Viscoelastic Characterization in Muscle using Group Speed Analysis and Volumetric Shear Wave Elasticity Imaging

Shear wave elasticity imaging (SWEI) typically assumes a homogeneous, isotropic, elastic material to relate shear wave speed (SWS) to stiffness; however muscle is anisotropic and viscoelastic (VE). To characterize this complex tissue, we combine 3D SWEI (accumulated with an axially rotating transducer performing 2D SWEI acquisitions) with a novel variant of our previously published robust group SWS dispersion analysis method. We evaluated this system in isotropic elastic and VE phantoms, and in human muscle in vivo. We determined the group SWS of the particle velocity maxima (peak SWS) and minima (trough SWS) at each transducer rotation angle, fit an ellipse to both the peak and trough SWS, and determined the major and minor ellipse axes. A look up table was generated to relate the peak and trough SWS at the principal axes to VE parameters assuming a linear attenuation model characterized by the phase velocity (c) and dispersion slope (c') at a reference frequency of 200 Hz. The results from the isotropic phantoms are isotropic, demonstrate dispersion in the VE phantom, and agree with previous measurements. Our preliminary in vivo data indicate slower speeds across muscle fibers than along the fibers and minor dispersion in both directions. The 3D SWEI method allows for repeatable SWS measurements across multiple in vivo acquisitions.