Jacques Ohayon

Viscoelastic characterization of soft tissue from dynamic finite element models

An iterative solution to the inverse problem of elasticity and viscosity is proposed in this paper. A new dynamic finite element model that is consistent with known rheological models has been derived to account for the viscoelastic changes in soft tissue. The model assumes known lumped masses at the nodes, and comprises two vectors of elasticity and viscosity parameters that depend on the material elasticity and viscosity distribution, respectively. Using this deformation model and the observed dynamic data for harmonic excitation, the inverse problem is solved to reconstruct the viscosity and elasticity in the medium by using a Gauss-Newton-based approach. As in other inverse problems, previous knowledge of the parameters on the boundaries of the medium is necessary to assure uniqueness and convergence and to obtain an accurate map of the viscoelastic properties. The sensitivity of the solutions to noise, model and boundary conditions has been studied through numerical simulations. Experimental results are also presented. The viscosity and elasticity of a gelatin-based phantom with inclusion of known properties have been reconstructed and have been shown to be close to the values obtained using standard rheometry.

Noninvasive Vascular Elastography With Plane Strain Incompressibility Assumption Using Ultrafast Coherent Compound Plane Wave Imaging

Plane strain tensor estimation using non-invasive vascular ultrasound elastography (NIVE) can be difficult to achieve using conventional focus beamforming due to limited lateral resolution and frame rate. Recent developments in compound plane wave (CPW) imaging have led to high speed and high resolution imaging. In this study, we present the performance of NIVE using coherent CPW. We show the impact of CPW beamforming on strain estimates compared to conventional focus sequences. To overcome the inherent variability of lateral strains, associated with the low lateral resolution of linear array transducers, we use the plane strain incompressibility to constrain the estimator. Taking advantage of the approximate tenfold increase in frame rate of CPW compared with conventional focus imaging, we introduce a time-ensemble estimation approach to further improve the elastogram quality. By combining CPW imaging with the constrained Lagrangian speckle model estimator, we observe an increase in elastography quality (~10 dB both in signal-to-noise and contrast-to-noise ratios) over a wide range of applied strains (0.02 to 3.2%).

Indentation for Estimating the Human Tongue Soft Tissues Constitutive Law: Application to a 3D Biomechanical Model

A 3D biomechanical model of the tongue is presented here. Its goal is to evaluate the speech control model. This model was designed considering three constraints: speech movement speed, tongue movements and tongue soft tissue mechanical properties. A model of the tongue has thus been introduced taking into account the non linear biomechanical behavior of its soft tissues in a large deformation analysis. Preliminary results showed that the finite element model is able to simulate the main movements of the tongue during speech data. It seems that the model may be used in the estimation of glossectomy impacts on patient speak and on different surgery approaches.

A sequential Bayesian based method for tracking and strain palpography estimation of arteries in intravascular ultrasound images

This paper investigates the task of tracking and strain estimation of arteries in intravascular ultrasound images. A tracking method is proposed to extract the inner and the outer contours of the vessel wall (lumen/intima-media and intima-media/adventitia interfaces, respectively), and the deformations along them. This estimation is carried out by a non parametric sequential Bayesian method. The Bayesian modeling holds three ingredients: the prior, which is given by a manually defined segmentation of the contours on the first image; the transition, which is assumed to follow a Markovian random walk; and the likelihood, which is a distance between patches distributed along the contours. The underlying Bayesian posterior distribution is approximated using a sequential Monte Carlo approach. Experiments on three PVA-C phantoms present direct readings of the deformations along the lumen/intima-media contour.

A pseudo active kinematic constraint for a biological living soft tissue: An effect of the collagen network

Recent studies in mammalian hearts show that left ventricular wall thickening is an important mechanism for systolic ejection and that during contraction the cardiac muscle develops significant stresses in the muscular cross-fiber direction. We suggested that the collagen network surrounding the muscular fibers could account for these mechanical behaviors. To test this hypothesis we develop a model for large deformation response of active, incompressible, nonlinear elastic and transversely isotropic living soft tissue (such as cardiac or arteries tissues) in which we include a coupling effect between the connective tissue and the muscular fibers. Then, a three-dimensional finite element formulation including this internal pseudo-active kinematic constraint is derived. Analytical and finite element solutions are in a very good agreement. The numerical results show this wall thickening effect with an order of magnitude compatible with the experimental observations.

Non-invasive vascular modulography: An inverse problem method for imaging the local elasticity of atherosclerotic carotid plaques

Quantifying biomechanical properties of atherosclerotic plaques may help preventing strokes. Non-invasive vascular elastography (NIVE) in superficial carotid arteries has the potential to assess such properties and to discriminate plaque components (e.g., fibrosis, lipid and calcium) through elastograms (i.e., spatial strain distribution). However, the elasticity and morphology of the vessel wall, cannot be assessed directly from strain maps since the stress distribution remains unknown. In this study, we describe an unsupervised inverse problem for elasticity mapping (non invasive vascular modulography), which is capable of reconstructing a heterogeneous Youngs modulus distribution of a plaque. High resolution elastograms were computed from ultrasound compounded plane wave images using the constrained lagrangian speckle model estimator (constrained LSME). Von mises strain maps were combined with a stress map, evaluated using a parametric finite element model (PFEM), and used to highlight mechanical heterogeneities and compute Young modulus maps (Modulograms).

Optimization of the strain tensor estimation for cross sectional non-invasive elastography of the carotid using plane wave imaging

Full-2D strain tensor estimation using non-invasive vascular elastography (NIVE) can be difficult to achieve using conventional beamforming methods due to its low lateral resolution. Angular compounding can help improving estimates of lateral components. However, using conventional focused imaging, compounding decreases dramatically the frame rate thus increasing the probability of decorrelation artefacts. As it can reconstruct the full field of view (FOV), using a single unfocused emission, plane wave imaging (PWI) can reach very high frame rate without significant loss of signal-to-noise ratio (SNR). In this study, we investigated the performance of compounding elastography using the lagrangian speckle model estimator (LSME) on tilted plane wave images. We introduced two compounding methods based on oriented strain fields without any mechanical assumptions. We evaluate our methods on both simulation and in-vitro data. It was found that, as well as conventional focused imaging, PWI can provide high quality axial strain and shear and significantly better lateral strain and shear. Moreover, the combination of compound elastography with tilted PWI improved significantly the quality of elastograms.