Shear wave propagation in a prestressed and transversely isotropic viscoelastic material: Inverse modeling challenges and strategies.

The functional role and structure of some biological tissues, such as skeletal muscle, result in anisotropy in both material properties and imposed stresses. Dynamic elastography reconstruction methods for estimating soft tissue viscoelastic properties that are rooted in assumptions of isotropy in both the material properties and the imposed mechanical stress field may produce inaccurate estimates. The superposition of axially aligned orthotropy (transverse isotropy) and uniaxial prestress due to passive stretch or muscle activation makes it difficult to independently discern what part of the apparent anisotropy is due to material versus stress field anisotropy. Separation of this could have clinical value by differentiating changes in muscle tissue structure versus muscle loading conditions, both of which will be uniquely affected by disease, injury, and response to therapy. A strategy for decoupling material and stress-based anisotropy is introduced and evaluated with a series of numerical finite element and experimental elastography studies. Transverse wave motion is studied in a polymeric muscle phantom with either isotropic or transverse isotropic material properties and subjected to uniaxial prestress. The proposed reconstruction approach to decouple material anisotropy from stress field anisotropy works well for the controlled phantom study. Challenges and strategies for future in vivo adaptation are discussed.