Source: https://www.institut-langevin.espci.fr/journal_articles_2012?lang=fr
Timestamp: 2019-04-20 06:11:24+00:00

Document:
Photoacoustic-guided ultrasound therapy with a dual-mode ultrasound array.
Prost, A., A. Funke, M. Tanter, J. F. Aubry, and E. Bossy.
Journal of Biomedical Optics 17, no. 6 (2012).
Résumé: Photoacoustics has recently been proposed as a potential method to guide and/or monitor therapy based on high-intensity focused ultrasound (HIFU). We experimentally demonstrate the creation of a HIFU lesion at the location of an optical absorber, by use of photoacoustic signals emitted by the absorber detected on a dual mode transducer array. To do so, a dedicated ultrasound array intended to both detect photoacoustic waves and emit HIFU with the same elements was used. Such a dual-mode array provides automatically coregistered reference frames for photoacoustic detection and HIFU emission, a highly desired feature for methods involving guidance or monitoring of HIFU by use of photoacoustics. The prototype is first characterized in terms of both photoacoustic and HIFU performances. The probe is then used to perform an idealized scenario of photoacoustic-guided therapy, where photoacoustic signals generated by an absorbing thread embedded in a piece of chicken breast are used to automatically refocus a HIFU beam with a time-reversal mirror and necrose the tissue at the location of the absorber. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE).
Ammari, H., E. Bossy, J. Garnier, and L. Seppecher.
SIAM Journal on Applied Mathematics 72, no. 5 (2012): 1592–1617.
Résumé: The aim of this paper is to develop a mathematical framework for acousto-electromagnetic tomography and to introduce an efficient reconstruction algorithm. In electromagnetic wave imaging, the resolution is limited by the Rayleigh criterion, that is, half the operating wavelength. By mechanically perturbing the medium, we show that it is possible to achieve a significant resolution enhancement. We provide a new inversion formula for the permittivity distribution from crosscorrelations between the electromagnetic boundary measurements in the perturbed medium and those in the unperturbed one. We present numerical results to illustrate the resolution and the stability performances of the proposed reconstruction algorithm. © 2012 Society for Industrial and Applied Mathematics.
Spatiotemporal focusing in opaque scattering media by wave front shaping with nonlinear feedback.
Aulbach, J., B. Gjonaj, P. Johnson, and A. Lagendijk.
Optics Express 20, no. 28 (2012): 29237–29251.
Résumé: We experimentally demonstrate spatiotemporal focusing of light on single nanocrystals embedded inside a strongly scattering medium. Our approach is based on spatial wave front shaping of short pulses, using second harmonic generation inside the target nanocrystals as the feedback signal. We successfully develop a model both for the achieved pulse duration as well as the observed enhancement of the feedback signal. The approach enables exciting opportunities for studies of light propagation in the presence of strong scattering as well as for applications in imaging, micro- and nanomanipulation, coherent control and spectroscopy in complex media. © 2012 Optical Society of America.
Spreading dynamics of drop impacts.
Lagubeau, G., M. A. Fontelos, C. Josserand, A. Maurel, V. Pagneux, and P. Petitjeans.
Journal of Fluid Mechanics 713 (2012): 50–60.
Résumé: We present an experimental study of drop impact on a solid surface in the spreading regime with no splashing. Using the space-time-resolved Fourier transform profilometry technique, we can follow the evolution of the drop shape during the impact. We show that a self-similar dynamical regime drives the drop spreading until the growth of a viscous boundary layer from the substrate selects a residual minimal film thickness. Finally, we discuss the interplay between capillary and viscous effects in the spreading dynamics, which suggests a pertinent impact parameter. © 2012 Cambridge University Press.
Ultrasound contrast plane wave imaging.
Couture, O., M. Fink, and M. Tanter.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 59, no. 12 (2012): 2676–2683.
Résumé: Background: Monitoring the accumulation of microbubbles within tissue vasculature with ultrasound allows both molecular and perfusion imaging. Unfortunately, conventional imaging with focused pulses can destroy a large fraction of the microbubbles it is trying to follow. Using coherent synthetic summation, ultrafast plane wave imaging could attain similar image quality, while reducing the peak acoustic pressure and bubble disruption. Method: In these experiments, microbubbles were flowed in a wall-less vessel phantom. Images were obtained on a programmable clinical scanner with a set of line-per-line focused pulses for conventional contrast imaging and with compounded plane wave transmission adapted for nonlinear imaging. Imaging was performed between 14 and 650 kPa peak negative pressure at 7.5 MHz. The disruption of the microbubbles was evaluated by comparing the microbubble intensity before and after acquisition of a set of 100 images at various pressures. Results: The acoustic intensity required to disrupt 50% of the microbubbles was 24 times higher with plane-wave imaging compared with conventional focused pulses. Although both imaging approaches yield similar resolution, at the same disruption level, plane-wave imaging showed better contrast. In particular, at similar disruption ratio (50% after 100 images), contrast-pulse sequencing (CPS) performed with plane waves displayed an improvement of 11 dB compared with conventional nonlinear imaging. Conclusion: In each resolution cell of the image, plane-wave imaging spread the spatial peak acoustic intensity over more pulses, reducing the peak pressure and, hence, preserving the microbubbles. This method could contribute to molecular imaging by allowing the continuous monitoring of the accumulation of microbubbles with improved contrast. © 2012 IEEE.
Perrard, S., Y. Couder, E. Fort, and L. Limat.
EPL 100, no. 5 (2012).
Résumé: Levitating a liquid over a vapor film was limited to droplets. Here we show that on curved substrates a larger quantity of fluid can be suspended. This opens a new possibility for exploring free-liquid-surface phenomena without any contact with a solid. In one of the simplest possible situations, a large fluid torus is levitated over a circular trough. A poloidal flow inside the ring generates a wave on its inner side, making it polygonal. This wave is described by a solitonic model which balances surface tension and the pressure depletion due to the distortion of the poloidal flow. © Copyright EPLA, 2012.
Shear Wave Elastography Quantification of Blood Elasticity During Clotting.
Bernal, M., J. - L. Gennisson, P. Flaud, and M. Tanter.
Ultrasound in Medicine and Biology 38, no. 12 (2012): 2218–2228.
Résumé: Deep venous thrombosis (DVT) affects millions of people worldwide. A fatal complication occurs when the thrombi detach and create a pulmonary embolism. The diagnosis and treatment of DVT depends on clot's age. The elasticity of thrombi is closely related to its age. Blood was collected from pigs and anticoagulated using ethylenediaminetetraacetic acid (EDTA). Coagulation was initiated using calcium ions. Supersonic shear wave imaging was used to generate shear waves using 100 μs tone bursts of 8 MHz. Tracking of the shear waves was done by ultrafast imaging. Postprocessing of the data was done using Matlab®. Two-dimensional (2-D) maps of elasticity were obtained by calculating the speed of shear wave propagation. Elasticity varied with time from around 50 Pa at coagulation to 1600 Pa at 120 min after which the elasticity showed a natural decreased (17%) because of thrombolytic action of plasmin. Ejection of the serum from the clot showed a significant decrease in the elasticity of the clot next to the liquid pool (65% decrease), corresponding to the detachment of the clot from the beaker wall. The use of a thrombolytic agent (Urokinase) on the coagulated blood decreased the shear elasticity close to the point of injection, which varied with time and distance. Supersonic imaging proved to be useful mapping the 2-D clot's elasticity. It allowed the visualization of the heterogeneity of mechanical properties of thrombi and has potential use in predicting thrombi breakage as well as in monitoring thrombolytic therapy. © 2012 World Federation for Ultrasound in Medicine & Biology.
The variance of quantitative estimates in shear wave imaging: Theory and experiments.
Deffieux, T., J. - L. Gennisson, B. Larrat, M. Fink, and M. Tanter.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 59, no. 11 (2012): 2390–2410.
Résumé: In this paper, we investigate the relationship between the estimated shear modulus produced in shear wave imaging and the acquisition parameters. Using the framework of estimation theory and the Cramer¿Rao lower bound applied both to the estimation of the velocity field variance and to the estimation of the shear wave travel time, we can derive the analytical formulation of the shear modulus variance Σ2μ using relevant physical parameters such as the shear wave frequency, bandwidth, and ultrasonic parameters. This variance corresponds to the reproducibility of shear modulus reconstruction for a deterministic, quasi-homogeneous, and purely elastic medium. We thus consider the shear wave propagation as a deterministic process which is then corrupted during its observation by electronic noise and speckle decorrelation caused by shearing. A good correlation was found between analytical, numerical, and experimental results, which indicates that this formulation is well suited to understand the parameters¿ influence in those cases. The analytical formula stresses the importance of high-frequency and wideband shear waves for good estimation. Stiffer media are more difficult to assess reliably with identical acquisition signal-to-noise ratios, and a tradeoff between the reconstruction resolution of the shear modulus maps and the shear modulus variance is demonstrated. We then propose to use this formulation as a physical ground for a pixel-based quality measure that could be helpful for improving the reconstruction of real-time shear modulus maps for clinical applications. © 2012 IEEE.
Time-reversal method and cross-correlation techniques by normal mode theory: A three-point problem.
Montagner, J. - P., C. Larmat, Y. Capdeville, M. Fink, H. Phung, B. Romanowicz, E. Clévédé, and H. Kawakatsu.
Geophysical Journal International 191, no. 2 (2012): 637–652.
Résumé: Since its beginning in acoustics, the Time-Reversal method (hereafter referred as TR) has been explored by different studies to locate and characterize seismic sources in elastic media. But few authors have proposed an analytical analysis of the method, especially in the case of an elastic medium and for a finite body such as the Earth. In this paper, we use a normal mode approach (for general 3-D case and degenerate modes in 1-D reference model) to investigate the convergence properties of the TR method. We first investigate a three-point problem, with two fixed points which are the source and the receiver and a third one corresponding to a changing observation point. We extend the problem of a single channel TR experiment to a multiple channel and multiple station TR experiment. We show as well how this problem relates to the retrieval of Green's function with a multiple source cross-correlation and also the differences between TR method and cross-correlation techniques. Since most of the noise sources are located close to the surface of the Earth, we show that the time derivative of the cross-correlation of long-period seismograms with multiple sources at the surface is different from the Green's function. Next, we show the importance of a correct surface-area weighting of the signal resent by the stations according to a Voronoi tessellation of the Earth surface. We use arguments based on the stationary phase approximation to argue that phase-information is more important than amplitude information for getting a good focusing in TR experiment. Finally, by using linear relationships between the time-reversed displacement (resp. strain wavefields) and the components of a vector force source (resp. a moment tensor source), we show how to retrieve force (or moment tensor components) of any long period tectonic or environmental sources by time reversal. © 2012 The Authors Geophysical Journal International © 2012 RAS.
Electric near field measurements and electromagnetic simulations of a bianisotropic metamaterial composed of split ring resonator (SRR) particles.
Talleb, H., Z. E. Djeffal, D. Lautru, A. Ourir, and V. F. Hanna.
Applied Physics A: Materials Science and Processing 109, no. 3 (2012): 693–698.
Résumé: In this paper, we confirm that a structure composed only of split ring resonator (SRR) particles can produce backward waves. We present numerical and experimental results for electric near fields that give rise to the backward-wave behaviour. This phenomenon is expressed by an effective wavelength λeff higher than λ. This structure behaves as a bianisotropic metamaterial when the SRRs reduce simultaneously to electric and magnetic dipoles. We show that the produced asymmetric current distribution in such configuration leads to a backward-wave behaviour. An extraction method based on a homogenization procedure shows a good concordance of the index of refraction between the experimental and simulation results. This method is used to quantify the magnetoelectric parameter. © Springer-Verlag Berlin Heidelberg 2012.
Surface waves radiation by finite arrays of magnetoelectric resonators.
Jouvaud, C., A. Ourir, and J. de Rosny.
Progress in Electromagnetics Research 132 (2012): 177–198.
Résumé: We study the propagation of waves on infinite and finite size arrays made of subwavelength magnetoelectric resonators. We propose an analytical study where each magnetoelectric resonator is modelled simultaneously by an electric and a magnetic dipole. We show how near field coupling and wavenumber quantification due to the finite size of the structure induce a frequency splitting of the resonator fundamental mode.We theoretically demonstrate that despite a spatial period of the waves smaller than half wavelength (in vacuum), the structure can efficiently emits radiations. An analytic expression of the Q factor associated to the radiation losses is proposed. To correctly estimate this factor, we show that not only near but also far field interaction terms between the dipoles must to be considered.
Measuring aberrations in the rat brain by coherence-gated wavefront sensing using a Linnik interferometer.
Wang, J., J. - F. Léger, J. Binding, A. Claude Boccara, S. Gigan, and L. Bourdieu.
Biomedical Optics Express 3, no. 10 (2012): 2510–2525.
Résumé: Aberrations limit the resolution, signal intensity and achievable imaging depth in microscopy. Coherence-gated wavefront sensing (CGWS) allows the fast measurement of aberrations in scattering samples and therefore the implementation of adaptive corrections. However, CGWS has been demonstrated so far only in weakly scattering samples. We designed a new CGWS scheme based on a Linnik interferometer and a SLED light source, which is able to compensate dispersion automatically and can be implemented on any microscope. In the highly scattering rat brain tissue, where multiply scattered photons falling within the temporal gate of the CGWS can no longer be neglected, we have measured known defocus and spherical aberrations up to a depth of 400 μm. © 2012 Optical Society of America.
Shivakiran Bhaktha, B. N., N. Bachelard, X. Noblin, and P. Sebbah.
Applied Physics Letters 101, no. 15 (2012).
Résumé: Random lasing is reported in a dye-circulated structured polymeric microfluidic channel. The role of disorder, which results from limited accuracy of photolithographic process, is demonstrated by the variation of the emission spectrum with local-pump position and by the extreme sensitivity to a local perturbation of the structure. Thresholds comparable to those of conventional microfluidic lasers are achieved, without the hurdle of state-of-the-art cavity fabrication. Potential applications of optofluidic random lasers for on-chip sensors are discussed. Introduction of random lasers in the field of optofluidics is a promising alternative to on-chip laser integration with light and fluidic functionalities. © 2012 American Institute of Physics.
Quantitative elastography of renal transplants using supersonic shear imaging: A pilot study.
Grenier, N., S. Poulain, S. Lepreux, J. - L. Gennisson, B. Dallaudière, Y. Lebras, E. Bavu, A. Servais, V. Meas-Yedid, M. Piccoli et al.
European Radiology 22, no. 10 (2012): 2138–2146.
Résumé: Purpose To evaluate the reliability of quantitative ultrasonic measurement of renal allograft elasticity using supersonic shear imaging (SSI) and its relationship with parenchymal pathological changes. Materials and methods Forty-three kidney transplant recipients (22 women, 21 men) (mean age, 51 years; age range, 18-70 years) underwent SSI elastography, followed by biopsy. The quantitative measurements of cortical elasticity were performed by two radiologists and expressed in terms of Young's modulus (kPa). Intra- and inter-observer reproducibility was assessed (Kruskal-Wallis test and Bland-Altman analysis), as well as the correlation between elasticity values and clinical, biological and pathological data (semi-quantitative Banff scoring). Interstitial fibrosis was evaluated semiquantitatively by the Banff score and measured by quantitative image analysis. © European Society of Radiology 2012.
Matching Pursuits with random sequential subdictionaries.
Moussallam, M., L. Daudet, and G. Richard.
Signal Processing 92, no. 10 (2012): 2532–2544.
Résumé: Matching Pursuits are a class of greedy algorithms commonly used in signal processing, for solving the sparse approximation problem. They rely on an atom selection step that requires the calculation of numerous projections, which can be computationally costly for large dictionaries and burdens their competitiveness in coding applications. We propose using a non-adaptive random sequence of subdictionaries in the decomposition process, thus parsing a large dictionary in a probabilistic fashion with no additional projection cost nor parameter estimation. A theoretical modeling based on order statistics is provided, along with experimental evidence showing that the novel algorithm can be efficiently used on sparse approximation problems. An application to audio signal compression with multiscale time-frequency dictionaries is presented, along with a discussion of the complexity and practical implementations. © 2012 Elsevier B.V. All rights reserved.
High-intensity therapeutic ultrasound: Metrological requirements versus clinical usage.
Metrologia 49, no. 5 (2012): S259–S266.
Résumé: High-intensity therapeutic ultrasound (HITU) is an appealing non-invasive, non-ionizing therapeutic modality with a wide range of tissue interactions ranging from transient permeabilization of cell membranes to thermal ablation. The ability to guide and monitor the treatment with an associated ultrasonic or magnetic resonance imaging device has resulted in a dramatic rise in the clinical use of therapeutic ultrasound in the past two decades. Nevertheless, the range of clinical applications and the number of patients treated has grown at a much higher pace than the definition of standards. In this paper the metrological requirements of the therapeutic beams are reviewed and are compared with the current clinical use of image-guided HITU mostly based on a practical approach. Liver therapy, a particularly challenging clinical application, is discussed to highlight the differences between some complex clinical situations and the experimental conditions of the metrological characterization of ultrasonic transducers. © 2012 BIPM & IOP Publishing Ltd.
Dynamic study of blood-brain barrier closure after its disruption using ultrasound: A quantitative analysis.
Marty, B., B. Larrat, M. Van Landeghem, C. Robic, P. Robert, M. Port, D. Le Bihan, M. Pernot, M. Tanter, F. Lethimonnier et al.
Journal of Cerebral Blood Flow and Metabolism 32, no. 10 (2012): 1948–1958.
Résumé: Delivery of therapeutic or diagnostic agents to the brain is majorly hindered by the blood-brain barrier (BBB). Recently, many studies have demonstrated local and transient disruption of the BBB using low power ultrasound sonication combined with intravascular microbubbles. However, BBB opening and closure mechanisms are poorly understood, especially the maximum gap that may be safely generated between endothelial cells and the duration of opening of the BBB. Here, we studied BBB opening and closure under magnetic resonance (MR) guidance in a rat model. First, MR contrast agents (CA) of different hydrodynamic diameters (1 to 65 nm) were employed to estimate the largest molecular size permissible across the cerebral tissues. Second, to estimate the duration of the BBB opening, the CA were injected at various times post-BBB disruption (12 minutes to 24 hours). A T 1 mapping strategy was developed to assess CA concentration at the ultrasound (US) focal point. Based on our experimental data and BBB closure modeling, a calibration curve was obtained to compute the half closure time as a function of CA hydrodynamic diameter. These findings and the model provide an invaluable basis for optimal design and delivery of nanoparticles to the brain. © 2012 ISCBFM All rights reserved.
Imaging velocities of a vibrating object by stroboscopic sideband holography.
Verpillat, F., F. Joud, M. Atlan, and M. Gross.
Optics Express 20, no. 20 (2012): 22860–22871.
Résumé: We propose here to combine sideband holography with stroboscopic illumination synchronized with the vibration of an object. By sweeping the optical frequency of the reference beam such a way the holographic detection is tuned on the successive sideband harmonic ranks, we are able to image the instantaneous velocities of the object. Since the stroboscopic illumination is made with an electronic device, the method is compatible with fast (up to several MHz) vibration motions. The method is demonstrated with a vibrating clarinet reed excited sinusoidally at 2 kHz, and a stroboscopic illumination with cyclic ratio 0.15. Harmonic rank up to n = ±100 are detected, and a movie of the instantaneous velocities is reported. © 2012 Optical Society of America.
In situ generation of surface plasmon polaritons using a near-infrared laser diode.
Costantini, D., L. Greusard, A. Bousseksou, R. Rungsawang, T. P. Zhang, S. Callard, J. Decobert, F. Lelarge, G. - H. Duan, Y. De Wilde et al.
Nano Letters 12, no. 9 (2012): 4693–4697.
Résumé: We demonstrate a semiconductor laser-based approach which enables plasmonic active devices in the telecom wavelength range. We show that optimized laser structures based on tensile-strained InGaAlAs quantum wells-coupled to integrated metallic patternings-enable surface plasmon generation in an electrically driven compact device. Experimental evidence of surface plasmon generation is obtained with the slit-doublet experiment in the near-field, using near-field scanning optical microscopy measurements. © 2012 American Chemical Society.
Homodimerization of Amyloid Precursor Protein at the Plasma Membrane: A homoFRET Study by Time-Resolved Fluorescence Anisotropy Imaging.
Devauges, V., C. Marquer, S. Lécart, J. - C. Cossec, M. - C. Potier, E. Fort, K. Suhling, and S. Lévêque-Fort.
PLoS ONE 7, no. 9 (2012).
Résumé: Classical FRET (Förster Resonance Energy Transfer) using two fluorescent labels (one for the donor and another one for the acceptor) is not efficient for studying the homodimerization of a protein as only half of the homodimers formed can be identified by this technique. We thus resorted to homoFRET detected by time-resolved Fluorescence Anisotropy IMaging (tr-FAIM). To specifically image the plasma membrane of living cells, an original combination of tr-FAIM and Total Internal Reflection Fluorescence Lifetime Imaging Microscope (TIRFLIM) was implemented. The correcting factor accounting for the depolarization due to the high numerical aperture (NA) objective, mandatory for TIRF microscopy, was quantified on fluorescein solutions and on HEK293 cells expressing enhanced Green Fluorescence Protein (eGFP). Homodimerization of Amyloid Precursor Protein (APP), a key mechanism in the etiology of Alzheimer's disease, was measured on this original set-up. We showed, both in epifluorescence and under TIRF excitation, different energy transfer rates associated with the homodimerization of wild type APP-eGFP or of a mutated APP-eGFP, which forms constitutive dimers. This original set-up thus offers promising prospects for future studies of protein homodimerization in living cells in control and pathological conditions. © 2012 Devauges et al.
Near-field acoustic holography using sparse regularization and compressive sampling principles.
Chardon, G., L. Daudet, A. Peillot, F. Ollivier, N. Bertin, and R. Gribonval.
Journal of the Acoustical Society of America 132, no. 3 (2012): 1521–1534.
Résumé: Regularization of the inverse problem is a complex issue when using near-field acoustic holography (NAH) techniques to identify the vibrating sources. This paper shows that, for convex homogeneous plates with arbitrary boundary conditions, alternative regularization schemes can be developed based on the sparsity of the normal velocity of the plate in a well-designed basis, i.e., the possibility to approximate it as a weighted sum of few elementary basis functions. In particular, these techniques can handle discontinuities of the velocity field at the boundaries, which can be problematic with standard techniques. This comes at the cost of a higher computational complexity to solve the associated optimization problem, though it remains easily tractable with out-of-the-box software. Furthermore, this sparsity framework allows us to take advantage of the concept of compressive sampling; under some conditions on the sampling process (here, the design of a random array, which can be numerically and experimentally validated), it is possible to reconstruct the sparse signals with significantly less measurements (i.e., microphones) than classically required. After introducing the different concepts, this paper presents numerical and experimental results of NAH with two plate geometries, and compares the advantages and limitations of these sparsity-based techniques over standard Tikhonov regularization. © 2012 Acoustical Society of America.
Supersonic Shear Wave Elastography of In Vivo Pig Kidney: Influence of Blood Pressure, Urinary Pressure and Tissue Anisotropy.
Gennisson, J. - L., N. Grenier, C. Combe, and M. Tanter.
Ultrasound in Medicine and Biology 38, no. 9 (2012): 1559–1567.
Résumé: The in vivo influence of renal anisotropy and of urinary and vascular pressure on elasticity values using ultrasonic supersonic shear wave elastography was studied in pigs. Experiments were conducted in agreement with the European Commission guidelines and directives of the French Research Ministry. Six kidneys in three pigs were studied in vivo. Elasticity of renal cortex and medulla was quantified through the shear modulus (μ) by using the supersonic shear imaging technique with an 8 MHz linear ultrasound probe. All measurements were done peroperatively both in the axis and perpendicular to the main axis of pyramids, in normal condition, after progressive increase of urinary pressure, and after renal artery and renal vein ligation. In normal conditions, cortical (C) and medullary (M) elasticity values were always higher when acquisitions were realized with the ultrasound main axis perpendicular to main pyramid axis (C//: 7.7 ± 2.3 kPa; M//: 8.7 ± 2.5 kPa) than parallel (C⊥: 6.9 ± 1.4 kPa; M⊥: 6.6 ± 2.3 kPa), demonstrating an effect of renal anisotropy. In renal cortex, two bands were separated, inner cortex showing higher elasticity values (IC⊥: 8.1 ± 1.9 kPa) than outer cortex (OC⊥: 6.9 ± 1.4 kPa). Renal artery and renal vein ligation induced a decrease and an increase of elasticity respectively. Parenchymal elasticity increased linearly with elevation of urinary pressure. Intrarenal elasticity values vary with tissue anisotropy and, with vascular and urinary pressure levels. These parameters have to be taken into account for interpretation of tissue changes. Separation of outer and inner cortex could be attributable to perfusion differences. © 2012 World Federation for Ultrasound in Medicine & Biology.
Aberration correction by time reversal of moving speckle noise.
Osmanski, B. - F., G. Montaldo, M. Tanter, and M. Fink.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 59, no. 7 (2012): 1575–1583.
Résumé: Focusing a wave through heterogeneous media is an important problem in medical ultrasound imaging. In such aberrating media, in the presence of a small number of point reflectors, iterative time reversal is a well-known method able to focus on the strongest reflector. However, in presence of speckle noise generated by many non-resolved scatterers, iterative time reversal alone does not work. In this paper, we propose the use of the echoes coming from moving particles in a flow, such as red blood cells, to generate a virtual point reflector by iterative time reversal. The construction of the virtual point reflector is performed by a coherent addition of independent realizations of speckle coming from moving particles. After focusing on a virtual point reflector, ultrasound images can be locally corrected inside an isoplanatic patch. An application for the correction of power Doppler images is presented. A theoretical analysis shows that this iterative method allows focusing on the point of maximal insonification of the uncorrected beam. © 1986-2012 IEEE.
Ultrasound elastic tensor imaging: Comparison with MR diffusion tensor imaging in the myocardium.
Lee, W. - N., B. Larrat, M. Pernot, and M. Tanter.
Physics in Medicine and Biology 57, no. 16 (2012): 5075–5095.
Résumé: We have previously proven the feasibility of ultrasound-based shear wave imaging (SWI) to non-invasively characterize myocardial fiber orientation in both in vitro porcine and in vivo ovine hearts. The SWI-estimated results were in good correlation with histology. In this study, we proposed a new and robust fiber angle estimation method through a tensor-based approach for SWI, coined together as elastic tensor imaging (ETI), and compared it with magnetic resonance diffusion tensor imaging (DTI), a current gold standard and extensively reported non-invasive imaging technique for mapping fiber architecture. Fresh porcine (n = 5) and ovine (n = 5) myocardial samples (20×20×30mm 3) were studied. ETI was firstly performed to generate shear waves and to acquire the wave events at ultrafast frame rate (8000 fps). A 2.8MHz phased array probe (pitch = 0.28mm), connected to a prototype ultrasound scanner, was mounted on a customized MRI-compatible rotation device, which allowed both the rotation of the probe from90°to 90°at 5°increments and co-registration between two imaging modalities. Transmural shear wave speed at all propagation directions realized was firstly estimated. The fiber angles were determined from the shear wave speed map using the least-squares method and eigen decomposition. The test myocardial sample together with the rotation device was then placed inside a 7T MRI scanner. Diffusion was encoded in six directions. A total of 270 diffusion-weighted images (b = 1000s mm 2, FOV = 30mm, matrix size = 60×64, TR = 6s, TE = 19ms, 24 averages) and 45 B 0images were acquired in 14h 30 min. The fiber structure was analyzed by the fiber-tracking module in software, MedINRIA. The fiber orientation in the overlapped myocardial region which both ETI and DTI accessed was therefore compared, thanks to the co-registered imaging system. Results from all ten samples showed good correlation (r 2= 0.81, p < 0.0001) and good agreement (3.05°bias) between ETI and DTI fiber angle estimates. The average ETI-estimated fractional anisotropy (FA) values decreased from subendocardium to subepicardium (p < 0.05, unpaired, one-tailed t-test, N = 10) by 33%, whereas the corresponding DTI-estimated FA values presented a change of10% (p > 0.05, unpaired, one-tailed t-test, N = 10). In conclusion, we have demonstrated that the fiber orientation estimated by ETI, which assesses the shear wave speed (and thus the stiffness), was comparable to that measured by DTI, which evaluates the preferred direction of water diffusion, and have validated this concept within the myocardium. Moreover, ETI was shown capable of mapping the transmural fiber angles with as few as seven shear wave propagation directions. © 2012 Institute of Physics and Engineering in Medicine.
Ultrafast doppler imaging of blood flow dynamics in the myocardium.
Osmanski, B. - F., M. Pernot, G. Montaldo, A. Bel, E. Messas, and M. Tanter.
IEEE Transactions on Medical Imaging 31, no. 8 (2012): 1661–1668.
Résumé: Imaging intramyocardial vascular flows in real-time could strongly help to achieve better diagnostic of cardiovascular diseases. To date, no standard imaging modality allows describing accurately myocardial blood flow dynamics with good spatial and temporal resolution. We recently introduced a novel ultrasonic Doppler imaging technique based on compounded plane waves transmissions at ultrafast frame rate. The high sensitivity of this ultrafast Doppler technique permits to image the intramyocardial blood flow and its dynamics. A dedicated demodulation-filtering process is implemented to compensate for the large tissue velocity of the myocardium during the cardiac cycle. A signed power Doppler processing provides the discrimination between arterial and venous flows. Experiments were performed in vivo in a large animal open chest model (N = 5 sheep) using a conventional ultrasonic probe placed at the surface of the heart. Results show the capability of the technique to image intramyocardial vascular flows in normal physiological conditions with good spatial (200 μm) and temporal resolution (10 ms). Flow dynamics over the cardiac cycle were investigated and the imaging method demonstrated a phase opposition of flow waveforms between arterial and venous flows. Finally, ultrafast Doppler combined with tissue motion compensation was found able to reveal vascular flow disruption in ischemic regions during occlusion of the main diagonal coronary artery. © 2012 IEEE.
Time reversal of water waves.
Przadka, A., S. Feat, P. Petitjeans, V. Pagneux, A. Maurel, and M. Fink.
Physical Review Letters 109, no. 6 (2012).
Résumé: We present time reversal experiments demonstrating refocusing of gravity-capillary waves in a water tank cavity. Owing to the reverberating effect of the cavity, only a few channels are sufficient to reconstruct the surface wave at the point source, even if the absorption is not negligible. Space-time-resolved measurements of the waves during the refocusing allow us to quantitatively demonstrate that the quality of the refocusing increases linearly with the number of reemitting channels. Numerical simulations corresponding to water waves at larger scales, with negligible damping, indicate the possibility of very high quality refocusing. © 2012 American Physical Society.
Tunable time-reversal cavity for high-pressure ultrasonic pulses generation: A tradeoff between transmission and time compression.
Arnal, B., M. Pernot, M. Fink, and M. Tanter.
Applied Physics Letters 101, no. 6 (2012).
Résumé: This Letter presents a time reversal cavity that has both a high reverberation time and a good transmission factor. A multiple scattering medium has been embedded inside a fluid-filled reverberating cavity. This allows creating smart ultrasonic sources able to generate very high pressure pulses at the focus outside the cavity with large steering capabilities. Experiments demonstrate a 25 dB gain in pressure at the focus. This concept will enable us to convert conventional ultrasonic imaging probes driven by low power electronics into high power probes for therapeutic applications requiring high pressure focused pulses, such as histotripsy or lithotripsy. © 2012 American Institute of Physics.
Transformation optics and subwavelength control of light.
Pendry, J. B., A. Aubry, D. R. Smith, and S. A. Maier.
Science 337, no. 6094 (2012): 549–552.
Résumé: Our intuitive understanding of light has its foundation in the ray approximation and is intimately connected with our vision. As far as our eyes are concerned, light behaves like a stream of particles. We look inside the wavelength and study the properties of plasmonic structures with dimensions of just a few nanometers, where at a tenth or even a hundredth of the wavelength of visible light the ray picture fails. We review the concept of transformation optics that manipulates electric and magnetic field lines, rather than rays; can provide an equally intuitive understanding of subwavelength phenomena; and at the same time can be an exact description at the level of Maxwell's equations.
Acousto-optical coherence tomography with a digital holographic detection scheme.
Benoit A La Guillaume, E., S. Farahi, E. Bossy, M. Gross, and F. Ramaz.
Optics Letters 37, no. 15 (2012): 3216–3218.
Résumé: Acousto-optical coherence tomography (AOCT) consists in using random phase jumps on ultrasound and light to achieve a millimeter resolution when imaging thick scattering media. We combined this technique with heterodyne off-axis digital holography. Two-dimensional images of absorbing objects embedded in scattering phantoms are obtained with a good signal-to-noise ratio. We study the impact of the phase modulation characteristics on the amplitude of the acousto-optic signal and on the contrast and apparent size of the absorbing inclusion. © 2012 Optical Society of America.
In vivo targeted delivery of large payloads with an ultrasound clinical scanner.
Couture, O., A. Urban, A. Bretagne, L. Martinez, M. Tanter, and P. Tabeling.
Medical Physics 39, no. 8 (2012): 5229–5237.
Résumé: Purpose: Performing drug-delivery with an ultrasonic imaging scanner in situ could drastically simplify treatment and improve its specificity. Our objective is to deliver large amounts of an encapsulated agent in vivo using a clinical ultrasound scanner with a millimetric resolution. This study describes the encapsulation of fluorescein within ultrasound-inducible composite droplets and its targeted release in predefined zones in the liver of rats. Methods: An aqueous solution of fluorescein was encapsulated within perfluorocarbon liquid in 4 μm monodisperse droplets using a microfluidic system. The agent was then injected within the femoral vein of 12 rats. After exploratory ultrasound imaging, the sonographer defined five zones in the liver and a release sequence was initiated on the same apparatus. The surface of the liver was observed under fluorescence macroscopy and intraoperative fluorescence camera in vivo, before liver samples were sliced for pathology. Results: Following the conversion of the droplets, a 25 dB increase in contrast was observed in the zones selected by the sonographer. These hyperechoic regions were colocalized with the bright fluorescent spots observed on the surface of the liver. A minimum peak-negative pressure of 2.6 MPa, which is within regulations for imaging pulses, was required for the delivery of the content of the droplets. The tissue and cellular structures were not affected by the exposure to the release sequence. Conclusions: Since composite droplets can carry various therapeutic and imaging agents, they could deliver such agents specifically in any organ accessible to ultrasound. © 2012 American Association of Physicists in Medicine.
Erratum: Propagation of guided waves through weak penetrable scatterers (Journal of the Acoustical Society of America (2012) 131:3 (1874-1889)).
Maurel, A., and J. - F. Mercier.
Journal of the Acoustical Society of America 132, no. 2 (2012): 1230.
Monitoring of cornea elastic properties changes during UV-A/riboflavin-induced corneal collagen cross-linking using supersonic shear wave imaging: A pilot study.
Nguyen, T. - M., J. - F. Aubry, D. Touboul, M. Fink, J. - L. Gennisson, J. Bercoff, and M. Tanter.
Investigative Ophthalmology and Visual Science 53, no. 9 (2012): 5948–5954.
Résumé: Purpose. Keratoconus disease or post-LASIK corneal ectasia are increasingly treated using UV-A/riboflavin-induced corneal collagen cross-linking (CXL). However, this treatment suffers from a lack of techniques to provide an assessment in real-time of the CXL effects. Here, we investigated the potential interest of corneal elasticity as a biomarker of the efficacy of this treatment. Methods. For this purpose, supersonic shear wave imaging (SSI) was performed both ex vivo and in vivo on porcine eyes before and after CXL. Based on ultrasonic scanners providing ultrafast frame rates (~30 kHz), the SSI technique generates and tracks the propagation of shear waves in tissues. It provides two- and three-dimensional (2-D and 3-D) quantitative maps of the corneal elasticity. Results. After CXL, quantitative maps of corneal stiffness clearly depicted the cross-linked area with a typical 200-μm lateral resolution. The CXL resulted in a 56% ± 15% increase of the shear wave speed for corneas treated in vivo (n = 4). Conclusions. The in vivo CXL experiments performed on pigs demonstrated that the quantitative estimation of local stiffness and the 2-D elastic maps of the corneal surface provide an efficient way to monitor the local efficacy of corneal cross-linking. © 2012 The Association for Research in Vision and Ophthalmology, Inc.
Colon tumor growth and antivascular treatment in mice: Complementary assessment with MR elastography and diffusion-weighted MR imaging.
Jugé, L., B. - T. Doan, J. Seguin, M. Albuquerque, B. Larrat, N. Mignet, G. G. Chabot, D. Scherman, V. Paradis, V. Vilgrain et al.
Radiology 264, no. 2 (2012): 436–444.
Résumé: Purpose: To investigate the potential value of magnetic resonance (MR) elastography and diffusion-weighted (DW) MR imaging in the detection of microstructural changes of murine colon tumors during growth and antivascular treatment. Materials and Methods: The study was approved by the regional ethics committee for animal care. Sixty Balb-C mice, bearing ectopic and orthotopic colon tumors, were monitored for 3 weeks with high-resolution T2-weighted MR imaging, threedimensional steady-state MR elastography, and DW MR imaging at 7 T. The same imaging protocol was performed 24 hours after injection of combretastatin A4 phosphate (CA4P) in 12 mice. The absolute value of the complex shear modulus (|G*|) and the apparent diffusion coefficient (ADC) were measured in the viable zones of tumors and compared with microvessel density (MVD), cellularity, and micronecrosis by using the Pearson correlation coefficient. Results: During tumor growth, |G*| increase was correlated with MVD (r = 0.70 [P = .08] and r = 0.78 [P = .002], for both the ectopic and orthotopic models, respectively). Moreover, the ectopic tumors displayed decreased ADC, which correlated with increased cellularity (r = 0.77, P = .04), whereas no changes in ADC and cellularity were observed in orthotopic tumors. After CA4P administration, |G*| decreased in the ectopic model (P < .0001), similar to the MVD evolution (P = .03), whereas no significant changes in |G*| (P = .7) and MVD (P = .6) were observed in the orthotopic model. ADC increased in both models (P = .047 and P = .01 for the ectopic and the orthotopic models, respectively) in relation to increased micronecrosis. Conclusion: Imaging of mechanical properties and diffusivity provide complementary information during tumor growth and regression that are respectively linked to vascularity and tumor cell alterations, including cellularity and micronecrosis. © RSNA, 2012.
Planar metamaterial based on hybridization for directive emission.
Ourir, A., R. Abdeddaim, and J. De Rosny.
Optics Express 20, no. 16 (2012): 17545–17551.
Résumé: We present the first experimental demonstration of a highdirectivity using a mu and epsilon near zero (MENZ) metamaterial. We use the hybridization principles to design a planar MENZ structure based on the fishnet unit cell. Resonant mode engineering achieves an effective permittivity and permeability that approaches zeros around 10.5 GHz simultaneously. We use this metamaterial as a superstrate of a microstrip patch antenna. We show that the directivity of the antenna is effectively enhanced compared to that of the patch antenna alone at the desired frequency. © 2012 Optical Society of America.
Taming random lasers through active spatial control of the pump.
Bachelard, N., J. Andreasen, S. Gigan, and P. Sebbah.
Physical Review Letters 109, no. 3 (2012).
Résumé: Active control of the spatial pump profile is proposed to exercise control over random laser emission. We demonstrate numerically the selection of any desired lasing mode from the emission spectrum. An iterative optimization method is employed, first in the regime of strong scattering where modes are spatially localized and can be easily selected using local pumping. Remarkably, this method works efficiently even in the weakly scattering regime, where strong spatial overlap of the modes precludes spatial selectivity. A complex optimized pump profile is found, which selects the desired lasing mode at the expense of others, thus demonstrating the potential of pump shaping for robust and controllable single mode operation of a random laser. © 2012 American Physical Society.
Time resolved three-dimensional acousto-optic imaging of thick scattering media.
Farahi, S., E. Benoit, A. A. Grabar, J. - P. Huignard, and F. Ramaz.
Optics Letters 37, no. 13 (2012): 2754–2756.
Résumé: Acousto-optic imaging is based on light interaction with focused ultrasound in a scattering medium. Thanks to photorefractive holography combined with pulsed ultrasound, we perform a time-resolved detection of ultrasound- modulated photons in the therapeutic window (780 nm). A high-gain SPS:Te crystal is used for this purpose and enables us to image through large optical thickness (500 mean free paths). We are able to generate threedimensional (3D) acousto-optic images by translating a multielement ultrasound probe in only one direction. A 3D absorbing object is imaged through a 3 cm thick phantom. © 2012 Optical Society of America.
Acousto-optic laser optical feedback imaging.
Jacquin, O., W. Glastre, E. Lacot, O. Hugon, H. G. De Chatellus, and F. Ramaz.
Optics Letters 37, no. 13 (2012): 2514–2516.
Résumé: We present a photon noise and diffraction-limited imaging method combining an imaging laser and ultrasonic waves. The laser optical feedback imaging (LOFI) technique is an ultrasensitive imaging method for imaging objects through or embedded within a scattering medium. However, LOFI performances are dramatically limited by parasitic optical feedback occurring in the experimental setup. In this Letter, we have tagged the ballistic photons by an acousto-optic effect in order to filter the parasitic feedback effect and to reach the theoretical and ultimate sensitivity of the LOFI technique. We present the principle and the experimental setup of the acousto-optic laser optical feedback imaging technique, and we demonstrate the suppression of the parasitic feedback. © 2012 Optical Society of America.
Boltzmann machine and mean-field approximation for structured sparse decompositions.
Dremeau, A., C. Herzet, and L. Daudet.
IEEE Transactions on Signal Processing 60, no. 7 (2012): 3425–3438.
Résumé: Taking advantage of the structures inherent in many sparse decompositions constitutes a promising research axis. In this paper, we address this problem from a Bayesian point of view. We exploit a Boltzmann machine, allowing to take a large variety of structures into account, and focus on the resolution of a marginalized maximum a posteriori problem. To solve this problem, we resort to a mean-field approximation and the “variational Bayes expectation- maximization” algorithm. This approach results in a soft procedure making no hard decision on the support or the values of the sparse representation. We show that this characteristic leads to an improvement of the performance over state-of-the-art algorithms. © 2012 IEEE.
Characterization of mechanical properties of a hollow cylinder with zero group velocity Lamb modes.
Cès, M., D. Royer, and C. Prada.
Journal of the Acoustical Society of America 132, no. 1 (2012): 180–185.
Résumé: Hollow cylinders used in the industry must be regularly inspected. Elastic guided waves, similar to Lamb modes in a plate, can propagate in the axial direction or around the circumference. They are sensitive to geometrical and mechanical parameters of the cylindrical shell. The objective of this paper is to show that zero group velocity (ZGV) Lamb modes can be used to bring out anisotropy and to measure elastic constants of the material. This study provides experimental and numerical investigations on a Zirconium alloy tube extensively used by the nuclear industry in reactor core components. A non-contact method, based on laser ultrasound techniques and ZGV Lamb modes, demonstrates that the difference observed between axial and circumferential guided waves cannot be explained by an isotropic model. Then, a transverse isotropic model is used for the Zircaloy tube. Four of the five elastic constants are directly extracted from ZGV resonance frequencies. The last one is deduced from the measured dispersion spectra. With this complete set of constants, a good agreement is obtained between theoretical and experimental dispersion curves for both axially and circumferentially propagating guided waves. © 2012 Acoustical Society of America.
Direct phase projection and transcranial focusing of ultrasound for brain therapy.
Pinton, G. F., J. - F. Aubry, and M. Tanter.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 59, no. 6 (2012): 1149–1159.
Résumé: Ultrasound can be used to noninvasively treat the human brain with hyperthermia by focusing through the skull. To obtain an accurate focus, especially at high frequencies (>500 kHz), the phase of the transmitted wave must be modified to correct the aberrations introduced by the patient's individual skull morphology. Currently, three-dimensional finite-difference time-domain simulations are used to model a point source at the target. The outward-propagating wave crosses the measured representation of the human skull and is recorded at the therapy array transducer locations. The signal is then time reversed and experimentally transmitted back to its origin. These simulations are resource intensive and add a significant delay to treatment planning. Ray propagation is computationally efficient because it neglects diffraction and only describes two propagation parameters: the wave's direction and the phase. We propose a minimal method that is based only on the phase. The phase information is projected from the external skull surface to the array locations. This replaces computationally expensive finite-difference computations with an almost instantaneous direct phase projection calculation. For the five human skull samples considered, the phase distribution outside of the skull is shown to vary by less than λ/20 as it propagates over a 5 cm distance and the validity of phase projection is established over these propagation distances. The phase aberration introduced by the skull is characterized and is shown to have a good correspondence with skull morphology. The shape of this aberration is shown to have little variation with propagation distance. The focusing quality with the proposed phase-projection algorithm is shown to be indistinguishable from the gold-standard full finite-difference simulation. In conclusion, a spherical wave that is aberrated by the skull has a phase propagation that can be accurately described as radial, even after it has been distorted. By combining finitedifference simulations with a phase-projection algorithm, the time required for treatment planning is significantly reduced. The correlation length of the phase is used to validate the algorithm and it can also be used to provide guiding parameters for clinical array transducer design in terms of transducer spacing and phase error. © 2012 IEEE.
Level splitting at macroscopic scale.
Eddi, A., J. Moukhtar, S. Perrard, E. Fort, and Y. Couder.
Physical Review Letters 108, no. 26 (2012).
Résumé: A walker is a classical self-propelled wave particle association moving on a fluid interface. Two walkers can interact via their waves and form orbiting bound states with quantized diameters. Here we probe the behavior of these bound states when setting the underlying bath in rotation. We show that the bound states are driven by the wave interaction between the walkers and we observe a level splitting at macroscopic scale induced by the rotation. Using the analogy between Coriolis and Lorentz forces, we show that this effect is the classical equivalent to Zeeman splitting of atomic energy levels. © 2012 American Physical Society.
Sub-wavelength energy concentration with electrically generated mid-infrared surface plasmons.
Bousseksou, A., A. Babuty, J. - P. Tetienne, I. Moldovan-Doyen, R. Braive, G. Beaudoin, I. Sagnes, Y. De Wilde, and R. Colombelli.
Optics Express 20, no. 13 (2012): 13738–13747.
Résumé: While freely propagating photons cannot be focused below their diffraction limit, surface-plasmon polaritons follow the metallic surface to which they are bound, and can lead to extremely sub-wavelength energy volumes. These properties are lost at long mid-infrared and THz wavelengths where metals behave as quasi-perfect conductors, but can in principle be recovered by artificially tailoring the surface-plasmon dispersion. We demonstrate – in the important mid-infrared range of the electromagnetic spectrum – the generation onto a semiconductor chip of plasmonic excitations which can travel along long distances, on bent paths, to be finally focused into a sub-wavelength volume. The demonstration of these advanced functionalities is supported by full near-field characterizations of the electromagnetic field distribution on the surface of the active plasmonic device. © 2012 Optical Society of America.
Localized mode hybridization by fine tuning of two-dimensional random media.
Labonté, L., C. Vanneste, and P. Sebbah.
Optics Letters 37, no. 11 (2012): 1946–1948.
Résumé: We study numerically the interaction of spatially localized modes in strongly scattering two-dimensional (2D) media. We move eigenvalues in the complex plane by changing gradually the index of a single scatterer. When spatial and spectral overlap is sufficient, localized states couple, and avoided level crossing is observed. We show that local manipulation of the disordered structure can couple several localized states to form an extended chain of hybridized modes crossing the entire sample, thus changing the nature of certain modes from localized to extended in a nominally localized disordered system. We suggest such a chain in 2D random systems is the analog of one-dimensional necklace states, the occasional open channels predicted by Pendry [Physics 1, 20 (2008).] through which the light can sneak through an opaque medium. © 2012 Optical Society of America.
Journal of biomedical optics 17, no. 6 (2012): 061205.
Résumé: Photoacoustics has recently been proposed as a potential method to guide and/or monitor therapy based on high-intensity focused ultrasound (HIFU). We experimentally demonstrate the creation of a HIFU lesion at the location of an optical absorber, by use of photoacoustic signals emitted by the absorber detected on a dual mode transducer array. To do so, a dedicated ultrasound array intended to both detect photoacoustic waves and emit HIFU with the same elements was used. Such a dual-mode array provides automatically coregistered reference frames for photoacoustic detection and HIFU emission, a highly desired feature for methods involving guidance or monitoring of HIFU by use of photoacoustics. The prototype is first characterized in terms of both photoacoustic and HIFU performances. The probe is then used to perform an idealized scenario of photoacoustic-guided therapy, where photoacoustic signals generated by an absorbing thread embedded in a piece of chicken breast are used to automatically refocus a HIFU beam with a time-reversal mirror and necrose the tissue at the location of the absorber.
Measurement of pulsatile motion with millisecond resolution by MRI.
Souchon, R., J. - L. Gennisson, M. Tanter, R. Salomir, J. - Y. Chapelon, and O. Rouvière.
Magnetic Resonance in Medicine 67, no. 6 (2012): 1787–1793.
Résumé: We investigated a technique based on phase-contrast cine MRI combined with deconvolution of the phase shift waveforms to measure rapidly varying pulsatile motion waveforms. The technique does not require steady-state displacement during motion encoding. Simulations and experiments were performed in porcine liver samples in view of a specific application, namely the observation of transient displacements induced by acoustic radiation force. Simulations illustrate the advantages and shortcomings of the methods. For experimental validation, the waveforms were acquired with an ultrafast ultrasound scanner (Supersonic Imagine Aixplorer), and the rates of decay of the waveforms (relaxation time) were compared. With bipolar motion-encoding gradient of 8.4 ms, the method was able to measure displacement waveforms with a temporal resolution of 1 ms over a time course of 40 ms. Reasonable agreement was found between the rate of decay of the waveforms measured in ultrasound (2.8 ms) and in MRI (2.7-3.3 ms). © 2011 Wiley-Liss, Inc.
Probabilities and trajectories in a classical wave-particle duality.
Couder, Y., and E. Fort.
Journal of Physics: Conference Series 361, no. 1 (2012).
Résumé: Several recent experiments were devoted to walkers, structures that associate a droplet bouncing on a vibrated liquid with the surface waves it excites. They reveal that a form of wave-particle duality exists in this classical system with the emergence of quantum-like behaviours. Here we revisit the single particle diffraction experiment and show the coexistence of two waves. The measured probability distributions are ruled by the diffraction of a quantumlike probability wave. But the observation of a single walker reveals that the droplet is driven by a pilot wave of different spatial structure that determines its trajectory in real space. The existence of two waves of these types had been proposed by de Broglie in his “double solution” model of quantum mechanics. A difference with the latter is that the pilot-wave is, in our experiment, endowed with a “path memory”. When intrusive measurements are performed, this memory effect induces transient chaotic individual trajectories that generate the resulting statistical behaviour.
Usual Anderson localization restored in bilayered left- and right-handed structures.
Maurel, A., A. Ourir, J. - F. Mercier, and V. Pagneux.
Physical Review B – Condensed Matter and Materials Physics 85, no. 20 (2012).
Résumé: We present a study of the attenuation length in a one-dimensional array of alternating left- and right-handed materials in which both the permittivities and the permeabilities are disordered. This type of structure has been shown to present an anomaly in the attenuation length when only permeabilities are disordered. We derive a simple analytical expression of the attenuation length, when the disorder in the refraction index is due to perturbations in both the permeability and the permittivity. Our expression is able to explain the transition to the anomalous behavior when perturbation only in the permeability or only in the permittivity is considered. Besides, we show that the anomaly is dramatically affected when considering perturbations in permeability and permittivity. The coupling effects are able to restore the ordinary localization length. © 2012 American Physical Society.
Full-field near-field optical microscope for cell imaging.
Barroca, T., K. Balaa, S. Lévêque-Fort, and E. Fort.
Physical Review Letters 108, no. 21 (2012).
Résumé: We report a new full-field fluorescence microscopy method for imaging live cell membranes based on supercritical near-field emission. This technique consists of extracting the self-interference between undercritical and supercritical light by simple image subtraction. In the objective back focal plane, this is equivalent to adding a virtual mask blocking the subcritical emission. We show that this virtual mask is radically different from a real physical mask, enabling a 100 nm axial confinement and enhancing the image sensitivity without damaging the lateral resolution. This technique is easy to implement and simultaneously provides images of the inner parts of the cell and its membrane with standard-illumination light. © 2012 American Physical Society.
Controlling waves in space and time for imaging and focusing in complex media.
Mosk, A. P., A. Lagendijk, G. Lerosey, and M. Fink.
Nature Photonics 6, no. 5 (2012): 283–292.
Résumé: In complex media such as white paint and biological tissue, light encounters nanoscale refractive-index inhomogeneities that cause multiple scattering. Such scattering is usually seen as an impediment to focusing and imaging. However, scientists have recently used strongly scattering materials to focus, shape and compress waves by controlling the many degrees of freedom in the incident waves. This was first demonstrated in the acoustic and microwave domains using time reversal, and is now being performed in the optical realm using spatial light modulators to address the many thousands of spatial degrees of freedom of light. This approach is being used to investigate phenomena such as optical super-resolution and the time reversal of light, thus opening many new avenues for imaging and focusing in turbid media. © 2012 Macmillan Publishers Limited. All rights reserved.
Magnetic resonance thermometry at 7T for real-time monitoring and correction of ultrasound induced mild hyperthermia.
Fite, B. Z., Y. Liu, D. E. Kruse, C. F. Caskey, J. H. Walton, C. - Y. Lai, L. M. Mahakian, B. Larrat, E. Dumont, and K. W. Ferrara.
PLoS ONE 7, no. 4 (2012).
Résumé: While Magnetic Resonance Thermometry (MRT) has been extensively utilized for non-invasive temperature measurement, there is limited data on the use of high field (≥7T) scanners for this purpose. MR-guided Focused Ultrasound (MRgFUS) is a promising non-invasive method for localized hyperthermia and drug delivery. MRT based on the temperature sensitivity of the proton resonance frequency (PRF) has been implemented in both a tissue phantom and in vivo in a mouse Met-1 tumor model, using partial parallel imaging (PPI) to speed acquisition. An MRgFUS system capable of delivering a controlled 3D acoustic dose during real time MRT with proportional, integral, and derivative (PID) feedback control was developed and validated. Real-time MRT was validated in a tofu phantom with fluoroptic temperature measurements, and acoustic heating simulations were in good agreement with MR temperature maps. In an in vivo Met-1 mouse tumor, the real-time PID feedback control is capable of maintaining the desired temperature with high accuracy. We found that real time MR control of hyperthermia is feasible at high field, and k-space based PPI techniques may be implemented for increasing temporal resolution while maintaining temperature accuracy on the order of 1°C. © 2012 Fite et al.
Characterization of circumferential guided waves in a cylindrical cortical bone-mimicking phantom.
Nauleau, P., E. Cochard, J. - G. Minonzio, Q. Grimal, P. Laugier, and C. Prada.
Journal of the Acoustical Society of America 131, no. 4 (2012): EL289–EL294.
Résumé: The femoral neck cortical shell was recently demonstrated to act like a waveguide for circumferential waves. Femoral neck assessment with ultrasound could be enhanced by guided waves measurement. In this study, the decomposition of the time reversal operator (DORT) method is used to measure the phase velocities of circumferential guided modes in a circular tube with dimensions characteristic of femoral neck. The tube is made of a bone-mimicking material. Five guided modes are obtained and compared to theoretical predictions. The work substantiates the feasibility of measuring guided waves in a relatively thick tube of attenuating material with the DORT method. © 2012 Acoustical Society of America.
Application of 1-d transient elastography for the shear modulus assessment of thin-layered soft tissue: Comparison with supersonic shear imaging technique.
Brum, J., J. - L. Gennisson, T. - M. Nguyen, N. Benech, M. Fink, M. Tanter, and C. Negreira.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 59, no. 4 (2012): 703–714.
Résumé: Elasticity estimation of thin-layered soft tissues has gained increasing interest propelled by medical applications like skin, corneal, or arterial wall shear modulus assessment. In this work, the authors propose one-dimensional transient elastography (1DTE) for the shear modulus assessment of thin-layered soft tissue. Experiments on three phantoms with different elasticities and plate thicknesses were performed. First, using 1DTE, the shear wave speed dispersion curve inside the plate was obtained and validated with finite difference simulation. No dispersive effects were observed and the shear wave speed was directly retrieved from time-of-flight measurements. Second, the supersonic shear imaging (SSI) technique (considered to be a gold standard) was performed. For the SSI technique, the propagating wave inside the plate is guided as a Lamb wave. Experimental SSI dispersion curves were compared with finite difference simulation and fitted using a generalized Lamb model to retrieve the plate bulk shear wave speed. Although they are based on totally different mechanical sources and induce completely different diffraction patterns for the shear wave propagation, the 1DTE and SSI techniques resulted in similar shear wave speed estimations. The main advantage of the 1DTE technique is that bulk shear wave speed can be directly retrieved without requiring a dispersion model. © 2012 IEEE.
Quantitative imaging of nonlinear shear modulus by combining static elastography and shear wave elastography.
Latorre-Ossa, H., J. - L. Gennisson, E. De Brosses, and M. Tanter.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 59, no. 4 (2012): 833–839.
Résumé: The study of new tissue mechanical properties such as shear nonlinearity could lead to better tissue characterization and clinical diagnosis. This work proposes a method combining static elastography and shear wave elastography to derive the nonlinear shear modulus by applying the acoustoelasticity theory in quasi-incompressible soft solids. Results demonstrate that by applying a moderate static stress at the surface of the investigated medium, and by following the quantitative evolution of its shear modulus, it is possible to accurately and quantitatively recover the local Landau (A) coefficient characterizing the shear nonlinearity of soft tissues. © 2012 IEEE.
Optical properties of metamaterials: Influence of electric multipoles, magnetoelectric coupling, and spatial dispersion.
Guth, N., B. Gallas, J. Rivory, J. Grand, A. Ourir, G. Guida, R. Abdeddaim, C. Jouvaud, and J. De Rosny.
Physical Review B – Condensed Matter and Materials Physics 85, no. 11 (2012).
Résumé: We present a study on the optical properties of metamaterial layers containing U-shaped resonators with resonances in the near infrared. We show that the description of the optical properties requires taking into account permittivity, permeability, and chirality tensors. The elements of these tensors were tentatively attributed to surface plasmon resonances of the arms, magnetoelectric coupling, and spatial dispersion. We show that measurements of the Mueller matrix of the sample in appropriately chosen conditions allow unambiguous discrimination of the contribution of the different elements of the tensors to the optical properties of the metamaterial. Emphasis is put on the description of transfers of polarization and symmetries among the Mueller matrix elements. The findings of the theoretical part are confirmed by spectroscopic ellipsometric measurements made on this sample. The spectral dispersion of the different elements of permittivity, permeability, and chirality, modeled using oscillators with Lorentzian line shape, allows correct adjustment of measurements. © 2012 American Physical Society.
Propagation of guided waves through weak penetrable scatterers.
Journal of the Acoustical Society of America 131, no. 3 (2012): 1874–1889.
Résumé: The scattering of a scalar wave propagating in a waveguide containing weak penetrable scatterers is inspected in the Born approximation. The scatterers are of arbitrary shape and present a contrast both in density and in wavespeed (or bulk modulus), a situation that can be translated in the context of SH waves, water waves, or transverse electric/transverse magnetic polarized electromagnetic waves. For small size inclusions compared to the waveguide height, analytical expressions of the transmission and reflection coefficients are derived, and compared to results of direct numerical simulations. The cases of periodically and randomly distributed inclusions are considered in more detail, and compared with unbounded propagation through inclusions. Comparisons with previous results valid in the low frequency regime are proposed. © 2012 Acoustical Society of America.
Lee, W. - N., M. Pernot, M. Couade, E. Messas, P. Bruneval, A. Bel, A. A. Hagège, M. Fink, and M. Tanter.
IEEE Transactions on Medical Imaging 31, no. 3 (2012): 554–562.
Résumé: The assessment of disrupted myocardial fiber arrangement may help to understand and diagnose hypertrophic or ischemic cardiomyopathy. We hereby proposed and developed shear wave imaging (SWI), which is an echocardiography-based, noninvasive, real-time, and easy-to-use technique, to map myofiber orientation. Five in vitro porcine and three in vivo open-chest ovine hearts were studied. Known in physics, shear wave propagates faster along than across the fiber direction. SWI is a technique that can generate shear waves travelling in different directions with respect to each myocardial layer. SWI further analyzed the shear wave velocity across the entire left-ventricular (LV) myocardial thickness, ranging between 10 (diastole) and 25 mm (systole), with a resolution of 0.2 mm in the middle segment of the LV anterior wall region. The fiber angle at each myocardial layer was thus estimated by finding the maximum shear wave speed. In the in vitro porcine myocardium (n=5), the SWI-estimated fiber angles gradually changed from +80° ± 7° (endocardium) to +30° ± 13° (midwall) and-40° ± 10° (epicardium) with 0° aligning with the circumference of the heart. This transmural fiber orientation was well correlated with histology findings (r 2=0.91± 0.02, p<0.0001). SWI further succeeded in mapping the transmural fiber orientation in three beating ovine hearts in vivo. At midsystole, the average fiber orientation exhibited 71° ± 13° (endocardium), 27° ± 8° (midwall), and-26° ± 30° (epicardium). We demonstrated the capability of SWI in mapping myocardial fiber orientation in vitro and in vivo. SWI may serve as a new tool for the noninvasive characterization of myocardial fiber structure. © 2011 IEEE.
MR-guided adaptive focusing of therapeutic ultrasound beams in the human head.
Marsac, L., D. Chauvet, B. Larrat, M. Pernot, B. Robert, M. Fink, A. L. Boch, J. F. Aubry, and M. Tanter.
Medical Physics 39, no. 2 (2012): 1141–1149.
Résumé: Purpose: This study aims to demonstrate, using human cadavers the feasibility of energy-based adaptive focusing of ultrasonic waves using magnetic resonance acoustic radiation force imaging (MR-ARFI) in the framework of non-invasive transcranial high intensity focused ultrasound (HIFU) therapy. Methods: Energy-based adaptive focusing techniques were recently proposed in order to achieve aberration correction. The authors evaluate this method on a clinical brain HIFU system composed of 512 ultrasonic elements positioned inside a full body 1.5 T clinical magnetic resonance (MR) imaging system. Cadaver heads were mounted onto a clinical Leksell stereotactic frame. The ultrasonic wave intensity at the chosen location was indirectly estimated by the MR system measuring the local tissue displacement induced by the acoustic radiation force of the ultrasound (US) beams. For aberration correction, a set of spatially encoded ultrasonic waves was transmitted from the ultrasonic array and the resulting local displacements were estimated with the MR-ARFI sequence for each emitted beam. A noniterative inversion process was then performed in order to estimate the spatial phase aberrations induced by the cadaver skull. The procedure was first evaluated and optimized in a calf brain using a numerical aberrator mimicking human skull aberrations. The full method was then demonstrated using a fresh human cadaver head. Results: The corrected beam resulting from the direct inversion process was found to focus at the targeted location with an acoustic intensity 2.2 times higher than the conventional non corrected beam. In addition, this corrected beam was found to give an acoustic intensity 1.5 times higher than the focusing pattern obtained with an aberration correction using transcranial acoustic simulation-based on X-ray computed tomography (CT) scans. Conclusions: The proposed technique achieved near optimal focusing in an intact human head for the first time. These findings confirm the strong potential of energy-based adaptive focusing of transcranial ultrasonic beams for clinical applications. © 2012 American Association of Physicists in Medicine.
Fourier transform profilometry for water waves: How to achieve clean water attenuation with diffusive reflection at the water surface?
Przadka, A., B. Cabane, V. Pagneux, A. Maurel, and P. Petitjeans.
Experiments in Fluids 52, no. 2 (2012): 519–527.
Résumé: We present a study of the damping of capillary-gravity waves in water containing pigments. The practical interest comes from a recent profilometry technique (FTP for Fourier Transform Profilometry) using fringe projection onto the liquid-free surface. This experimental technique requires diffusive reflection of light on the liquid surface, which is usually achieved by adding white pigments. It is shown that the use of most paint pigments causes a large enhancement of the damping of the waves. Indeed, these paints contain surfactants which are easily adsorbed at the air-water interface. The resulting surface film changes the attenuation properties because of the resonance-type damping between capillary-gravity waves and Marangoni waves. We study the physicochemical properties of coloring pigments, showing that particles of the anatase (TiO 2) pigment make the water surface light diffusive while avoiding any surface film effects. The use of the chosen particles allows to perform space-time resolved FTP measurements on capillary-gravity waves, in a liquid with the damping properties of pure water. © 2011 Springer-Verlag.
Imaging changes in scattering media from Time Reversal of the Coda wave Difference (TRECOD).
Bonneau, L., C. Prada, M. Fink, and A. Tourin.
Waves in Random and Complex Media 22, no. 1 (2012): 109–120.
Résumé: We propose a new method for monitoring temporal changes in a complex scattering environment. It is referred to as Time Reversal of the Coda wave Difference (TRECOD). The impulse responses of the probed medium are recorded between all the emitter/receiver pairs of a transceiver array. A Fourier transform of these responses then produces the so-called backscattering transfer matrix at each frequency of the transceiver bandwidth. The matrix acquisition is repeated and the image of the temporal changes occurring between two matrix acquisitions is formed by propagating numerically the significant singular vectors of the matrix difference. Small-scale experiments with ultrasound are used to validate the method and demonstrate its interest for the monitoring of a fluid injection in a porous medium behind a strongly scattering obstacle. © 2012 Copyright Taylor and Francis Group, LLC.
Optimal spatiotemporal focusing through complex scattering media.
Aulbach, J., A. Bretagne, M. Fink, M. Tanter, and A. Tourin.
Physical Review E – Statistical, Nonlinear, and Soft Matter Physics 85, no. 1 (2012).
Résumé: We present an alternative approach for spatiotemporal focusing through complex scattering media by wave front shaping. Using a nonlinear feedback signal to shape the incident pulsed wave front, we show that the limit of a spatiotemporal matched filter can be achieved; i.e., the wave amplitude at the intended time and focus position is maximized for a given input energy. It is exactly what is also achieved with time reversal. Demonstrated with ultrasound experiments, our method is generally applicable to all types of waves. © 2012 American Physical Society.
Second-harmonic generation from coupled plasmon modes in a single dimer of gold nanospheres.
Slablab, A., L. Le Xuan, M. Zielinski, Y. De Wilde, V. Jacques, D. Chauvat, and J. - F. Roch.
Optics Express 20, no. 1 (2012): 220–227.
Résumé: We show that a dimer made of two gold nanospheres exhibits a remarkable efficiency for second-harmonic generation under femtosecond optical excitation. The detectable nonlinear emission for the given particle size and excitation wavelength arises when the two nanoparticles are as close as possible to contact, as in situ controlled and measured using the tip of an atomic force microscope. The excitation wavelength dependence of the second-harmonic signal supports a coupled plasmon resonance origin with radiation from the dimer gap. This nanometer-size light source might be used for high-resolution near-field optical microscopy. © 2011 Optical Society of America.
Detection of intrarenal microstructural changes with supersonic shear wave elastography in rats.
Derieppe, M., Y. Delmas, J. - L. Gennisson, C. Deminière, S. Placier, M. Tanter, C. Combe, and N. Grenier.
European Radiology 22, no. 1 (2012): 243–250.
Résumé: Objectives To evaluate, in a rat model of glomerulosclerosis, whether ultrasonic shear wave elastography detects kidney cortex stiffness changes and predicts histopatholog- ical development of fibrosis. Materials and methods Three groups were studied trans- versally: a control group (n=8), a group after 4 weeks of L-NAME administration (H4, n = 8), and a group after 6 weeks (H6, n =15). A fourth group was studied longitudinally (n=8) before, after 4 weeks and after 7 weeks of L-NAME administration. Shear modulus of renal cortex was quantified using supersonic shear imaging technique. Urine was analysed for dosage of protein/creatinine ratio. Kidneys were removed for histological quantification of fibrosis. Results Diseased rats showed an increased urinary protein/ creatinine ratio. Cortical stiffness expressed as median (interquartile range) was 4.0 kPa (3.3-4.5) in control kidneys. It increased in all but one pathological groups: H4: 7.7 kPa (5.5-8.6) (p<0.01); H6: 4.8 kPa (3.9-5.9) (not significant); in the longitudinal cohort, from 4.5 kPa (3.1-5.9) to 7.7 kPa (5.9-8.3) at week 4 (p<0.05) and to 6.9 kPa (6.1-7.8) at week 7 (p<0.05). Stiffness values were correlated with the proteinuria/creatininuria ratio (r=0.639, p<0.001). Conclusions Increased cortical stiffness is correlated with the degree of renal dysfunction. More experience in other models is necessary to understand its relationship with microstructural changes. © European Society of Radiology 2011.
Experimentally based description of harp plucking.
Chadefaux, D., J. - L. Le Carrou, B. Fabre, and L. Daudet.
Journal of the Acoustical Society of America 131, no. 1 (2012): 844–855.
Résumé: This paper describes an experimental study of string plucking for the classical harp. Its goal is to characterize the playing parameters that play the most important roles in expressivity, and in the way harp players recognize each other, even on isolated notes-what we call the acoustical signature of each player. We have designed a specific experimental setup using a high-speed camera that tracks some markers on the fingers and on the string. This provides accurate three-dimensional positioning of the finger and of the string throughout the plucking action, in different musical contexts. From measurements of ten harp players, combined with measurements of the soundboard vibrations, we extract a set of parameters that finely control the initial conditions of the string's free oscillations. Results indicate that these initial conditions are typically a complex mix of displacement and velocity, with additional rotation. Although remarkably reproducible by a single player-and the more so for professional players-we observe that some of these control parameters vary significantly from one player to another. © 2012 Acoustical Society of America.
Attenuation, scattering, and absorption of ultrasound in the skull bone.
Pinton, G., J. - F. Aubry, E. Bossy, M. Muller, M. Pernot, and M. Tanter.
Medical Physics 39, no. 1 (2012): 299–307.
Résumé: Purpose: Measured values of ultrasound attenuation in bone represent a combination of different loss mechanisms. As a wave is transmitted from a fluid into bone, reflections occur at the interface. In the bone, mode conversion occurs between longitudinal and shear modes and the mechanical wave is scattered by its complex internal microstructure. Finally, part of the wave energy is absorbed by the bone and converted into heat. Due to the complexity of the wave propagation and the difficulty in performing measurements that are capable of separating the various loss mechanisms, there are currently no estimates of the absorption in bone. The aim of this paper is, thus, to quantify the attenuation, scattering, and thermal absorption in bone. Methods: An attenuating model of wave propagation in bone is established and used to develop a three-dimensional finite difference time domain numerical algorithm. Hydrophone and optical heterodyne interferometer measurements of the acoustic field as well as a x-ray microtomography of the bone sample are used to drive the simulations and to measure the attenuation. The acoustic measurements are performed concurrently with an infrared camera that can measure the temperature elevation during insonication. A link between the temperature and the absorption via a three-dimensional thermal simulation is then used to quantify the absorption coefficients for longitudinal and shear waves in cortical bone. Results: We demonstrate that only a small part of the attenuation is due to absorption in bone and that the majority of the attenuation is due to reflection, scattering, and mode conversion. In the nine samples of a human used for the study, the absorption time constant for cortical bone was determined to be 1.04 μs ± 28%. This corresponds to a longitudinal absorption of 2.7 dB/cm and a shear absorption of 5.4 dB/cm. The experimentally measured attenuation across the approximately 8 mm thick samples was 13.3 ± 0.97 dB/cm. Conclusions: This first measurement of ultrasound absorption in bone can be used to estimate the amount of heat deposition based on knowledge of the acoustic field. © 2012 American Association of Physicists in Medicine.
Understanding the behavior of miniaturized metamaterial-based dipole antennas in leaky wave regime.
Zhou, L., H. H. Ouslimani, A. Priou, A. Ourir, and O. Maas.
Applied Physics A: Materials Science and Processing 106, no. 1 (2012): 145–149.
Résumé: We investigate the frequency behavior of a half-wave dipole antenna placed very close over a 2LC uni-planar compact electromagnetic bandgap (UC-EBG) structure. A very compact and high-gain antenna is realized at 1 GHz. The air distance between the dipole element and the UC-EBG surface is λ 0/100. We analyse the structure by using the optical model of Hansen, full-wave electromagnetic simulations (EM) and experimental characterizations. The analytical model of Hansen describes accurately the UC-EBG phase contribution to the total radiated field below 1 GHz. Above this frequency, the Hansen analytical model is in discrepancy with the measurements and full-wave simulations, which show split in the radiation patterns. We show that this phenomenon is induced by the power leakage of the fast-wave UC-EBG surface excited by the dipole source inside its leaky wave region. We propose an original model based on the Hansen optical analysis that takes into account the overall phenomena. The model includes the contribution of the weighted fields radiated by the cells of the UC-EBG. This model leads to very good agreements with measurements and full-wave simulations. © 2011 Springer-Verlag.
Numerical prediction of frequency dependent 3D maps of mechanical index thresholds in ultrasonic brain therapy.
Pinton, G., J. - F. Aubry, M. Fink, and M. Tanter.
Medical Physics 39, no. 1 (2012): 455–467.
Résumé: Purpose: Therapeutic ultrasound has been used in the brain for thrombolysis and high intensity focused ultrasound (HIFU) therapy. A low-frequency clinical study of sonothrombolysis, called the transcranial low-frequency ultrasound-mediated thrombolysis in brain ischemia (TRUMBI), has revealed an increased incidence of hemorrhage, which may have been caused by cavitation. The goal of this study is to determine if there is a comparable risk of generating cavitation during HIFU brain therapy at different frequencies. Methods: Two approaches are used to transmit acoustic energy through the skull to the brain: low-frequency ultrasound, with a wavelength that is larger than the skull thickness, and high frequency ultrasound, that is sensitive to aberrations and must use corrective techniques. At high frequency, the mechanical index (MI) is lower, which translates to a higher cavitation threshold. In addition to the nonfocused geometry of the 300 kHz sonothrombolysis treatment device, two types of focused therapeutic transducers were modeled: a low frequency 220 kHz transducer and a 1 MHz transducer that required aberration correction with a time-reversal approach, representing the lowest and highest frequencies currently used. The acoustic field was modeled with a finite difference fullwave acoustic code developed for large scale computations, that is, capable of simulating the entire brain volume. Various MI thresholds and device geometries were considered to determine the regions of the brain that have an increased probability of cavitation events. Results: For an equivalent energy deposition rate, it is shown that at a low frequency there is a significant volume of the brain that is above the MI thresholds. At a high frequency, the volume is over 3 orders of magnitude smaller, and it is entirely confined to a compact focal spot. Conclusions: The significant frequency dependence of the volumes with an increased probability of cavitation can be attributed to two factors: First, the volume encompassed by the focal region depends on the cube of the frequency. Second, the heat deposition increases with frequency. In conclusion, according to these simulations, the acoustic environment during HIFU brain therapy at 1 MHz is not conducive to a high probability of cavitation in extended regions of the brain. © 2012 American Association of Physicists in Medicine.

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