Wave propagation in prestressed and transversely isotropic viscoelastic structures: Inverse modeling challenges in elasticity imaging

The functional role and structure of skeletal muscle results in anisotropy in both material properties and imposed stresses, as well as waveguide effects. Dynamic elastography reconstruction methods for estimating muscle tissue viscoelastic properties that are rooted in assumptions of isotropy and bulk wave motion may produce inaccurate estimates. The superposition of axially aligned orthotropy (transverse isotropy) in material properties and axially aligned prestress conditions due to passive stretch or muscle activation makes it difficult to independently discern how much of the apparent anisotropy is due to the muscle material or the imposed stress field. Furthermore, this stress field may result in large strain conditions that require the use of higher-order terms in the stress–strain relationship. The significance of these confounding conditions and strategies for decoupling material and stress-based anisotropy are investigated with a series of numerical finite element studies based on simple and morphological image-informed geometries, and experimental elastography studies using scanning laser Doppler vibrometry and magnetic resonance elastography.