Experimental assessment of four ultrasound scattering models for characterizing concentrated tissue-mimicking phantoms

International audienceTissue-mimicking phantoms with high scatterer concentrations were examined using quantitative ultrasound techniques based on four scattering models: the Gaussian Model (GM), the Faran Model (FM), the Structure Factor Model (SFM) and the Particle Model (PM). Experiments were conducted using 10- and 17.5-MHz focused transducers on tissue-mimicking phantoms with scatterer concentrations ranging from 1 to 25%. Theoretical BSCs were first compared with the experimentally measured BSCs in the forward problem framework. The measured BSC versus scatterer concentration relationship was predicted satisfactorily by the SFM and the PM. The FM and the PM overestimated the BSC magnitude at actual concentrations greater than 2.5% and 10%, respectively. The SFM was the model that better matched the BSC magnitude at all the scatterer concentrations tested. Secondly, the four scattering models were compared in the inverse problem framework to estimate the scatterer size and concentration from the experimentally measured BSCs. The FM did not predict the concentration accurately at actual concentrations greater than 12.5%. The SFM and PM need to be associated with another quantitative parameter to differentiate between low and high concentrations. In that case, the SFM predicted the concentration satisfactorily with relative errors below 38% at actual concentrations ranging from 10 to 25%

Non-linear ultrasonic tomography of high-contrasted materials

International audienceThis study focuses on the ultrasonic characterization and imaging of elastic materials like cylinders or tubes. In this case, ultrasonic wave propagation is greatly perturbed by the difference in the acoustic impedance between the scatterer and the surrounding medium (soft tissues, water or coupling gel), which results in considerable parasite events such as the refraction, attenuation and scattering of the waves. The aim of this work is then to solve a non-linear inverse scattering problem. Analytical or algebraic approaches may be applied generally involving in a "classical" problem of minimization of the differences between modeling data and measurements. Several strategies can be used to model the forward problem and to solve the inverse problem simply, efficiently and accurately. The distorted diffraction tomography is an inversion iterative method and belongs to the class of algebraic reconstruction algorithms. This method was developed to increase the order of application of the Born approximation (in the case of weakly contrasted media) to higher orders. The iterations are performed numerically by solving the forward and inverse problems at every iteration after calculating an appropriate Green's function; the previous iteration serves in each case to define the surrounding medium with a variable background. This yields quantitative information about the scatterer, such as the speed of sound and the attenuation. Quantitative ultrasonic imaging techniques of this kind are of great potential value in fields such as medicine, underwater acoustics and non-destructive testing

Active control of scattered acoustic radiation: a real-time implementation for a three-dimensional object

International audienceThis paper presents an active noise control experiment designed to validate a real-time control strategy for reduction of the noise scattered from a three-dimensional body. The control algorithm relies on estimating the scattered noise by linear filtering of the total noise measured around the body; suitable filters are identified from off-line measurements. A modified Filtered-Error Least-Mean-Squares algorithm then leads to the adaptive filters which drive the secondary sources. The paper provides the numerical simulations using a Boundary Element Method which helped in designing a feasible experiment in an anechoic chamber with a limited number of control sources. Eventually a real-time pure-tone implementation with 14 ordinary loudspeakers and a large body is shown to yield on average a 10~dB reduction of the scattered noise at the error sensors, which is close to the optimum reduction predicted by the numerical simulations for the sensor arrangement

Ultrasonic Computed Tomography

Ultrasonic Computed Tomography (UCT) is a full digital imaging technique, which consists in numerically solving the inverse scattering problem associated to the forward scattering problem describing the interaction of ultrasonic waves with inhomogeneous media. For weakly inhomogeneous media such as soft tissues, various approximations of the solution of the forward problem (straight ray approximation, Born approximation...), leading to easy-to-implement approximations of the inverse scattering problem (back-projection or back-propagation algorithms) can be used. In the case of highly heterogeneous media such as bone surrounded by soft tissues, such approximations are no more valid. We present here two non-linear inversion schemes based on high-order approximations. These methods are conceived like the prolongation of the methods implemented in the weakly inhomogeneous case for soft tissues. The results show the feasibility of this UCT approach to bones and its potential to perform measurements in vivo

Distorted Born diffraction tomography: limits and applications to inverse the ultrasonic field scattered by an non-circular infinite elastic tube

International audienceThis study focuses on the application of ultrasonic diffraction tomography to noncircular 2D-cylindrical objects immersed in an infinite fluid. The distorted Born iterative method used to solve the inverse scattering problem be longs to the class of algebraic reconstruction algorithms. This method was developed to increase the order of application of the Born approximation (in the case of weakly-contrasted media) to higher orders. This yields quantitative in formation about the scatterer, such as the speed of sound and the attenuation. Quantitative ultrasonic imaging techniques of this kind are of great potential value in fields such as medicine, under water acoustics and non destructive testing

Two-dimensional ultrasonic computed tomography of growth bone

International audienceUltrasonography is the main first-line imaging technique used to diagnose various pediatric pathologies. Pediatric radiologists are very keen on ultrasonography since it is a non-invasive, non-irradiant, painless, inexpensive imaging modality, which is also practicable on bedside. In bone diseases, this technique has also proved to be a very effective tool to assess congenital (i.e., hip dysphasia), infectious (i.e., sub-periostal abscess), inflammatory (i.e., chronic arthritis), and even traumatic (i.e., ankle sprain) processes. However, with standard devices, this exam is not suitable for diagnosis or monitoring bone tumors, classically explored by other more effective techniques, which presented various disadvantages (cost, irradiation, sedation, accessibility). Many authors have dealt with the ultrasonic imaging of bones. Their main aim has usually been to assess the thickness of the diaphysis and to calculate the speed of sound of a wave crossing the structure. Our group has been focusing on the cross-sectional radial imaging process, using ultrasonic computed tomography. Known to be a potentially valuable method of imaging objects with a similar acoustical impedance, difficulties arise when it is proposed to obtain quantitative tomograms of more highly contrasted media (hard bone tissues). Finding solutions here involves either using non-linear schemes and/or performing extensive studies on the limitation of the initial approximation. In this paper, we recall the limits of the ultrasonic computed tomography when dealing with highly contrasted scatterers, using a high-order tomographic method. The performances and limitations are evaluated in the basis of simulated and experimental data, obtained using an ultrasonic scanner, to one geometrical-mimicking phantom and to a real children fibula. Sample thicknesses and speed-of-sound varied as a function of radial position and the experimental procedure fulfilled the criteria of the diffraction tomography. Results are promising and suggest that the geometrical and acoustical characteristics of the bones, which are known to be one of the main factors contributing to the growth bone disease, can be determined using this ultrasonic computed tomography

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses

Acoustic signals generated by filamentation of ultrashort TW laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.Comment: 9 pages, 12 figure

Utilisation d'une source laser pulsée à haute energie comme source acoustique large bande en milieu liquide Source acoustique générée par un laser pulsé intense

International audienceIn this work the non-linear opto-acoustic problem which consists in generating an acoustic signal in water from an intense ultra short laser pulse has been studied. The acoustic source obtained could be related to the phenomenon of filamentation which produces a contraction of the initial beam accompanied by the formation of plasma. Relatively recent work has shown that lasers of this type could be used to produce remote acoustic sources with interesting applications to underwater acoustics. The spectrum of the sound source obtained was investigated and its directivity pattern in both planes (plane of the filament and plane 96 TS. Volume 33 – n° 1/2016 perpendicular to the filament) was measured. The sound level of the source as a function of energy, duration, and wavelength of the laser pulse was also measured.Ce travail est relatif à l'étude expérimentale d'un problème d'opto-acoustique non linéaire, consistant à générer un signal acoustique dans l'eau à partir d'un laser pulsé térawatt (TW). La source acoustique obtenue a pu être reliée au phénomène de filamentation qui produit une contraction du faisceau initial, accompagnée de la formation d'un plasma. Des travaux relativement récents ont montré que les lasers de ce type pouvaient être utilisés pour produire des sources acoustiques déportées, avec des applications intéressantes pour l'acoustique sous-marine. Le spectre de la source acoustique obtenue a été étudié à l'aide de plusieurs hydrophones couvrant une très large bande de fréquence, et le diagramme de directivité a été mesuré dans deux plans (plan du filament et plan perpendiculaire au filament). Le niveau acoustique de la source en fonction de l'énergie, de la durée, et de la longueur d'onde de l'impulsion laser, a également été étudi