Stéphane G. Conti

Passive in vivo elastography from skeletal muscle noise

Measuring the in vivo elastic properties of muscles (e.g., stiffness) provides a means for diagnosing and monitoring muscular activity. The authors demonstrated a passive in vivo elastography technique without an active external radiation source. This technique instead uses cross correlations of contracting skeletal muscle noise recorded with skin-mounted sensors. Each passive sensor becomes a virtual in vivo shear wave source. The results point to a low-cost, noninvasive technique for monitoring biomechanical in vivo muscle properties. The efficacy of the passive elastography technique originates from the high density of cross paths between all sensor pairs, potentially achieving the same sensitivity obtained from active elastography methods.

Validation of the stochastic distorted-wave Born approximation model with broad bandwidth total target strength measurements of Antarctic krill

Total-scattering cross-sections (σ t ) of Antarctic krill (Euphausia superba) were measured over a broad bandwidth (36-202 kHz) using a new technique based on acoustical reverberation in a cavity. From 18 February to 9 March 2002, mean total target strengths (TTS = 10 log(σ t /4π)), were measured from groups of 57-1169 krill (average standard length=31.6 mm; standard deviation=6.6 mm) at the Cape Shirreff field station, Livingston Island, Antarctica, and aboard RV “Yuzhmorgeologiya”. Chirp pulses were transmitted sequentially by an omni-directional emitter into one of three glass carboys containing groups of krill swimming in 9.3, 19.3, or 45.9 liters of seawater (0.6°C≤temperature≤4.0°C). Between each pulse the krill moved within the fixed-boundary tank and the modulated reverberations were sensed bi-statically with three omni-directional receivers. At each center frequency (f c ), the coherent energy in 200-pulse ensembles identified sound scattered by the tank. The incoherent energy described total sound scattering from the krill. Thus, the TTS at each f c was extracted from a correlation analysis of energy reverberated in the tank. Measurement bias was determined to be ±0.4 dB from an experiment using metal sphere reference targets, and the precision was estimated as ±0.8 dB from the variability in the krill TTS (f c ) measurements. The empirical estimates of mean σ t corroborated a krill-scattering model based on the distorted-wave Born approximation (DWBA), enhanced by the authors to account for the stochastic nature of sound scattering (SDWBA), integrated over all scattering angles and averaged over all incident angles (SDWBA TTS ). The SDWBA, solved for target strength of Antarctic krill, may be the best predictor of backscatter for this important species and may also provide backscattering spectra for improving their acoustic identification. These advances may help to reduce uncertainty in krill-biomass estimation using multi-frequency echosounder data and echo-integration methods.

Reconciling theoretical versus empirical target strengths of krill: effects of phase variability on the distorted-wave Born approximation

A model was recently proposed to predict the target strengths (TS) of Antarctic krill, Euphausia superba, versus incidence angle (θ) (Deep Sea Res. II 45(7) (1998) 1273). Based on the distorted-wave Born approximation (DWBA), the model depends on the coherent summation of scattering from elements of a discretized-bent cylinder. It was empirically validated at 120 kHz near-broadside incidence (θ≈90°), but large discrepancies were observed at other angles away from the main lobe. As the side-lobe measurements were both higher than the model predictions and above the noise floor, the authors noted that the differences were not entirely due to noise. In this study, the accuracy of the DWBA model is further explored. Results indicate that phase variability in the scatter from elements of a discretized-bent cylinder (krill model) causes a dramatic flattening in the side-lobe regions of TS(θ), while negligibly affecting the main scattering lobe. These results are consistent with the krill TS measurements reported by McGehee et al. Thus, by accounting for phase variability in the solution of the DWBA model, a more accurate and thus practical tool is developed for predicting krill TS.

Wide-bandwidth acoustical characterization of anchovy and sardine from reverberation measurements in an echoic tank

The total-scattering cross-sections (σ t ) of anchovy (Engraulis mordax) and sardine (Sardinops sagax caerulea) were measured acoustically over a wide bandwidth (0.5-202 kHz) from ensembles of reverberation time-series. Measurements were made sequentially in two cylindrical, galvanized-steel tanks containing filtered seawater (21±1°C). The lower-frequency measurements were made from two groups of fish (35 anchovy and 10 sardine) in a 1000-l tank, and those at higher-frequency (≥36 kHz) from 10 individual fish in a 100-l tank for each species. Thus, wide-bandwidth, total target strengths (TTS = 10 log 10 (σ t /4π)) were estimated for multiple sizes of these important pelagic species of the California Current. The TTS frequency dependence (TTS(f)) is significantly different for these two species. For sardine, it first increases and then decreases over the frequency range, whereas for anchovy it increases monotonically with frequency. Moreover, at 38, 70, 120, and 200 kHz the variations of TTS with fish length and weight were markedly non-linear. Empirical estimates of TTS were statistically compared with theoretical predictions derived using the Kirchhoff ray-mode model. When surveying with echo-integration methods these measurements should be useful in the acoustical identification and classification of anchovy and sardine and for estimating their sizes.

Near-field time-reversal amplification

The spatial resolution of the focused field of a classical time-reversal mirror has a wavelength-order lambda diffraction limit. Previously reported results for subwavelength focus require either the full knowledge of the original source or the evanescent waves in the near field. Here it is shown that subwavelength focusing can be achieved without a priori knowledge of the original probe source. If the field is recorded at a few wavelengths away from the probe source, where the amplitude of the near field is too low for subwavelength focusing, it is shown that the low amplitude near field can be amplified and the spatial resolution improved, using the near-field time reversal (NTR) procedure introduced here. The NTR is performed from the phase of the spatial spectrum of the field recorded on an array around the original probe source using an analytical continuation for the amplitude of the spatial spectrum. Following theory, lambda/20 resolution is experimentally demonstrated with audible acoustic wavefields in the air.

Absolute measurements of total target strength from reverberation in a cavity

A new method was developed to acoustically measure the density and total scattering cross-section (sigma(t)) or total target strength [TTS = 10log10(sigma(t)/4pi)] of objects in motion in a highly reflective cavity [J. De Rosny and P. Roux, J. Acoust. Soc. Am. 109, 2587-2597 (2001)]. From an ensemble of pulse-echo recordings, the average contribution of the scatterer(s) to the reverberation within the cavity provides a measurement of the scattering mean free path. The latter was shown through theory and experiment to be proportional to the volume of the cavity and inversely proportional the product of the mean sigma(t) and number of scatterers. Here, the TTS measurement uncertainty is characterized using standard metal spheres as references. Theoretical TTS was calculated for multiple copper and tungsten carbide standard spheres (Cu: 60.0 30.05 and 23 mm and WC: 38.1 and 33.4 mm diameters, respectively), using well-described theory for scattering from elastic spheres and the optical theorem. Measurements of TTS were made over a wide bandwidth (30-120 kHz) and compared to their theoretical values. Measurements were made in a corrugated, cylindrical, galvanized-steel tank with 25 or 50 l of fresh water at a temperature of 21 +/- 1 degrees C. The results indicate the method can provide TTS measurements that are accurate to at least 0.4 dB with an average precision of +/-0.7 dB (95% confidence interval). Discussed are the requisite cavity volumes and signal-to-noise ratios for quality measurements of TTS, tank volume, and/or numerical abundance of mobile targets. Also discussed are multiple potential applications of this technique in bioacoustical oceanography.

Experimental demonstration of a high-frequency forward scattering acoustic barrier in a dynamic coastal environment

Detecting a target by measuring its forward scattered field is of interest for harbor surveillance because target strength levels are generally higher in the forward direction than in the backward direction for simple geometries. An acoustic barrier based on forward scattering was demonstrated in a nearly range-independent shallow water environment. The experimental location was characterized by high reverberation, low temporal signal coherence, and, as a result, few stable multipath arrivals due to the fluctuating sea surface. This high-frequency experiment utilized a vertical source array, broadcasting broadside pulses, and a vertical receiver array spanning the water column. The signal of interest was the aberration (in space and time) caused by the acoustic forward scattering field of crossing targets (2-m-long aluminum cylinder, 1-m-diameter steel sphere and pair of scuba tanks). Hence, the spatial and temporal coherence of the recorded acoustic signals was first investigated to assess the stability of the early acoustic arrivals in this rapidly fluctuating coastal environment. A principal component analysis of the stable portion of the recorded acoustic signals was then used to determine the crossing time of the target and to isolate some of its scattered wavefield components.

Measurement of the scattering and absorption cross sections of the human body

Presented here are absolute measurements of the acoustic intensity scattered and absorbed by humans. The total scattering and absorption cross sections, σT and σa, were obtained for individual humans walking randomly in a room, using long-duration acoustic reverberation. Within the audible range, the sound scattering spectra of the human body is similar to that of a hard ellipsoid with same volume (dimensions proportional to the mass to the one-third power). Moreover, increasing amounts of clothing have little effect on scattering while absorption is greatly increased.

Depth and range shifting of a focal spot using a time-reversal mirror in an acoustic waveguide

We present a technique based on time reversal to focus an acoustic field at any depth and range in a waveguide. We take advantage of the signal received from an acoustic beacon in the guide to build a time-reversed modified version of the field that refocuses at a different point in the waveguide. This method is based on the application of the images theorem in the guide and, as in the classical time-reversal experiment, it does not require a priori knowledge of the waveguide characteristics. Ultrasonic laboratory experiments and underwater acoustic simulations are presented and compared to classical time-reversal focusing results.

Diffuse reverberant acoustic wave spectroscopy with absorbing scatterers

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.