Characterizing Shear and Tensile Anisotropy in Skeletal Muscle using Ultrasonic Rotational 3D Shear Wave Elastography

Shear wave elastography imaging (SWEI) of skeletal muscle is of great interest to the medical community, as there is a large need for a non-invasive, quantitative biomarker of muscle health that relates to muscle function. SWEI measures mechanical properties by generating quantitative images of tissue stiffness using an acoustic radiation force (ARF) excitation in the material and measuring the resulting shear waves that propagate outward. Most SWEI tools assume an isotropic, linear, elastic material, however skeletal muscle is commonly modeled as transversely isotropic (TI) due to the alignment of the muscle fibers. This means that shear wave speed (c, SWS) is dependent on the direction of the traveling shear wave relative to the fibers in skeletal muscle.

If muscle is assumed to be incompressible and transversely isotropic (ITI) it can be described with three parameters: the longitudinal shear modulus μ_L, the transverse shear modulus μ_T, and a single parameter combining longitudinal and transverse Young's moduli (E_L and E_T) called tensile anisotropy χ_E. In an elastic ITI material, there are two shear wave modes with different polarizations that can be excited: the shear horizontal (SH) and the shear vertical (SV). Shear moduli μ_L and μ_T can be measured solely based on the observation of the SH mode, however to quantify tensile anisotropy χ_E using SWEI, it is necessary to observe the SV mode. This thesis explores characterizing skeletal muscle as an ITI material and explores factors that affect the measurement of the SV mode wave. In order to evaluate the use of χ_E as a biomarker of muscle health we must understand the factors that affect its measurement using SWEI.

Chapter 3 demonstrates feasibility of measuring both the SH and SV modes using a 3D rotational SWEI system in the vastus lateralis muscle in vivo. We develop and validate methodology to estimate μ_L, μ_T, and χ_E and describe measurements these parameters in vivo.

Chapter 4 explores the factors that affect the SV mode waves, using Green's function simulations to perform a parametric analysis to determine the optimal interrogation parameters to facilitate visualization and quantification of SV waves in muscle. We evaluate the impact of five factors: μ_L, μ_T, and χ_E as well as fiber tilt angle θ_tilt and F-number of the push geometry on SV mode speed, amplitude, and rotational distribution.

Chapter 5 extends the work in Chapter 4 to understand SH and SV wave propagation in 3D by simulating multiple observation tilt angles and all 3 components of displacement. Tilting the observation plane to particular angles allows for maximization of the strength of the SH or SV waves, demonstrating that observation of these tilted planes in in vivo data would increase opportunities for estimation of SH and SV waves.

The work presented in this thesis explores using 3D SWEI to better characterize skeletal muscle as an ITI material, specifically by assessing the SH and SV mode shear wave speed. This work also investigates factors that affect measurement of SV mode waves, and thus the ability to estimate χ_E, towards a better understanding of χ_E for use as a potential biomarker of muscle health.