Quantifying the effect of fiber pennation angle on shear wave viscoelastography estimates: <i>In silico</i> and phantom studies

Ultrasound shear wave elastography can be useful for assessing muscle pathology. The effect of anisotropy on shear wave elasticity estimates of skeletal muscle has been reported previously. However, muscle is inherently viscoelastic, and hence, tissue viscosity is also an important material parameter to assess. The goal of this study was to systematically quantify the effect of fiber pennation angle on shear wave viscoelasticity imaging estimates. Numerical phantom simulations of skeletal muscle-mimicking phantoms were analyzed. Anisotropic polyvinyl alcohol phantoms embedded with polysulfone fibers were developed to mimic the viscoelasticity and appearance of muscle in B-mode images. Shear wave dispersion analysis, assuming a Kelvin-Voigt model, was performed to estimate the shear modulus and viscosity of the phantoms along the fibers (in-plane) and across the fibers (cross-plane) with varying pennation angles (0°-30°). A decreasing trend was observed in shear modulus estimates with increasing fiber pennation angle in the in-plane orientation for all phantoms. Notably, simulations showed that viscosity estimates decreased with increasing angle. These results provide a systematic quantification of the effect of fiber pennation angle on viscoelastic estimates under controlled conditions, which will be useful for assessing the performance of shear wave viscoelasticity imaging approaches for muscle assessment.