Viscoelastic response (VisR)-derived relative elasticity and relative viscosity reflect true elasticity and viscosity, in silico

VisR ultrasound characterizes the viscoelastic properties of tissue by fitting acoustic radiation force (ARF)-induced displacements in the region of ARF excitation to a 1D mass-spring-damper (MSD) model. By so doing, viscosity and elasticity are found separately from each other but relative to the applied ARF amplitude. We refer to these parameters as `relative elasticity (RE)' and `relative viscosity (RV)'. We herein test the hypothesis that RE and RV linearly correlate to true elasticity and viscosity, in silico. VisR imaging was simulated in 144 homogeneous, isotropic, viscoelastic materials with varying elasticity and viscosity using finite element model (FEM) and Field II simulation. RE linearly correlated with shear elasticity of the modeled materials, with R² = 1.0. For a given elasticity, RE values varied by 2.5% on average when the shear viscosity was changed from 0.1 to 1.3 Pa.s. Similarly, RV linearly correlated with shear viscosity (R² ≥ 0.99); however, for a given viscosity, RV values varied by 112.7% on average when shear elasticity changed from 3.33 to 20 kPa. This high degree of variation in RV was due to complex 3D system inertia, with greater inertial effects in stiffer materials. This confounding impact of elasticity on RV was compensated by using the natural frequency parameter, ω. After so compensating, corrected RV varied by <; 1.0% on average for a given viscosity when the shear elasticity changed from 3.3 to 20 kPa. These results suggest the potential of VisR ultrasound for independently evaluating elastic and viscous properties in tissue.