Articles 2005

Résumé: Three-dimensional numerical simulations of ultrasound transmission were performed through 31 trabecular bone samples measured by synchrotron microtomography. The synchrotron microtomography provided high resolution 3D mappings of bone structures, which were used as the input geometry in the simulation software developed in our laboratory. While absorption (i.e. the absorption of ultrasound through dissipative mechanisms) was not taken into account in the algorithm, the simulations reproduced major phenomena observed in real through-transmission experiments in trabecular bone. The simulated attenuation (i.e. the decrease of the transmitted ultrasonic energy) varies linearly with frequency in the MHz frequency range. Both the speed of sound (SOS) and the slope of the normalized frequency-dependent attenuation (nBUA) increase with the bone volume fraction. Twenty-five out of the thirty-one samples exhibited negative velocity dispersion. One sample was rotated to align the main orientation of the trabecular structure with the direction of ultrasonic propagation, leading to the observation of a fast and a slow wave. Coupling numerical simulation with real bone architecture therefore provides a powerful tool to investigate the physics of ultrasound propagation in trabecular structures. As an illustration, comparison between results obtained on bone modelled either as a fluid or a solid structure suggested the major role of mode conversion of the incident acoustic wave to shear waves in bone to explain the large contribution of scattering to the overall attenuation. © 2005 IOP Publishing Ltd.

Résumé: A novel laser-based ultrasonic technique for the inspection of thin plates and membranes is presented, in which a modulated continuous-wave laser source is used to excite narrow bandwidth Lamb waves. The dominant feature in the acoustic spectrum is a sharp resonance peak that occurs at the minimum frequency of the first-order symmetric Lamb mode, where the group velocity of the Lamb wave goes to zero while the phase velocity remains finite. Experimental results with the laser source and receiver on epicenter demonstrate that the zero-group velocity resonance generated with a low-power modulated excitation source can be detected using a Michelson interferometer coupled to a lock-in amplifier. This resonance peak is sensitive to the thickness and mechanical properties of plates and may be suitable, for example, for the measurement and mapping of nanoscale thickness variations. © 2005 American Institute of Physics.

Résumé: [1] Time reversal (TR) communication in various configurations (single input, single output (SISO); multiple inputs, single output (MISO); or multiple inputs, multiple outputs (MIMO)) is studied. In particular, we report an experimental demonstration of time reversal focusing with electromagnetic waves in a SISO scheme. An antenna transmits a 1 μs electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity. A spatial focusing of the compressed pulse is also shown. This experiment is the first step for a communication scheme based on time reversal. It would be very interesting for ultrawideband communication in complex media since TR would permit compensation for delay spreading. MISO and MIMO TR communications are discussed on the basis of small-scale experiments with ultrasound. In particular, the binary error rate of the method is studied as a function of both data rate and external noise. A simple theoretical approach explains the results. Copyright 2005 by the American Geophysical Union.

Résumé: Strong interest has been shown on the propagation of transient ultrasonic waves inside a high reverberating cavity filled with moving scatterers. A diffuse model theoretically justifies that the elastic cross section and the dynamic of the scatterers can be deduced from field correlations. Recently, experiments have been performed to also obtain the inelastic cross section of the scatterers. In this letter, we propose justifying this extension within the diffuse model. Experimental results obtained inside a 1.5 liter reverberant water tank at 900 kHz central working frequency with spheres made of materials of different absorption are presented. © 2005 American Institute of Physics.

Résumé: Ruthenium can easily be incorporated into Bi12SiO20 (BSO) and an unusually high (1.3 cm-1) photorefractive gain was measured in the diffusion regime with a krypton laser at 647 nm. One particular experiment demonstrated that electron and hole gratings could eventually be formed with different time constants, thus leading to a reduction of the gain. Several complementary spectroscopic techniques were used to characterise our crystals. Magnetic circular dichroism (MCD) demonstrated that Ru substitutes under several valences for Bi in the pseudo-octahedron formed by bismuth and oxygen atoms. This was confirmed by electron paramagnetic resonance detected either classically (EPR at 9 GHz) or optically (ODMR, 35 GHz). Photochromism was investigated via a series of absorption and MCD experiments on oxidized and reduced samples. Our results in the visible spectral range could be understood via a correlation with the behaviour of the Ru5+ and Ru3+ MCD features in the near-IR, under similar illuminations. The primary process with red light is the ionisation of electrons from the VB to the Ru 4+/3+ acceptor level and the subsequent capture of the left holes at the Ru5+/4+ level. Under blue light, paramagnetic Bi Si 4+ is formed via the ionisation of BiSi 3+. The charge transfer transitions of Ru5+ were assigned via additional experiments on Ru-doped lithium niobate and garnets. The build-up and decay of photochromism were investigated, three different behaviours being observed, depending upon the initial conditions. The three-valence-two-level model is not adequate to explain the bi-exponential temporal behaviour of photochromism. It is suggested that a third relatively shallow level, possibly associated to iron, plays an important role. © 2004 Elsevier B.V. All rights reserved.

Résumé: We present a new high speed full-field optical coherence tomography (OCT) instrument, the first full-field OCT system that is capable of in vivo ocular imaging. An isotropic resolution of ∼ 1 μm is achieved thanks to the use of a xenon arc lamp source and relatively high numerical aperture microscope objectives in a Linnik-type interferometer. Full-field illumination allows the capture of two-dimensional en face images in parallel, using a fast CMOS camera as detector array. Each en face image is acquired in a 4 ms period, at a maximum repetition rate of 250 Hz. Detection sensitivity per en face image is 71 dB. Higher sensitivity can be achieved by image correlation and averaging, although frame rate is reduced. We present the first preliminary results of in vivo imaging in the anterior segment of the rat eye, which reveal some cellular features in the corneal layers. © 2005 Optical Society of America.

Résumé: We study the coherent propagation of an elastic wave in a two-dimensional continuous elastic medium filled with dislocation arrays randomly distributed and oriented in space. This configuration reasonably mimics grain boundaries in polycrystals. Interest is in evaluating the plastic contributions to the multiple scattering of waves in polycrystals that may superpose to other known scattering processes, like scattering due to inhomogeneities of elastic properties among grains. Calculations are performed in a multiple scattering formalism, based on the derivation of the so-called mass operator, in the approximation of weak scattering. We find that sound attenuation increases when the frequency decreases, a trend opposite to the usual behavior, suggesting that dislocations could sensibly modify the acoustic properties of materials at low frequency. © 2005 Elsevier B.V. All rights reserved.

Résumé: We present the acoustic analog of resonant tunneling through a double barrier in quantum mechanics. Pairs of identical phononic crystals, in both 2D and 3D, were assembled and separated by a uniform spacer, forming a resonant cavity. The ultrasonic transmission exhibits resonant peaks at frequencies inside the band gaps, where ultrasound tunneling through each phononic crystal occurs. On resonance, the measured group time is large and even predicted to increase exponentially with the thickness of the crystals in the absence of absorption, while off resonance very fast speeds are found. © EDP Sciences.

Résumé: Usually, time reversal is studied with pulsed emissions. Here, the properties of time reversal of the acoustic field emitted by noise sources in a reverberation room are studied numerically, theoretically, and experimentally. A time domain numerical simulation of a two-dimensional enclosure shows that the intensity of a time-reversed noise is strongly enhanced right on the initial source position. A theory based on the link that exists between time reversal of noise and the "well-known" time reversal of short pulse is developed. One infers that the focal spot size equals half a wavelength and the signal to noise ratio only depends on the number of transceivers in the time reversal mirror. This last property is characteristic of the time reversal of noise. Experimental results are obtained in a 5 × 3 × 3 m3 reverberation room. The working frequency range varies from 300 Hz to 2 kHz. The ability of the time reversal process to physically reconstruct the image of two noise sources is studied. To this end, care is given to the technique to separate two close random sources, and also to the influence of temperature fluctuations on the focusing quality. © 2005 Acoustical Society of America.

Résumé: Elastography is a technique to assess the viscoelastic properties of tissue by measuring an acoustic wave propagating though the object. Here, the technique is applied in the course of standard MR mammography to 15 patients with different pathologies (six breast cancer cases, six fibroadenoma cases and three mastopathy cases). Low-frequency mechanical waves are coupled longitudinally into the tissue in order to obtain sufficient wave amplitude throughout the entire breast. This leads to the presence of a substantial fraction of compressional waves, which contribute to the total displacement field. It is shown theoretically that the correct evaluation of these contributions from the compressional wave is rather difficult due to the almost incompressible nature of tissue. To overcome this problem, it is proposed to apply the curl-operator to the measured displacement field in order to completely remove contributions from the compressional wave. Results from simulations and a breast phantom demonstrate the feasibility of the technique. The in vivo results show a good separation between breast cancer and benign fibroadenoma utilizing the shear modulus. Breast cancer appears on average 2.2 (P<.001) times stiffer. All breast cancer cases showed a good delineation to the surrounding breast tissue with an average elevation of a factor of 3.3 (P<1.4×10 -6). The results as obtained for the shear viscosity do not indicate to be useful for separating benign from malignant lesions. © 2005 Elsevier Inc. All rights reserved.

Résumé: MR-elastography is a new technique for assessing the viscoelastic properties of tissue. One current focus of elastography is the provision of new physical parameters for improving the specificity in breast cancer diagnosis. This analysis describes a technique to extend the reconstruction to anisotropic elastic properties in terms of a so-called transversely isotropic model. Viscosity is treated as being isotropic. The particular model chosen for the anisotropy is appealing because it is capable of describing elastic shear anisotropy of parallel fibers. The dependence of the reconstruction on the particular choice of Poisson's ratio is eliminated by extracting the compressional displacement contribution using the Helmholtz-Hodge decomposition. Results are presented for simulations, a polyvinyl alcohol breast phantom, excised beef muscle, and measurements in two patients with breast lesions (invasive ductal carcinoma and fibroadenoma). The results show enhanced anisotropic and viscous properties inside the lesions and an indication for preferred fiber orientation. © 2005 Wiley-Liss, Inc.