Ultrafast ultrasonic imaging coupled to rheometry: principle and illustration.

We describe a technique coupling standard rheology and ultrasonic imaging with promising applications to characterization of soft materials under shear. Plane wave imaging using an ultrafast scanner allows to follow the local dynamics of fluids sheared between two concentric cylinders with frame rates as high as 10 000 images per second, while simultaneously monitoring the shear rate, shear stress, and viscosity as a function of time. The capacities of this "rheo-ultrasound" instrument are illustrated on two examples: (i) the classical case of the Taylor-Couette instability in a simple viscous fluid and (ii) the unstable shear-banded flow of a non-Newtonian wormlike micellar solution.

A passive inverse filter for Green's function retrieval.

Passive methods for the recovery of Green's functions from ambient noise require strong hypotheses, including isotropic distribution of the noise sources. Very often, this distribution is nonisotropic, which introduces bias in the Green's function reconstruction. To minimize this bias, a spatiotemporal inverse filter is proposed. The method is tested on a directive noise field computed from an experimental active seismic data set. The results indicate that the passive inverse filter allows the manipulation of the spatiotemporal degrees of freedom of a complex wave field, and it can efficiently compensate for the noise wavefield directivity.

Passive elastography: shear-wave tomography from physiological-noise correlation in soft tissues.

Inspired by seismic-noise correlation and time reversal, a shear-wave tomography of soft tissues using an ultrafast ultrasonic scanner is presented here. Free from the need for controlled shear-wave sources, this passive elastography is based on Green's function retrieval and takes advantage of the permanent physiological noise of the human body.

Coherent backscattering enhancement in cavities. Highlights of the role of symmetry.

Through experiments and simulations, the consequences of symmetry on coherent backscattering enhancement (CBE) are studied in cavities. Three main results are highlighted. First, the CBE outside the source is observed: (a) on a single symmetric point in a one-dimensional (1-D) cavity, in a disk and in a symmetric chaotic plate; (b) on three symmetric points in a two-dimensional (2-D) rectangle; and (c) on seven symmetric points in a three-dimensional (3-D) parallelepiped cavity. Second, the existence of enhanced intensity lines and planes in 2-D and 3-D simple-shape cavities is demonstrated. Third, it is shown how the anti-symmetry caused by the special boundary conditions is responsible for the existence of a coherent backscattering decrement with a dimensional dependence of R = (½)(d), with d = 1,2,3 as the dimensionality of the cavity.

Electromechanical delay in biceps brachii assessed by ultrafast ultrasonography.

Using ultrasound we tested the utility of determining the relative contribution of the main muscle structures/mechanisms to the electromechanical delay in the biceps brachii. Nine subjects underwent electrically evoked contractions with the echographic probe maintained over the muscle and the myotendinous junction. No difference was found between the onset of muscle fascicle motion (Dm, 5.57 ± 1.37 ms) and the onset of myotendinous junction motion (Dt, 5.47 ± 1.38 ms), whereas significant differences were found between Dm/Dt and electromechanical delay (approximately 10 ms). Electromechanical delay can be used as a model for studying the effects of neuromuscular disorders or various constraints that affect excitation-contraction coupling and/or muscle force transmission.

1-D elasticity assessment in soft solids from shear wave correlation: the time-reversal approach.

One-channel time-reversal (TR) experiments allow focalization of waves in reverberant cavities. According to the Rayleigh criterion, the focal spot width is directly related to the wavelength and therefore depends on the mechanical properties of the medium. Thus, the general idea of this work is to extract quantitative estimations of these mechanical properties using a time-reversal approach based on cross-correlations of the wave field. An external source creates mechanical waves in the audible frequency range. One component of the vectorial field is measured along a line as function of time with signal processing developed in the field of 1-D elastography. The shear wavelength information is deduced from these mechanical waves using spatiotemporal correlations and interpreted in the frame of the time-reversal symmetry. The impact of wave attenuation in soft solids is reduced using a spatial average of the correlation field. The result is shown to be suitable for global elasticity estimation. The advantage is that the technique is almost independent of the source kind, shape, and time excitation function. This robustness as regard to shear wave source allows translation of this technique to applications in the medical field, including deep or moving organs.

Electromechanical delay revisited using very high frame rate ultrasound.

Electromechanical delay (EMD) represents the time lag between muscle activation and muscle force production and is used to assess muscle function in healthy and pathological subjects. There is no experimental methodology to quantify the actual contribution of each series elastic component structures that together contribute to the EMD. We designed the present study to determine, using very high frame rate ultrasound (4 kHz), the onset of muscle fascicles and tendon motion induced by electrical stimulation. Nine subjects underwent two bouts composed of five electrically evoked contractions with the echographic probe maintained over 1) the gastrocnemius medialis muscle belly (muscle trials) and 2) the myotendinous junction of the gastrocnemius medialis muscle (tendon trials). EMD was 11.63 +/- 1.51 and 11.67 +/- 1.27 ms for muscle trials and tendon trials, respectively. Significant difference (P < 0.001) was found between the onset of muscle fascicles motion (6.05 +/- 0.64 ms) and the onset of myotendinous junction motion (8.42 +/- 1.63 ms). The noninvasive methodology used in the present study enabled us to determine the relative contribution of the passive part of the series elastic component (47.5 +/- 6.0% of EMD) and each of the two main structures of this component (aponeurosis and tendon, representing 20.3 +/- 10.7% and 27.6 +/- 11.4% of EMD, respectively). The relative contributions of the synaptic transmission, the excitation-contraction coupling, and the active part of the series elastic component could not be directly quantified with our results. However, they suggest a minor role of the active part of the series elastic component that needs to be confirmed by further experiments.