Sylvain Yon

Sound focusing in rooms: The time-reversal approach

New perspectives in audible range acoustics, such as virtual sound space creation and active noise control, rely on the ability of the rendering system to recreate precisely a desired sound field. This ability to control sound in a given volume of a room is directly linked to the capacity to focus acoustical energy both in space and time. However, sound focusing in rooms remains a complicated problem, essentially because of the multiple reflections on obstacles and walls occurring during propagation. In this paper, the technique of time-reversal focusing, well known in ultrasound, is experimentally applied to audible range acoustics. Compared to classical focusing techniques such as delay law focusing, time reversal appears to considerably improve quality of both temporal and spatial focusing. This so-called super-resolution phenomenon is due to the ability of time reversal to take into account all of the different sound paths between the emitting antenna and the focal point, thus creating an adaptive spatial and temporal matched filter for the considered propagation medium. Experiments emphasize the strong robustness of time-reversal focusing towards small modifications in the medium, such as people in motion or temperature variations. Sound focusing through walls using the time-reversal approach is also experimentally demonstrated.

Sound focusing in rooms. II. The spatio-temporal inverse filter.

The potential of time reversal processing for room acoustics has been extensively investigated in the companion of this paper [J. Acoust. Soc. Am. 113(3), 1533-1543 (2003)]. In particular, a simple implementation of a loudspeaker time reversal antenna able to take advantage of the multiple reflections in reverberating rooms demonstrates its potential for audible range acoustics while improving focusing both in space and time. However, loss of information (e.g., sound absorption in walls or nonequalized bandwidths of the loudspeakers) during a time reversal experiment degrades the quality of time reversal focusing. In this paper, a more sophisticated technique called spatio-temporal inverse filtering is investigated that achieves time and space deconvolution of the propagation operator between the loudspeakers antenna and a set of microphones embedded inside the insonified volume. Theoretical and experimental comparisons between time reversal and inverse filter focusing are presented. Finally, advantages and limitations of both focusing approaches are highlighted.

Transient elastography: changing clinical practice in hepatology

Transient elastography is one of several new approaches that have recently been proposed to manage liver diseases. This quantitative method is used in clinical practice to assess liver fibrosis noninvasively and rapidly. The technique consists in generating low‐frequency elastic shear waves through the liver and measuring their velocity using ultrafast pulse‐echo ultrasound acquisition. Liver stiffness is directly related to shear waves velocity. Nowadays more than 400 devices (Fibroscan®, Echosens, Paris, France) are being used worldwide in clinical practice. Studies reported a strong correlation between liver stiffness and the fibrosis stage obtained by liver biopsy in patients with chronic liver diseases: hepatitis B and/or C, HCV co‐infection, HIV, alcoholism, etc. Stiffness measurements obtained using transient elastography are in good agreement with measurements performed using other elastography techniques (magnetic resonance elastography and radiation force). Main limitations of transient elastogr...

Acoustic cavitation sub-millisecond signal processing for ultrasound surgery

Cavitation in Focused Ultrasound Surgery (FUS) is a stochastic phenomenon that can evolve very rapidly at the high intensities typically employed. Cavitation activity can be monitored analyzing its acoustic signals in the frequency domain. Cavities act as strong scatterers of incident acoustic energy, potentially enhancing different therapeutic effects, e.g., thermal or mechanical, if properly controlled. We report on the development on an instrument capable of controlling acoustic cavitation at sub-millisecond speed. The cavitation noise signal has been recorded and processed by an FPGA coupled with a feedback loop, capable of modulating the driving field every few hundreds of microseconds to maintain an ideal level of cavitation activity and differentiate between several types of cavitation and the onset of boiling. The system has been tested in tissue phantom, ex-vivo tissue and in-vivo and the impact on lesion sizes has been analysed. We also used the experimental data as input for a non-linear simula...

Characterization of HIFU ablation using DNA fragmentation labeling as apoptosis stain

The goal of this work was to compare modalities to precisely quantify the extent of thermally induced lesions: gross pathology vs. histopathology vs. devascularization. Liver areas of 14 rabbits were targeted with HIFU and RF ablations in an acute study. Contrast enhanced computorized tomography (CE-CT) scan images were acquired two hours after HIFU and RF treatment to obtain the devascularized volumes of the livers. The animals were then euthanized and deep frozen. The livers were sliced and each slice was photographed and stacked yielding a volume of gross pathology. The volume VGP of the HIFU lesions were derived. The area AGP of the lesions were computed on a particular slice. The lesions were segmented as hypo intense (devascularized) regions on CE-CT images and their volumes VC were computed. The ratios VC/VGP were computed for all the HIFU lesions on all the 14 subjects with a mean value of 1.2. Histology was performed on the livers using Hematoxyline Eosine Staining (HES) and DNA Fragmentation lab...

Sound focusing in reverberating rooms: The time‐reversal approach

Time reversal is already known as an efficient ultrasonic method to focus through inhomogeneous or multidiffusive media. The presence of pointlike reflectors in these media allows applications of this technique in fields like nondestructive testing, medical imaging, or underwater acoustics. An extension of this technique in audible range acoustics is presented. An array of 70 microphone/loudspeaker couples is used to refocus sound inside of a reverberating room. At a desired focal point, directivity patterns are measured and compared with those obtained by focusing with a cylindrical beamforming technique through the same antenna. Time reversal is shown to strongly improve the focal spot pattern (beamwidth and sidelobes level). These results are related to the ability of time reversal to compensate for reverberation and scattering induced in the studied room. The process acts as a spatio‐temporally matched filter to the propagation transfer function of the desired focal point through the room. It makes this technique an auto‐adaptive focusing system to a random 3‐D cavity. Theory, experimental results, and future applications will be described.