Jean-Pierre Remenieras

Exploration of trabecular bone nonlinear elasticity using time-of-flight modulation

Bone tissue contains microcracks that may affect its mechanical properties as well as the whole trabecular structure. The relationship between crack density and bone strength is nevertheless poorly understood. Linear ultrasound techniques being almost insensitive to the level of damage, we propose a method to measure acoustic non- linearity in trabecular bone using time-of-flight modulation (TOFM) measurements. Ultrasonic short bursts times-of- flight (TOF) are modulated as a result of nonlinear interaction with a low-frequency (LF) wave in the medium. TOF variations are directly related to elastic modulus variations. Classical and nonclassical nonlinear parameters beta, delta, and alpha can be derived from these measurements. The method was validated in materials with classical, quadratic, nonlinear elasticity. In dense trabecular bone region, TOFM related to classical, quadratic, nonlinear elasticity as a function of the LF pressure exhibits tension-compression asymmetry. The TOFM amplitude measured in dense areas of trabecular bone is almost one order of magnitude higher than in a low-density area, but the linear parameters show much smaller variations: 5% for ultrasound propagation velocity and 100% for broadband ultrasonic attenuation (BUA). In high-density trabecular bone regions, beta depends on the LF pressure amplitude and can reach 400 at 50 kPa.

Temporal analysis of tissue displacement induced by a transient ultrasound radiation force

One of the stress sources that can be used in dynamic elastography imaging methods is the acoustic radiation force. However, displacements of the medium induced by this stress field are generally not fully understood in terms of spatial distribution and temporal evolution. A model has been developed based on the elastodynamic Greens function describing the different acoustic waves generated by focused ultrasound. The function is composed of three terms: two far-field terms, which correspond to a purely longitudinal compression wave and a purely transverse shear wave, and a coupling near-field term which has a longitudinal component and a transverse component. For propagation distances in the shear wavelength range, the predominant term is the near field term. The displacement duration corresponds to the propagation duration of the shear wave between the farthest source point and the observation point. This time therefore depends on the source size and the local shear modulus of the tissue. Evolution of the displacement/time curve profile, which is directly linked to spatial and temporal source profiles, is computed at different radial distances, for different durations of force applications and different shear elastic coefficients. Experimental results performed with an optical interferometric method in a homogeneous tissue-mimicking phantom agreed with the theoretical profiles.

High-frequency estimation of the ultrasonic attenuation coefficient slope obtained in human skin: simulation and in vivo results.

In vivo ultrasonic characterization of the skin was performed at 40 MHz by estimating the slope of the attenuation coefficient in the human dermis. The centroid algorithm was first tested on simulated backscattered RF lines with a second-order autoregressive model to carry out the spectral analysis. A relative error of less than 8.5% and a relative precision of less than 6% were predicted for a 2-mm tissue thickness and for temporal window sizes ranging from 0.25 to 0.45 micros. In vivo measurements performed on 138 healthy volunteers yielded values of the attenuation coefficient slope ranging from 0.8 to 3.6 dB/cm MHz. A decrease was observed with advancing age, but no significant difference appeared between men and women. The results from this study suggest that this acoustic parameter shows the effect of the ageing process on normal skin tissue in vivo.

Methodology for developing a high-precision ultrasound flow meter and fluid velocity profile reconstruction

This article reports the methodology used to develop a high-precision ultrasound transit time flow meter dedicated to liquid hydrocarbons. This kind of flow meter is designed for custody transfer applications requiring accuracy better than 0.15% of reading. We focus here on certain specific points to achieve this accuracy. The transit time method needs to estimate accurately the time delay between signals received by a pair of transducers. In this study, we review different ways of estimating this time delay. We also propose a specific configuration of the flow meter paths. In particular, this configuration compensates for the swirl phenomenon, which has a significant impact on the accuracy of the flow meter. We also propose a theoretical parametric profile to reconstruct the fluid velocity profile in order to perform in situ diagnosis of the flow. The parameters of the model are estimated from the measurements of the flow meter. Simulations and experimental results showed that this method provides characterization of the flow in disturbed and undisturbed flow conditions.

High resolution processing techniques for ultrasound Doppler velocimetry in the presence of colored noise. II. Multiplephase pipe-flow velocity measurement

For pt.I see ibid., vol.50, no.3, p.267-78 (2003). This paper presents an application of continuous wave ultrasound Doppler velocity measurements to two-phase flow in pipes. In many petroleum wells, the multiphase flow is separated into two phases: the first is a liquid phase and the second is a gas phase with small scatterers. The problem of multiphase velocity profile measurements has not been satisfactorily solved by classical approaches due to the multiphase nature of the fluid and the presence of colored noise, which introduces a significant bias in classical frequency estimators. We propose the use of resolution frequency techniques to overcome the classical limitations. Direct estimation of Doppler frequency then obtained using either time frequency maximum frequency or arguments of poles of the parametric model that identifies the Doppler part of the signal is discussed. The tests made with synthetic Doppler signals and two-phase flow have demonstrated the excellent performance of the high resolution techniques based on reassignment and parametric techniques.

Transit time ultrasonic flowmeter : velocity profile estimation

Over the last years, improvements in acoustics and signal processing allowed to measure the flow rate with ultrasonic flowmeter at a very high accuracy. Transit time ultrasonic method is based on a well known principle: let A and B be 2 locations of transducers on each side of a pipe, then the apparent difference of the sound speed on the path AB and on the path BA is proportional to the fluid velocity averaged over the path. The estimation of the flow rate needs the conversion of this path velocity to a velocity averaged over the entire cross-section of the pipe containing the flowing fluid under investigation. Two phenomena have a particularly important impact on flowmeter performance: on the one hand, swirl which is the whole of nonflowing transverse velocities and on the other hand, the fluid velocity profile which can be asymmetric downstream an elbow for example. To date, the correct estimation of the fluid velocity profile and the compensation of swirl is not satisfactory solved. For these purposes we investigated two main directions. To overcome the problem of swirl, we have developed a geometrical configuration of flowmeter paths, which fully compensates for this phenomenon. Concerning the fluid velocity profile, we have defined a parametric model able to describe both symmetric and asymmetric flow velocity profiles. This theoretical parametric model was tested on numerical simulations and validated on data coming from experimental petroleum set up loop. These results show that our approach is a promising way for performance improvement of existing ultrasonic flowmeter accuracy.

Ultrasound Brain Tissue Pulsatility is decreased in middle aged and elderly type 2 diabetic patients with depression.

We used Tissue Pulsatility Imaging (TPI) to compare the Brain Tissue Pulsatility (BTP) in depressed (n=11) and non-depressed (n=13) type-2 diabetic non-demented patients aged 50 years and older. Both maximum and mean BTP were significantly decreased in depressed diabetic subjects compared to non-depressed diabetic subjects.

Simulation of shear wave propagation in a soft medium using a pseudospectral time domain method

Elastography applications require the use of efficient models to simulate the propagation of shear waves in soft media such as human tissues. These models are needed to improve understanding of the measured displacement field, to reconstruct the viscoelasticity of heterogeneous tissues, and to test inversion algorithms. This paper reports a numerical model based on a pseudospectral time domain method developed to simulate shear and compression wave propagation in an axisymmetric heterogeneous viscoelastic medium. This model was adapted to the study of soft tissues where the ratio between the compression and the shear wave velocity was about a thousand and validated in the homogeneous situation by comparison with an analytical model based on elastodynamic Greens functions. Displacements obtained experimentally using transient elastography are presented, compared with simulation results, and discussed.

Ultrasound measurements of brain tissue pulsatility correlate with the volume of MRI white-matter hyperintensity.

White-matter hyperintensity (WMH) is frequently seen in magnetic resonance imaging (MRI), but the complete physiopathology of WMH remains to be elucidated. In this study, we sought to determine whether there is an association between the maximum brain tissue displacement (maxBTD), as assessed by ultrasound, and the WMH, as observed by MRI. Nine healthy women aged 60 to 85 years underwent ultrasound and MRI assessments. We found a significant negative correlation between maxBTD and WMH (ρ = -0.86, P<0.001), suggesting a link between cerebral hypoperfusion and WMH.

Estimation of the correlation amplitude of rf signals: application to microcirculation study

For the measurement of blood flow, signal processing based on temporal shift evaluation by cross-correlation of the RF signals is now widely used. The estimation of the normalized correlation coefficient is becoming a complementary tool. In the case of microcirculation (low flow rate, low SNR), the amplitude of the correlation peak can be used to detect the presence of blood flow and to discriminate between false and true detections (reliability index). We have numerically evaluated some statistical performances (bias and variance) of the cross-correlation algorithm used as a correlation peak amplitude estimator in severe conditions (short correlation window length, low SNR). Some of these results have been verified with in-vitro experimentation on a microcirculatory phantom and with in-vivo experimentation on the auriculares candales vein of the external ear of a rabbit.