Régine Guillermin

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

Tissue-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 backscatter coefficients (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. Second, 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%.

Scattering by an elastic sphere embedded in an elastic isotropic medium

The scattering of acoustic waves by an elastic sphere embedded in an elastic isotropic medium is investigated. Expressions for the scattered waves are given in terms of monostatic and bistatic scattering cross sections. The resonances of the solid sphere were determined numerically in the individual normal mode amplitudes; dispersion curves for the phase velocities of the circumferential waves were also obtained. Computations and experimental results for an aluminum sphere embedded in Plexiglas were in good agreement.

Distorted born diffraction tomography applied to inverting ultrasonic field scattered by noncircular infinite elastic tube

This 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 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. 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 nondestructive testing.

Inversion of synthetic and experimental acoustical scattering data for the comparison of two reconstruction methods employing the Born approximation.

This work is concerned with the reconstruction, from measured (synthetic or real) data, of a 2D penetrable fluid-like object of arbitrary cross-section embedded in a fluid of infinite extent and insonified by a plane acoustic wave. Greens theorem is used to provide a domain integral representation of the scattered field. The introduction therein of the Born approximation gives rise to a linearized form of the inverse problem. The actual inversion is carried out by two methods. The first diffraction tomography (DT), exhibits the contrast function very conveniently and explicitly in the form of a wave number/incident angle Fourier transform of the far backscattered field and thus requires measurements of this field for incident waves all around the object and at all frequencies. The second discretized domain integral equation with Born approximation method, is numerically more intensive, but enables a wider choice of configurations and requires less measurements (one or several frequencies, one or several incident waves, choice of measurement points) than the DT method. A comparison of the two methods is carried out by inversion of both simulated and experimental scattered field data.

Imaging an object buried in the sediment bottom of a deep sea by linearized inversion of synthetic and experimental scattered acoustic wavefields

This paper is concerned with the reconstruction, from measured (synthetic and experimental) data, of a 2D penetrable fluid-like cylindrical object of arbitrary cross-section imbedded in a fluid-like (sediment) half-space separated by a plane interface from another fluid half-space (deep water) wherein propagates a plane acoustic interrogating wave. The Green theorem is used to provide (1) a domain integral representation (DIR) of the scattered field and (2) a domain integral equation (DIE) for the pressure field in a test region containing the object. Both the DIE and DIR are discretized by collocation, thereby leading to a linear system of equations for the discretized pressure in the test region and a linear transform for the discretized pressure outside the test region. This is the means adopted herein for generating synthetic scattered field data. The inverse problem is linearized by replacing the (unknown) field in the test region by the (known) field which is established in the water/sediment system in the absence of the object. Using this Born approximation and minimizing the discrepancy between the measured and model scattered fields gives rise to a linear system of equations for the (unknown) discretized index-of-refraction contrast function in the test region. Due to its ill conditioned nature, the linear system is solved by a singular value decomposition technique. Images of the index-of-refraction contrast representation of the object obtained by inversion of both simulated and experimentally measured scattered field data are presented and compared.

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses

Acoustic signals generated by filamentation of ultrashort terawatt 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.

Underwater acoustic signals induced by intense ultrashort laser pulse

Acoustic signals generated in water by terawatt (TW) laser pulses undergoing filamentation are studied. The acoustic signal has a very broad spectrum, spanning from 0.1 to 10 MHz and is confined in the plane perpendicular to the laser direction. Such a source appears to be promising for the development of remote laser based acoustic applications.

High-Frequency Sound Reflection by Water-Saturated Sediment Interfaces

Sound reflection by water-saturated sands and glass beads with a flattened surface was studied under controlled laboratory conditions in a wide frequency range, from 200 kHz to 7 MHz. In the ¿low-frequency¿ domain and in the case of medium sand, the reflected sound level was found to be in good agreement with both classical sonar measurements and classical theories of reflection developed for fluid porous media (this reflected level is practically independent of the frequency); as the frequency increases, a large decrease in the reflected level occurs, possibly due to incoherent scattering. In the very high-frequency regime (>3 MHz), the sound level measured was more than 20 dB below the classical level, and it remained constant at higher frequencies. Similar experiments were carried out with coarse sand to study the effect of grain size on the reflection loss. The same behavior with only a frequency shift was observed. These effects were confirmed by repeating the experiments with glass beads of two sizes. The anomalies observed in the reflected levels measured seem to be directly connected to the ratio between the grain size and the wavelength. One of the main conclusions reached in this study was that for very coarse sand and gravels, the effects of the granular structure of the bottom should not be neglected, even at the usual sonar frequencies.

P3D-4 Two-Dimensional Ultrasonic Computed Tomography of Growing Bones

This study deals with the 2-D ultrasonic qualitative and quantitative imaging of child bone. The inverse problem is linearly and non-linearly solved via a Born-based procedure involving minimization of the discrepancies between measurements and modeling data. Inversions of experimental data are presented.

Laboratory study of high‐frequency scattering from water‐saturated granular sediments

Sound backscattering and reflection from water‐saturated granular sediments at frequencies from 200 kHz to 7 MHz were studied in controlled laboratory conditions. Two kinds of well‐sorted sandy sediments, fine and coarse sands, and two kinds of glass beads with corresponding sizes were chosen for the study. The two types of sand had narrow log‐normal size distributions of particles with the mean diameters 0.25 and 1.5 mm for fine and coarse sand, respectively. The sediments were degassed and their surface was flattened carefully. In these conditions, the grain‐scattering mechanism can be considered as a dominating factor controlling incoherent component of the field scattered from the sediment. Frequency dependencies for the backscattering strength at various grazing angles and the reflection coefficient at normal incidence were measured. The effects related to the sediment grain size are analyzed and their possible applications to remote sensing of marine sediments are discussed. [Work supported by ONR a...