Shear Wave Propagation in a Fiber-Laden Viscoelastic Muscle Phantom: Challenges in Inverse Modeling

The functional role of skeletal muscle and the hierarchal microstructure and arrangement of fibers within it results in anisotropy and inhomogeneity in both material properties and imposed stresses. Dynamic elastography reconstruction methods for estimating muscle tissue viscoelastic properties that are based on assumptions of homogeneity, isotropy and only bulk wave motion may produce inaccurate estimates. Biases may be introduced in reconstruction by homogenizing muscle with axially aligned fibers and approximating it as transversely isotropic. The significance of these biases, and their interplay with imposed stresses and confounding waveguide effects due to small cross-sectional dimensions, is quantified with a series of numerical finite element and experimental elastography studies on fiber-laden phantoms, with varying fiber dimensions. This study reveals how accurate homogenization is informed not only by cross-sectional area or volumetric information but depends on more detailed multiscale understanding of the heterogeneous material structure.