On the correlation between knee flexion and 3D shear wave speed and amplitude in in vivo vastus lateralis

We are investigating the potential for shear wave elasticity imaging (SWEI) derived material parameters to serve as biomarkers for skeletal muscle health. We consider muscle as a transversely isotropic (TI) material and use rotational 3D-SWEI acquisitions to characterize the shear wave propagation in directions along and across the muscle fibers at various passive stretch states. Data were collected in the vastus lateralis of the dominant leg of 10 healthy volunteers at various knee flexion angles (controlled by a BioDex system). The 3D-SWEI acquisitions were analyzed for both shear wave speed and amplitude in the directions along and across the muscle fibers. Relative to the values at 45 degrees knee flexion, the shear wave speed along the fibers changed an average of +78% at 105 degrees knee flexion and -11% at 0 degrees knee flexion, while the shear wave speed across the fibers changed +17% at 105 degrees knee flexion, with no clear change at 0 degrees knee flexion. These values support the need to control for subject positioning (joint angle) during skeletal muscle SWEI. Shear wave amplitude along the fibers increases by +110% from 45 degrees to 105 degrees knee flexion and changes by -43% from 45 to 0 degrees knee flexion. Interestingly, in the direction along the fibers, the shear wave amplitude increases with increasing flexion even while shear wave speed increases over this same flexion range, opposite the trend expected from isotropic materials. Across the fibers, there is no clear trend in shear wave amplitude from 45 to 105 degrees knee flexion, but amplitude changes by an average of +52% at 0 degrees knee flexion relative to 45 degrees knee flexion. This observation could provide insight for optimization of in vivo imaging protocols and understanding the higher order properties of skeletal muscle. Additionally, we explore the quantification of muscle's response to stretch through the rate of change in SWS with knee flexion at flexion angle above 45 degrees and find linear fits with high R-squared values and slopes of 0.023 +/- 0.0034 m/s/degrees.