation of ultrasonic waves in water from a multielement source to a multielement detector through a strongly scattering system of randomly placed vertical rods. Due to the strong scattering, the wave transport in the sample is in the diffusive regime. The correlation between the waves is induced when the distance between transducer elements is within the coherence region of the scattered sound. We measure the multichannel transfer matrix H, each element of which represents the signal strength between the m individual transmitters and n receivers. The observed eigenvalue distribution of the matrix H H† clearly shows the effect of correlations between channels and can be interpreted using random matrix theory. These results are of practical importance in many areas, e.g., for evaluating the information transfer capacity of such a complex scattering system. © 2008 The American Physical Society.


Timereversal acoustics. Fink, M. Journal of Physics: Conference Series 118, no. 1 (2008).
Résumé: Timereversal mirrors (TRMs) refocus an incident acoustic field to the position of the original source regardless of the complexity of the propagation medium. TRM's have now been implemented in a variety of physical scenarios from MHz ultrasonics with order centimeter aperture size to hundreds/thousands of Hz in ocean acoustics with order hundred meter aperture size. Common to this broad range of scales is a remarkable robustness exemplified by observations at all scales that the more complex the medium between the probe source and the TRM, the sharper the focus. The relation between the medium complexity and the size of the focal spot is studied in this paper. It is certainly the most exciting property of TRM compared to standard focusing devices. A TRM acts as an antenna that uses complex environments to appears wider than it is, resulting for a broadband pulse in a refocusing quality that does not depend of the TRM aperture. In this paper, we investigate the timereversal approach in various media of increasing complexity and we discuss the link existing between timereversal approach and local helioseismology where Green's functions can be extracted from diffusive noise. © 2008 IOP Publishing Ltd.


The adiabatic invariant of the ndegreeoffreedom harmonic oscillator. Devaud, M., V. Leroy, J.  C. Bacri, and T. Hocquet. European Journal of Physics 29, no. 4 (2008): 831–843.
Résumé: In this graduatelevel theoretical paper, we propose a general derivation of the adiabatic invariant of the ndegreeoffreedom harmonic oscillator, available whichever the physical nature of the oscillator and of the parametrical excitation it undergoes. This derivation is founded on the use of the classical Glauber variables and ends up with this simplest result: the oscillator's adiabatic invariant is just the sum of all the semiclassical quanta numbers associated with its different eigenmodes. © 2008 IOP Publishing Ltd.
MotsClés: Oscillators (mechanical); Adiabatic invariants; Eigenmodes; General (CO); Graduate levels; harmonic oscillators; physical nature; Semiclassical (SC); Oscillators (electronic)


Fluctuations of correlations and Green's function reconstruction: Role of scattering. Larose, E., P. Roux, M. Campillo, and A. Derode. Journal of Applied Physics 103, no. 11 (2008).
Résumé: Correlations of ambient seismic or acoustic vibrations are now widely used to reconstruct the impulse response between two passive receivers as if a source was placed at one of them. This provides the opportunity to do imaging without a source, or passive imaging. Applications include terrestrial and solar seismology, underwater acoustics, and structural health monitoring, to cite only a few. Nevertheless, for a given set of data, correlations do not only yield Green's function between the sensors. They also contain residual fluctuations that result from an imperfect time or source averaging that might eventually blur the images. In this article, we propose a heuristic model to describe the level of fluctuations of the correlations in the case of nonstationary wavefields, and more particularly in the case of scattering media. The work includes theoretical derivations and numerical simulations. The role of multiple scattering is quantitatively evaluated. The level of fluctuations decreases when the duration and intensity of the diffuse waves increase. The role of absorption is also discussed: absorption is properly retrieved by correlation, but the level of fluctuations is greater, thus degrading Green's function reconstruction. Discrepancies of our simple model in the case of strong multiple scattering (k * ≤18) are discussed. © 2008 American Institute of Physics.
MotsClés: Absorption; Acoustic receivers; Acoustics; Computer simulation; Correlation methods; Differential equations; Functions; Health; Impulse response; Mathematical models; Multiple scattering; Photoacoustic effect; Repair; Restoration; Scattering; Seismology; Set theory; Structural health monitoring; Underwater acoustics; Underwater audition; Acoustic vibrations; American Institute of Physics (AIP); Diffuse waves; Function reconstruction; Healthmonitoring; Heuristic models; Non stationary; Numerical simulations; passive imaging; Passive receivers; Residual fluctuations; Scattering medium; Simple modeling; Wavefields; Green's function


Transcostal highintensityfocused ultrasound: Ex vivo adaptive focusing feasibility study. Aubry, J.  F., M. Pernot, F. Marquet, M. Tanter, and M. Fink. Physics in Medicine and Biology 53, no. 11 (2008): 2937–2951.
Résumé: Ex vivo experiments have been conducted through excised pork rib with bone, cartilage, muscle and skin. The aberrating effect of the ribcage has been experimentally evaluated. Adaptive ultrasonic focusing through ribs has been studied at low power. Without any correction, the pressure fields in the focal plane were both affected by 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 of the half width of the main lobe and up to 20 dB increase of the secondary lobe level. Thanks to timereversal focusing, a 5 dB decrease in the secondary lobes was obtained and the ratio between the energy deposited at the target location and the total amount of energy emitted by the therapeutic array was six times higher than that without correction. Timereversal minimizes the heating of the ribs by automatically sonicating between the ribs, as demonstrated by temperature measurements using thermocouples placed at different locations on the ribcage. It is also discussed how this aberration correction process could be achieved noninvasively for clinical application. © 2008 Institute of Physics and Engineering in Medicine.
MotsClés: Decision making; Drug therapy; Focusing; Health; Heating; Photoacoustic effect; Planning; Resource allocation; Technology; Temperature measurement; Tissue engineering; Ultrasonics; (e ,3e) process; (p ,p ,t) measurements; Aberration corrections; Adaptive focusing; Clinical applications; Energy deposited; Exvivo; Feasibility studies; Focal planes; Focused ultrasound (FUS); half widths; Inhomogeneous attenuation; Institute of Physics; low powers; Main effects; phase distortions; Pressure fields; target location; Time reversal (TR); Time reversal focusing; Total amount; Location; article; controlled study; energy absorption; high intensity focused ultrasound; in vivo study; mathematical model; nonhuman; pork; priority journal; rib; temperature measurement; transducer; Animals; Artifacts; Cartilage; Computer Simulation; Feasibility Studies; Muscles; Ribs; Skin; Swine; Ultrasonic Therapy


Power law decay of zero group velocity Lamb modes. Prada, C., D. Clorennec, and D. Royer. Wave Motion 45, no. 6 (2008): 723–728.
Résumé: Elastic plates or cylinders can support guided modes with zero group velocity (ZGV) for nonzero wave numbers. At these ZGVpoints of the dispersion curves the acoustic energy does not propagate in the waveguide, resulting in sharp resonance effects. In this paper, using laserbased ultrasonic techniques, we investigate the timedecay of the mechanical displacement for ZGV Lamb modes excited by a pulsed laser in various thin metallic plates. In the first microseconds of the local plate vibration, we observed a t 1 / 2 decay due to the second order term in the dispersion relation. This effect is dominant because the first order term, proportional to the group velocity, vanishes for ZGVmodes. After this power law decay, the mechanical displacement undergoes an exponential decay corresponding to the wave damping. Then, the local attenuation of the plate material can be estimated at the ZGVresonance frequency. © 2008.
MotsClés: Lamb modes; Time decay; Zero group velocity; Attenuation equalizers; Damping; Estimation; Natural frequencies; Wave equations; Elastic plates; Laserbased ultrasonic techniques; Mechanical displacement; Wave damping; Zero group velocity (ZGV); Elastic waves; Attenuation equalizers; Damping; Elastic waves; Estimation; Natural frequencies; Wave equations; damping; estimation method; laser method; power law; resonance; ultrasonics; wave attenuation


Lifetime fluctuations of a single emitter in a disordered nanoscopic system: The influence of the transition dipole orientation. FroufePérez, L. S., and R. Carminati. Physica Status Solidi (A) Applications and Materials Science 205, no. 6 (2008): 1258–1265.
Résumé: We study the fluctuations of the fluorescence decay rate of a single emitter in a random cluster of nanoparticles, in a regime dominated by nearfield scattering. Configurational changes of the environment induce statistical changes of the decay rate. Two regimes are considered which differ in terms of transition dipole orientation. In one regime, the orientation of the transition dipole is assumed to remain constant while the configuration of the cluster changes randomly. In another regime, the orientation of the transition dipole is assumed unknown and continuously averaged over the three directions of space. Using exact numerical simulations and a simple analytical model, we show that the statistical distributions of the spontaneous decay rate are substantially different in both regimes. In both cases, the decay rate fluctuations are strongly dependent on the level of absorption at the nanoscale. We discuss the impact of this result in terms of imaging in complex media. © 2008 WILEYVCH Verlag GmbH & Co. KGaA.
MotsClés: Analytical models; Complex medias; Decay rates; Fluorescence decay rates; Imaging; Nanoscale; Nanoscopic systems; Nearfield; Numerical simulations; Random clusters; Spontaneous decay rates; Statistical distributions; Three directions; Transition dipoles; Absorption; Computer simulation; Molecular vibrations; Nanotechnology; Optical waveguides; Statistical methods; Decay (organic)


MR elastography of liver fibrosis: Preliminary results comparing spinecho and echoplanar imaging. Huwart, L., N. Salameh, L. ter Beek, E. Vicaut, F. Peeters, R. Sinkus, and B. E. Van Beers. European Radiology 18, no. 11 (2008): 2535–2541.
Résumé: The purpose of this study was to prospectively compare the performance of magnetic resonance (MR) elastography using echoplanar and spinecho imaging for staging of hepatic fibrosis. Twentyfour patients who had liver biopsy for suspicion of chronic liver disease had MR elastography performed with both spinecho and echoplanar sequences. At histology, the fibrosis stage was assessed according to METAVIR. The data acquisition time was about 20 min using spinecho, and only 2 min using echoplanar imaging. The hepatic signaltonoise ratios were similar on both images (22.51±5.37 for spinecho versus 21.02 ± 4.76 for echoplanar, p = 0.33). The elasticity measurements and the fibrosis stages were strongly correlated. The Spearman correlation coefficients were r=0.91 (p < 0.01) with spinecho and r = 0.84 (p < 0.01) with echoplanar sequences. These correlation coefficients did not differ significantly (p = 0.17). A strong correlation was also observed between spinecho and echoplanar elasticity (r = 0.83, p < 0.001), without systematic bias. The results of our study showed that echoplanar imaging substantially decreased the data acquisition time of MR elastography, while maintaining the image quality and diagnostic performance for staging of liver fibrosis. This suggests that echoplanar MR elastography could replace spinecho MR elastography in clinical practice. © European Society of Radiology 2008.
MotsClés: Echoplanar imaging; Elasticity; Fibrosis; Liver cirrhosis; Magnetic resonance imaging; adult; aged; article; chronic liver disease; clinical article; controlled study; correlation coefficient; correlation function; diagnostic accuracy; diagnostic value; differential diagnosis; echo planar imaging; elastography; electron spin resonance; female; histopathology; human; human tissue; image display; image quality; intermethod comparison; liver biopsy; liver fibrosis; magnetic resonance elastography; male; priority journal; signal noise ratio; staging; Adult; Aged; Cyclic NOxides; EchoPlanar Imaging; Elasticity Imaging Techniques; Female; Humans; Liver Cirrhosis; Male; Middle Aged; Pilot Projects; Reproducibility of Results; Sensitivity and Specificity


Optical properties of photodetectors based on wurtzite quantum dot arrays. Tronc, P., K. S. Zhuravlev, V. G. Mansurov, G. F. Karavaev, S. N. Grinyaev, I. Milosevic, and M. Damnjanovic. Physical Review B – Condensed Matter and Materials Physics 77, no. 16 (2008).
Résumé: We show that two types of wurtzite quantum dots can be grown with the C3v symmetry. Their symmetry axis coincides with a threefold proper rotation and a 63 improper rotation axis of the wurtzite lattice, respectively. One, two, and threedimensional periodic structures made of such dots are considered. Most of them have the C3v point symmetry except some one and threedimensional structures that have the C6v one. The symmetry planes and possible glide planes for the dots and dot structures are those of the wurtzite matrix. It is shown that, under incident light propagating along the growth direction, the absorption is strong for valencetoconductionband transitions but, on the contrary, the absorption is weak in infrared detectors based on transitions within the conduction band. It makes it necessary to draw a grating to change the light direction within the device (as is also the case for infrared detectors based on wurtzite quantum wells or superlattices). © 2008 The American Physical Society.


Intracavity nearfield optical imaging of a midinfrared quantum cascade laser mode. Lemoine, P.  A., V. Moreau, M. Bahriz, Y. De Wilde, R. Colombelli, L. R. Wilson, and A. B. Krysa. Materials Science and Engineering B: SolidState Materials for Advanced Technology 149, no. 3 (2008): 270–274.
Résumé: We report the direct imaging of FabryPérot standing waves inside the cavity of a midinfrared quantum cascade laser via apertureless scanning nearfield optical microscopy. The quantum cascade devices employed present an evanescent wave at the top surface, whose magnitude is directly proportional to the cavity mode intensity in the device core region. Apertureless scanning nearfield optical microscopy measurements provide experimental results about the nature of this evanescent field in good agreement with calculations (effective index and electric field decay length). © 2007 Elsevier B.V. All rights reserved.
MotsClés: Evanescent fields; Nearfield microscopy; Quantum cascade lasers; Semiconductor lasers; SNOM; Electric fields; FabryPerot interferometers; Near field scanning optical microscopy; Semiconductor lasers; Standing wave meters; Apertureless scanning; Electric field decay length; Evanescent fields; Quantum cascade devices; Quantum cascade lasers


Characterization of an elastic cylinder and an elastic sphere with the timereversal operator: Application to the subresolution limit. Minonzio, J.  G., F. D. Philippe, C. Prada, and M. Fink. Inverse Problems 24, no. 2 (2008).
Résumé: The Décomposition de l'Opérateur de Retournement Temporel method applies to scattering analysis with arrays of transducers. It comprises the study of the timereversal invariants which correspond to the eigenvectors of the timereversal operator or to the singular vectors of the array response matrix K. In this paper, the decomposition of the scattered pressure into normal modes of vibrations is used to determine the theoretical timereversal invariants for a large elastic object such as a cylinder or sphere. For an N transducers onedimensional array, the timereversal operator is dimension N. It is shown that the dimension is reduced to 2k0a for a cylinder or a sphere, where a is the scatterer radius and k0 is the wave number in the surrounding fluid. Furthermore, this approach provides analytical expressions of symmetric and antisymmetric singular values and vectors in the subresolution limit, i.e. when the scatterer diameter is smaller than the array resolution cell. These results are verified experimentally and in good agreement with the original point of view: for a small scatterer, one singular value dominates and the associated singular vector focuses isotropically on the scatterer. © 2008 IOP Publishing Ltd.
MotsClés: Eigenvalues and eigenfunctions; Electromagnetic wave scattering; Matrix algebra; Transducers; Vectors; Elastic cylinders; Elastic sphere; Mathematical operators


Temperaturedependent diffusing acoustic wave spectroscopy with resonant scatterers. Leroy, V., and A. Derode. Physical Review E – Statistical, Nonlinear, and Soft Matter Physics 77, no. 3 (2008).
Résumé: The influence of a slight temperature change on the correlation of multiply scattered acoustic waves is studied, and experimental results are discussed. The technique presented here, similar to diffusingacousticwave spectroscopy, is based on the sensitivity of a multiply scattering medium to a slight change. Ultrasonic waves around 3 MHz are transmitted through a sample made of steel rods in water and recorded by an array of transducers at different temperatures. The cross correlations between highly scattered signals are computed. The main effect of the temperature change is a simple dilation of the times of arrival, due to a change of the sound velocity in water. But the scatterers also play a role in the progressive decorrelation of wave forms. An analysis resolved in both time and frequency shows that at some particular frequencies, the resonant behavior of the scatterers is responsible for a significantly larger decorrelation. Interestingly, the experimental results allow one to detect the presence of a small resonance that was not detected earlier on the same scatterers with classical measurement of the scattering mean free path. A simple model is proposed to interpret the experimental results. © 2008 The American Physical Society.
MotsClés: Acoustic spectroscopy; Scattering; Ultrasonic waves; Acoustic wave spectroscopy; Resonant scatterers; Steel rods; Acoustic wave effects


Potential of MRI and ultrasound radiation force in elastography: Applications to diagnosis and therapy. Sinkus, R., M. Tanter, J. Bercoff, K. Siegmann, M. Pernot, A. Athanasiou, and M. Fink. Proceedings of the IEEE 96, no. 3 (2008): 490–499.
Résumé: Elastography has many exciting new areas of application in the domains of diagnosis and therapy. We present in this overview the current gold standard given by MR elastography, which uses a full threedimensional approach to solve locally for the unknown complex shear modulus at one frequency. Clinical results for benign and malignant breast lesions are shown. Less rigorous in terms of data completeness, but significantly faster and easier to apply, we introduce the ultrasoundbased supersonic shear imaging technique, which uses acoustic radiation force to generate inside the medium planar shear waves. Subsequent ultrafast imaging of the propagating shear wave allows one to recuperate detailed timeofflight maps of invivo breast lesions. Lastly, we present initial results for using magnetic resonance imaging and acoustic radiation force together for highintensity focused ultrasound interventions. © 2006 IEEE.
MotsClés: Acoustic radiation force; Elastography; Highintensity focused ultrasound (HIFU); Magnetic resonance imaging (MRI); Supersonic shear imaging; Ultrasound; Acoustic radiation force; Elastography; Highintensity focused ultrasound (HIFU); Supersonic shear imaging; Ultrasound; Acoustic emissions; Acoustic radiators; Acoustic wave propagation; Gold compounds; Magnetic resonance; Magnetic resonance imaging; Shear waves; Sonochemistry; Three dimensional; Ultrasonic applications; Ultrasonic transmission; Ultrasonics; Ultrasonic imaging


A method of biological tissues elasticity reconstruction using magnetic resonance elastography measurements. Ammari, H., P. Garapon, H. Kang, and H. Lee. Quarterly of Applied Mathematics 66, no. 1 (2008): 139–175.
Résumé: Magnetic resonance elastography (MRE) is an approach to measuring material properties using external vibration in which the internal displacement measurements are made with magnetic resonance. A variety of simple methods have been designed to recover mechanical properties by inverting the displacement data. Currently, the remaining problems with all of these methods are that, in general, the homogeneous Helmholtz equation is used and therefore it fails at interfaces between tissues of different properties. The purpose of this work is to propose a new method for reconstructing both the shape and the shear modulus of a small anomaly with Lamé parameters different from the background ones using internal displacement measurements. © 2007 Brown University.
MotsClés: Elastic imaging; Layer potentials; Level set method; Quasiincompressible elasticity; Reconstruction; Small volume asymptotic expansions; Stokes system; Atoms; Biomechanics; Difference equations; Differentiation (calculus); Helmholtz equation; Histology; Ketones; Magnetic fields; Magnetic materials; Magnetic resonance; Magnetism; Mechanical properties; (p ,p ,t) measurements; (PL) properties; Biological tissues; Brown University; Displacement measurements; elasticity reconstruction; External vibrations; General (CO); Helmholtz; Magnetic (CE); Magnetic resonance elastography (MRE); mate rial properties; New methods; Shear modulus; simple methods; Resonance


Symmetry of wurtzite nanostructures with the c axis in the layer plane. Tronc, P., and P. Vennéguès. Physical Review B – Condensed Matter and Materials Physics 77, no. 7 (2008).
Résumé: In wurtzitebased quantum wells and superlattices with the c axis parallel to the layer plane, the plane is parallel to either a symmetry plane of the wurtzite lattice (type I structures, the □ 1120 □ growth direction) or a glide plane parallel to the c axis (type II structures, the □ 1010 □ growth direction). We show that, in both cases, the space symmetry of the structure depends on the parity of the number of monolayers within the slab(s). The point symmetry is C2v except for the type II structures with odd number(s) of monolayers. The latter structures have the σv point symmetry and can present a builtin electric field. Quite different selection rules, depending on the structure symmetry, govern electron optical transitions and exciton radiative recombination as well as first and secondorder infrared absorption. The intensities of the various dipolar, infrared, and Raman lines are discussed. The effect of applied magnetic and electric fields is presented. A simple optical test is proposed to distinguish the structures with the σv point symmetry and those with the C2v one. © 2008 The American Physical Society.


Time reversal of elastic waves in soft solids. Catheline, S., N. Benech, J. Brum, and C. Negreira. Physical Review Letters 100, no. 6 (2008).
Résumé: When a scalar farfield wave is time reversed, it starts to converge toward its initial point source location, then collapses and finally diverges. Without evanescent waves, the symmetric focus spot is limited by the Rayleigh criterion. We present an experimental observation of a timereversal elastic wave in a soft solid cavity using the transient elastography technique. It is observed that the timereversed far field wave collapses and gives birth to near fieldlike effects. Elastodynamic Green's functions computation confirms the experimental conclusions: the diffraction limit implies a direction dependant Rayleigh criterion. © 2008 The American Physical Society.
MotsClés: Diffraction efficiency; Green's function; Rayleigh scattering; Evanescent waves; Soft solid cavity; Transient elastography technique; Elastic waves


Green's function estimation in speckle using the decomposition of the time reversal operator: Application to aberration correction in medical imaging. Robert, J.  L., and M. Fink. Journal of the Acoustical Society of America 123, no. 2 (2008): 866–877.
Résumé: The FDORT method (French acronym for decomposition of the time reversal operator using focused beams) is a time reversal based method that can detect point scatterers in a heterogeneous medium and extract their Green's function. It is particularly useful when focusing in a heterogeneous medium. This paper generalizes the theory of the FDORT method to random media (speckle), and shows that it is possible to extract Green's functions from the speckle signal using this method. Therefore it is possible to achieve a good focusing even if no point scatterers are present. Moreover, a link is made between FDORT and the Van CittertZernike theorem. It is deduced from this interpretation that the normalized first eigenvalue of the focused time reversal operator is a wellknown focusing criterion. The concept of an equivalent virtual object is introduced that allows the random problem to be replaced by an equivalent deterministic problem and leads to an intuitive understanding of FDORT in speckle. Applications to aberration correction are presented. The reduction of the variance of the Green's function estimate is discussed. Finally, it is shown that the method works well in the presence of strong interfering scatterers. © 2008 Acoustical Society of America.
MotsClés: Aberrations; Medical imaging; Problem solving; Speckle; Theorem proving; FDORT method; Time reversal operators; Green's function; article; decomposition; imaging system; mathematical model; methodology; priority journal; radiation scattering; signal detection; theory; Algorithms; Computer Simulation; Fourier Analysis; Humans; Image Enhancement; Interferometry; Models, Theoretical; Phantoms, Imaging; Rubber; Ultrasonography; UserComputer Interface


Stresses and displacements for some Rayleightype surface acoustic waves propagating on an anisotropic half space (L). Royer, D. Journal of the Acoustical Society of America 123, no. 2 (2008): 599–601.
Résumé: Specific relations between mechanical displacements and stresses for Rayleightype surface acoustic waves propagating on an anisotropic half space are demonstrated. For 16 symmetry configurations belonging to the orthorhombic, tetragonal, hexagonal and cubic systems, involving only two displacement and stress components, it is shown that the ratio between the shear and normal stresses inside the propagation media is equal to the ratio between the normal and inplane displacement components at the free surface. This result generalizes the previous one obtained in the case of an isotropic solid [W. Hassan and P. B. Nagy, J. Acoust. Soc. Am. 104, 31073110 (1998)]. © 2008 Acoustical Society of America.
MotsClés: Acoustic surface wave devices; Anisotropy; Mechanical variables measurement; Rayleigh scattering; Stress analysis; Anisotropic half spaces; Isotropic solids; Mechanical displacements; Acoustic wave propagation; acoustics; article; mechanical stress; priority journal; shear stress; sound; stress; surface property


Characterization of thermal conductivity degradation induced by heavy ion irradiation in ceramic materials. David, L., S. Gomès, G. Carlot, J.  P. Roger, D. Fournier, C. Valot, and M. Raynaud. Journal of Physics D: Applied Physics 41, no. 3 (2008).
Résumé: Thermal conductivity degradation of three semimetallic ceramics: titanium carbide, zirconium carbide and titanium nitride, and a covalent compound: 6H silicon carbide, induced by irradiation with energetic heavy ions at room temperature, is studied and quantified. Irradiations by 25.8 MeV krypton ions at 10 16 and 6 × 10 16 ions cm 2 doses were used to produce defects in the considered materials. Modulated thermoreflectance microscopy measurements were performed to characterize the resulting subsurfasic degradation of the thermal conductivity for each of the investigated materials. The study considers the two collision domains produced by the inelastic collisions and the elastic collisions that occur during an ion irradiation. A significant thermal conductivity degradation in the two collision domains for all materials is obtained. Elastic collisions are shown to degrade the thermal properties more strongly than the inelastic ones. The scattering of thermal energy carriers is larger in the elastic collision domain because displacement cascades produce a very high concentration of point defects. The degradation coming from electronic interactions is found to be more important in SiC, which can be explained by the presence of large populations of generated extended defects, facing generated individual point defects in the studied semimetallic materials. © 2008 IOP Publishing Ltd.
MotsClés: Ceramic materials; Degradation; Thermal conductivity; Titanium nitride; Heavy ion irradiation; Thermoreflectance microscopy measurements; Zirconium carbide; Ion bombardment


A MUSICtype algorithm for detecting internal corrosion from electrostatic boundary measurements. Ammari, H., H. Kang, E. Kim, K. Louati, and M. S. Vogelius. Numerische Mathematik 108, no. 4 (2008): 501–528.
Résumé: We establish an asymptotic representation formula for the steady state current perturbations caused by internal corrosive boundary parts of small surface measure. Based on this formula we design a noniterative method of MUSIC (multiple signal classification) type for localizing the corrosive parts from voltagetocurrent observations. We perform numerical experiments to test the viability of the algorithm and the results clearly demonstrate that the algorithm works well even in the presence of relatively high noise ratios. © 2007 SpringerVerlag.


Thermal conductivity degradation induced by heavy ion irradiation at room temperature in ceramic materials. Gomès, S., L. David, J.  P. Roger, G. Carlot, D. Fournier, C. Valot, and M. Raynaud. European Physical Journal: Special Topics 153, no. 1 (2008): 87–90.
Résumé: The thermal conductivity degradation induced by irradiation with energetic heavy ions at room temperature is studied and quantified. Three semimetallic systems: titanium and zirconium carbides, titanium nitride, as well as a covalent compound: 6H silicon carbide were irradiated by 25.8 MeV krypton ions at 10 16 and 6. 10 16 ions.cm 2 doses to produce defects. During ion irradiation, inelastic collisions and elastic collisions occur at a different depth in a material. Two collision domains can be defined. Modulated thermoreflectance microscopy measurements were performed at differing frequencies to characterize the thermal conductivity degradation in these two domains for each of the investigated materials. Our results reveal a significant thermal conductivity degradation in the two collision domains for all materials. Elastic collisions are shown to degrade more strongly the thermal properties than inelastic ones. Scattering of thermal energy carriers is larger in elastic collision domain because displacement cascades produce a very high concentration of point defects: vacancies, interstitials and implanted Kr ions. The degradation coming from electronic interactions that seems to be more important in SiC can be explained by the presence of large populations of generated extended defects, facing to generated individual point defects in TiC, TiN or ZrC. © EDP Sciences/Societé Italiana di Fisica/SpringerVerlag 2008.

