Javier Brum

RSNA/QIBA: Shear wave speed as a biomarker for liver fibrosis staging

An interlaboratory study of shear wave speed (SWS) estimation was performed. Commercial shear wave elastography systems from Fibroscan, Philips, Siemens and Supersonic Imagine, as well as several custom laboratory systems, were involved. Fifteen sites were included in the study. CIRS manufactured and donated 11 pairs of custom phantoms designed for the purposes of this investigation. Dynamic mechanical tests of equivalent phantom materials were also performed. The results of this study demonstrate that there is very good agreement among SWS estimation systems, but there are several sources of bias and variance that can be addressed to improve consistency of measurement results.

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 Greens function retrieval and takes advantage of the permanent physiological noise of the human body.

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.

Integrated Evaluation of Age-Related Changes in Structural and Functional Vascular Parameters Used to Assess Arterial Aging, Subclinical Atherosclerosis, and Cardiovascular Risk in Uruguayan Adults: CUiiDARTE Project

This work was carried out in a Uruguayan (South American) population to characterize aging-associated physiological arterial changes. Parameters markers of subclinical atherosclerosis and that associate age-related changes were evaluated in healthy people. A conservative approach was used and people with nonphysiological and pathological conditions were excluded. Then, we excluded subjects with (a) cardiovascular (CV) symptoms, (b) CV disease, (c) diabetes mellitus or renal failure, and (d) traditional CV risk factors (other than age and gender). Subjects (n = 388) were submitted to non-invasive vascular studies (gold-standard techniques), to evaluate (1) common (CCA), internal, and external carotid plaque prevalence, (2) CCA intima-media thickness and diameter, (3) CCA stiffness (percentual pulsatility, compliance, distensibility, and stiffness index), (4) aortic stiffness (carotid-femoral pulse wave velocity), and (5) peripheral and central pressure wave-derived parameters. Age groups: ≤20, 21–30, 31–40, 41–50, 51–60, 61–70, and 71–80 years old. Age-related structural and functional vascular parameters profiles were obtained and analyzed considering data from other populations. The work has the strength of being the first, in Latin America, that uses an integrative approach to characterize vascular aging-related changes. Data could be used to define vascular aging and abnormal or disease-related changes.

In vivo quantification of the shear modulus of the human Achilles tendon during passive loading using shear wave dispersion analysis.

The shear wave velocity dispersion was analyzed in the Achilles tendon (AT) during passive dorsiflexion using a phase velocity method in order to obtain the tendon shear modulus (C 55). Based on this analysis, the aims of the present study were (i) to assess the reproducibility of the shear modulus for different ankle angles, (ii) to assess the effect of the probe locations, and (iii) to compare results with elasticity values obtained with the supersonic shear imaging (SSI) technique. The AT shear modulus (C 55) consistently increased with the ankle dorsiflexion (N = 10, p < 0.05). Furthermore, the technique showed a very good reproducibility (all standard error of the mean values <10.7 kPa and all coefficient of variation (CV) values ⩽ 0.05%). In addition, independently from the ankle dorsiflexion, the shear modulus was significantly higher in the proximal location compared to the more distal one. The shear modulus provided by SSI was always lower than C55 and the difference increased with the ankle dorsiflexion. However, shear modulus values provided by both methods were highly correlated (R = 0.84), indicating that the conventional shear wave elastography technique (SSI technique) can be used to compare tendon mechanical properties across populations. Future studies should determine the clinical relevance of the shear wave dispersion analysis, for instance in the case of tendinopathy or tendon tear.

Shear elasticity estimation from surface wave: The time reversal approach

In this work the shear elasticity of soft solids is measured from the surface wave speed estimation. An external source creates mechanical waves which are detected using acoustic sensors. The surface wave speed estimation is extracted from the complex reverberated elastic field through a time-reversal analysis. Measurements in a hard and a soft gelatin-based phantom are validated by independent transient elastography estimations. In contrast with other elasticity assessment methods, one advantage of the present approach is its low sound technology cost. Experiments performed in cheese and soft phantoms allows one to envision applications in the food industry and medicine.

A New High-Resolution Spectral Approach to Noninvasively Evaluate Wall Deformations in Arteries

By locally measuring changes on arterial wall thickness as a function of pressure, the related Young modulus can be evaluated. This physical magnitude has shown to be an important predictive factor for cardiovascular diseases. For evaluating those changes, imaging segmentation or time correlations of ultrasonic echoes, coming from wall interfaces, are usually employed. In this paper, an alternative low-cost technique is proposed to locally evaluate variations on arterial walls, which are dynamically measured with an improved high-resolution calculation of power spectral densities in echo-traces of the wall interfaces, by using a parametric autoregressive processing. Certain wall deformations are finely detected by evaluating the echoes overtones peaks with power spectral estimations that implement Burg and Yule Walker algorithms. Results of this spectral approach are compared with a classical cross-correlation operator, in a tube phantom and “in vitro” carotid tissue. A circulating loop, mimicking heart periods and blood pressure changes, is employed to dynamically inspect each sample with a broadband ultrasonic probe, acquiring multiple A-Scans which are windowed to isolate echo-traces packets coming from distinct walls. Then the new technique and cross-correlation operator are applied to evaluate changing parietal deformations from the detection of displacements registered on the wall faces under periodic regime.

Estimation of PSD Shifts for High-Resolution Metrology of Thickness Micro-Changes with Possible Applications in Vessel Walls and Biological Membrane Characterization

Achieving accurate measurements of inflammation levels in tissues or thickness changes in biological membranes (e.g., amniotic sac, parietal pleura) and thin biological walls (e.g., blood vessels) from outside the human body, is a promising research line in the medical area. It would provide a technical basis to study the options for early diagnosis of some serious diseases such as hypertension, atherosclerosis or tuberculosis. Nevertheless, achieving the aim of non-invasive measurement of those scarcely-accessible parameters on patient internal tissues, currently presents many difficulties. The use of high-frequency ultrasonic transducer systems appears to offer a possible solution. Previous studies using conventional ultrasonic imaging have shown this, but the spatial resolution was not sufficient so as to permit a thickness evaluation with clinical significance, which requires an accuracy of a few microns. In this paper a broadband ultrasonic technique, that was recently developed by the authors to address other non-invasive medical detection problems (by integrating a piezoelectric transducer into a spectral measuring system), is extended to our new objective; the aim is its application to the thickness measurement of sub-millimeter membranes or layers made of materials similar to some biological tissues (phantoms). The modeling and design rules of such a transducer system are described, and various methods of estimating overtones location in the power spectral density (PSD) are quantitatively assessed with transducer signals acquired using piezoelectric systems and also generated from a multi-echo model. Their effects on the potential resolution of the proposed thickness measuring tool, and their capability to provide accuracies around the micron are studied in detail. Comparisons are made with typical tools for extracting spatial parameters in laminar samples from echo-waveforms acquired with ultrasonic transducers. Results of this advanced measurement spectral tool are found to improve the performance of typical cross-correlation methods and provide reliable and high-resolution estimations.

Application of 1-d transient elastography for the shear modulus assessment of thin-layered soft tissue: comparison with supersonic shear imaging technique

Elasticity estimation of thin-layered soft tissues has gained increasing interest propelled by medical applications like skin, corneal, or arterial wall shear modulus assessment. In this work, the authors propose one-dimensional transient elastography (1DTE) for the shear modulus assessment of thin-layered soft tissue. Experiments on three phantoms with different elasticities and plate thicknesses were performed. First, using 1DTE, the shear wave speed dispersion curve inside the plate was obtained and validated with finite difference simulation. No dispersive effects were observed and the shear wave speed was directly retrieved from time-of-flight measurements. Second, the supersonic shear imaging (SSI) technique (considered to be a gold standard) was performed. For the SSI technique, the propagating wave inside the plate is guided as a Lamb wave. Experimental SSI dispersion curves were compared with finite difference simulation and fitted using a generalized Lamb model to retrieve the plate bulk shear wave speed. Although they are based on totally different mechanical sources and induce completely different diffraction patterns for the shear wave propagation, the 1DTE and SSI techniques resulted in similar shear wave speed estimations. The main advantage of the 1DTE technique is that bulk shear wave speed can be directly retrieved without requiring a dispersion model.

Improvement of artery radii determination with single ultra sound channel hardware & in vitro artificial heart system

In several clinical and experimental circumstances, it is widely necessary to characterize the bio-mechanical changes induced by atherosclerosis to the arterial wall. In this context, the purpose of this paper is twofold. Firstly, to propose a low cost ultrasound setup to improve artery radii determination in elasticity experiments, based on two transducers using a single channel ultrasound hardware. Secondly, to present an in vitro artificial heart system developed in our laboratory, which provides a wide range of hemodynamic parameters in arterial elasticity assessment experiments. It can be used in a liquid, stand alone mode or blowing air to a Jarvik device. This system will be integrated in future works with the proposed ultrasound setup to provide real time elasticity measurements.