Anin silicoframework to analyze the anisotropic shear wave mechanics in cardiac shear wave elastography

Abstract

Shear wave elastography (SWE) is a potential tool to non-invasively assess cardiac muscle stiffness. This study focused on the effect of the orthotropic material properties and mechanical loading on the performance of cardiac SWE, as it is known that these factors contribute to complex 3D anisotropic shear wave propagation. To investigate the specific impact of these complexities, we constructed a finite element model with an orthotropic material law subjected to different uniaxial stretches to simulate SWE in the stressed cardiac wall. Group and phase speed were analyzed in function of tissue thickness and virtual probe rotation angle. Tissue stretching increased the group and phase speed of the simulated shear wave, especially in the direction of the muscle fiber. As the model provided access to the true fiber orientation and material properties, we assessed the accuracy of two fiber orientation extraction methods based on SWE. We found a higher accuracy (but lower robustness) when extracting fiber orientations based on the location of maximal shear wave speed instead of the angle of the major axis of the ellipsoidal group speed surface. Both methods had a comparable performance for the center region of the cardiac wall, and performed less well towards the edges. Lastly, we also assessed the (theoretical) impact of pathology on shear wave physics and characterization in the model. It was found that SWE was able to detect changes in fiber orientation and material characteristics, potentially associated with cardiac pathologies such as myocardial fibrosis. Furthermore, the model showed clearly altered shear wave patterns for the fibrotic myocardium compared to the healthy myocardium, which forms an initial but promising outcome of this modeling study.

Local myocardial stiffness variations identified by high frame rate shear wave echocardiography

“…Since shear waves in the myocardium could be affected by several factors such as shape, anisotropy, and inhomogeneity [27,28] the precise quantitative relation between the naturally-occurring shear waves' velocity and the intrinsic stiffness is difficult to establish directly. We therefore quantify only the propagation velocity of the shear waves, assuming a monotonic relation with the tissue stiffness [1,29].…”

Assessing cardiac stiffness using ultrasound shear wave elastography

“…Villemain et al (Villemain et al, 2018) reported a FA of 0.238±0.068 for the mid cardiac wall in healthy volunteers, and a decreased FA of 0.133±0.073 in patients with hypertrophic cardiomyopathy (HCM). (Caenen et al, 2018). The black dotted line in the slices represents the actual myocardial fiber orientation in the model.…”

Mathematical modelling of acoustic radiation force in transient shear wave elastography in the heart

“…In the context of this work (shear wave propagation in soft tissues) we choose L = 10 −2 m and T = 10 −2 s, since our main focus is the propagation of shear waves (1 m s −1 to 10 m s −1 ) in a rectangular slab of an organ (e.g. in some volume of 6 cm × 6 cm × 2 cm of the left ventricle of the heart [29]). To model the myocardial tissue, in accordance with [19,28,29,30,26] we choose…”

“…in some volume of 6 cm × 6 cm × 2 cm of the left ventricle of the heart [29]). To model the myocardial tissue, in accordance with [19,28,29,30,26] we choose…”