Yassine Mofid

In-vivo imaging of skin under stress: potential of high-frequency (20 MHz) static 2-D elastography

The aim of this study was to evaluate the potential of high-frequency static two-dimensional (2-D) elastography for in vivo exploration of the mechanical behavior of skin. Our device was based on the combination of a 20 MHz sonographer and a patented extensiometer device able to apply calibrated uniaxial stretching of the skin. We used a new algorithm to compute elastograms that improve elastographic signal-to-noise ratio (SNRe) without sacrificing resolution. Mechanical behavior was described according to the axial strain and lateral displacements induced in the tissue. The efficacy of the strain anpolyvinyl alcohol first evaluated in poly vinyl alcohol (PVA)-cryogel phantoms. Several in vivo experiments then were conducted, mainly with the multistretching averaging method, and demonstrated the potential of this technique in the evaluation of mechanical behavior of the dermis and the hypodermis under stress

Mechanical skin thinning-to-thickening transition observed in vivo through 2D high frequency elastography.

This study was based on two dimensional (2D) high frequency elastography to describe quantitatively the mechanical behavior of the human dermis in vivo. The study was conducted on the forearm skin and elastographic tests were performed using a combination of two devices: an extensiometer developed for the in vivo study of the mechanical behavior of the skin using uniaxial stretching stress, and a 20MHz real time sonographer (Dermcup 2020™) for ultrasound skin imaging. The staggered strain estimation algorithm (SSE) was used to produce elastograms. A temporal cumulative technique was applied to improve elastogram quality and to monitor variations in skin strain during stretching. The influence of the natural skin tension controlled by arm bending was studied and distinctive mechanical behavior was observed for low and high mechanical stress levels. In a preliminary analysis, the reproducibility of measurements was assessed by means of coefficient of variation (CV) in 5 selected healthy volunteers.Finally, two hypotheses linked to the geometrical and structural properties of the dermis are proposed to account for the new findings described in this study.

A human skin ultrasonic imaging to analyse its mechanical properties

The analysis of the skin mechanical behaviour is a key-point for different field of investigation. As the skin is a complex structure, studies are usually based on inverse methods that compare experimental and finite element numerical results. Besides the considered behaviour law, one of the most important question concerns the geometrical aspects of the skin tissue. In this paper, it is shown how high frequency ultrasound imaging helps the calculation of skin mechanical parameters. The hypodermis influence is firstly discussed through elastographic analyses. A specific procedure to measure the dermis thickness is then proposed to highlight that such a measurement must be considered to draw reliable conclusions. The obtained results are finally discussed to point out the interest of such simplifications for the study of more complex behaviour laws.

Axial Ultrasound B-Scans of the Entire Eye With a 20-MHz Linear Array: Correction of Crystalline Lens Phase Aberration by Applying Fermat's Principle

In ophthalmic ultrasonography the crystalline lens is known to be the main source of phase aberration, causing a significant decrease in resolution and distortion effects on axial B-scans. This paper proposes a computationally efficient method to correct the phase aberration arising from the crystalline lens, including refraction effects using a bending ray tracing approach based on Fermats principle. This method is used as a basis to perform eye-adapted beamforming (BF), with appropriate focusing delays for a 128-element 20-MHz linear array in both emission and reception. Implementation was achieved on an in-house developed experimental ultrasound scanning device, the ECODERM. The proposed BF was tested in vitro by imaging a wire phantom through an eye phantom consisting of a synthetic gelatin lens anatomically set up in an appropriate liquid (turpentine) to approach the in vivo velocity ratio. Both extremes of accommodation shapes of the human crystalline lens were investigated. The performance of the developed BF was evaluated in relation to that in homogeneous medium and compared to a conventional delay-and-sum (DAS) BF and a second adapted BF which was simplified to ignore the lens refraction. Global expectations provided by our method with the transducer array are reviewed by an analysis quantifying both image quality and spatial fidelity, as well as the detrimental effects of a crystalline lens in conventional reconstruction. Compared to conventional array imaging, the results indicated a two-fold improvement in the lateral resolution, greater sensitivity and a considerable reduction of spatial distortions that were sufficient to envisage reliable biometry directly in B-mode, especially phakometry.

Transient elastography in heterogeneous tissues

The Fibroscan® (Echosens, Paris, France) is a transient elastography based device used to quantify liver fibrosis by following the propagation of a low frequency shear wave and measuring the mean Youngs modulus of the liver. This device has been successfully applied to homogeneous tissues such as liver in patients with chronic hepatitis C. Current developments in transient elastography are now headed toward the characterization of heterogeneous tissues. The estimation of the shear wave velocity can be achieved by solving the elastic wave equation taking into account either the 1D, the 2D, or the 3D components of the displacement spatial derivatives. The objective of this study is to characterize focal nodules in human liver and to quantify heterogeneous fibrosis. We present the methods used to estimate the local shear wave velocity and the results of experiments conducted on heterogeneous phantoms and in the liver in vivo.

4J-6 In Vivo High Frequency Elastography for Mechanical Behavior of Human Skin Under Suction Stress: Elastograms and Kinetics of Shear, Axial and Lateral Strain Fields

The aim of this work was to show the potential of high-frequency elastography in depicting the mechanical behavior of human skin under suction stress in vivo. Our 20 MHz real time sonograph (DERMCUP2020trade), was combined with a circular suction device to measure, by elastography, different strain fields in the skin. The elastographic process was based on filtered SSE technique with 2D tracking. Elastograms were cumulated to reach high deformation percents with a good SNRe and to generate kinetic profiles. In-vivo experiments were conducted on the volar forearm healthy human skin. Shear, axial and lateral strain elastograms/kinetics were produced with cumulating small deformation steps. Axial, lateral and shear strain fields presented useful and complementary data to describe the mechanical behavior in different layer in the skin. The quantitative approach was to measure kinetic profiles of specific ROIs in elastograms and extract the useful information corresponding to each kind of strain

High frequency elastography for in-vivo study of the mechanical behavior of skin

The aim of this paper is to show the potential of high frequency elastography in the evaluation of the in-vivo mechanical behavior of human skin. In the first part we describe the filtered and staggered strain estimation (FSSE) which is a new elastographic algorithm based on the staggered strain estimation (SSE) method. This algorithm does not integrate the global stretching, commonly used in the case of global compression stress, but it uses a low-pass filtering to estimate the axial strain in a complex medium such as the skin. We present also a lateral displacement detection technique based on a parabolic interpolation. In the second part we expose a PVA-cryogel phantom and in-vivo experiments conducted with two combined devices: our 20 MHz real time sonographer (DERMCUP2020/spl reg/) used for ultrasound imaging and a patented device devoted to apply uniaxial stretching stress. The in-vivo mechanical behavior of human skin is illustrated using the multi-stretching FSSE technique and elastograms and lateral displacement images are shown in the results.

High frequency ultrasound imaging of whole blood gelation and retraction during in vitro coagulation.

Blood coagulation is a series of biochemical reactions resulting in the mechanical transformation of liquid blood into a gel. As a consequence, ultrasound, being mechanical waves, can provide specific details on the dynamics of coagulation. In fact, previous high-frequency ultrasound monitoring studies have shown drastic changes in ultrasound velocity and attenuation during whole blood coagulation and a model discussing the observed mechanical transformations was proposed. In this paper, a technique of visualization of the clotting mechanism is introduced, which complements and revises the previous hypotheses. This method is based on the monitoring of scatterers (red blood cells) movement through a time correlation of 20 MHZ rf signals. It allows the computing of both a displacement map revealing local details and disparities and a parameter quantifying the global structural behavior. Qualitative results for two typical samples show that the technique provides new insights on the gelation dynamics. A quantitative analysis computed from 12 healthy subjects found that the changes in the structural parameters are significantly correlated to the changes in velocity and attenuation, both dependent on the mechanical transformations in the sample. The previous model is therefore revised and a new way to measure gel and retraction times is proposed.

Assessment of the elastic properties of heterogeneous tissues using transient elastography: Application to the liver

Fibroscanreg (Echosens, Paris, France) is a parametric transient elastography based device used to quantify liver fibrosis by following the propagation of a low frequency shear wave and measuring the mean Youngs modulus of the liver. This device has been successfully applied to homogeneous tissues such as liver in patients with chronic hepatic diseases. Current developments in transient elastography target the characterization of heterogeneous tissues. The objective of this study is to investigate the potential of parametric transient elastography in the characterization of heterogeneous tissues such as liver with heterogeneous fibrosis or tumors. The estimation of the shear wave velocity can be achieved by solving the elastic wave equation taking into account either the 1D, the 2D or the 3D components of the displacement spatial derivatives. Two methods are investigated: A 1D direct local inversion is performed on data acquired with a standard Fibroscanreg probe composed of a 3.5 MHz transducer mounted on a low frequency vibrator and a 3D direct local inversion is achieved on data acquired using a new dedicated multi-elements probe connected to an ultra-fast ultrasonic device. The algorithms are validated on simulations and on heterogeneous phantoms and tested on data acquired in the liver in vivo.

P1B-2 In-Vivo Exploration of the Mechanical Properties of Healthy and Pathological Human Dermis with 2D High Resolution Elastography

This work was based on 2D high frequency elastography used to describe, quantitatively, the mechanical behavior of human dermis in vivo in terms of axial strain and lateral displacement. The experiment material was the combination of 2 devices. First, an extensiometer developed by LMARC laboratory for in-vivo skin applications using uniaxial stretching stress. Second, our 20 MHz real time sonographer, Dermcup 2020trade. Filtered and Staggered Strain Estimation algorithm (SSE + low-pass filtering), was used to produce elastograms. A temporal cumulative technique was applied and kinetics of the axial strain, epsiv(t), and lateral retraction were obtained for homogeneous ROIs in the dermis. Kinetic profiles were useful to study several characteristics: inter-individual variations, viscoelasticity and reproducibility. Results on pathological dermis were discussed in the conclusion