Finite element modeling for shear wave elastography

Abstract

Shear wave elastography is an important imaging modality to evaluate tissue mechanical properties and supplement conventional ultrasound diagnostic imaging. A 3D finite element model has been created in PZFlex for simulating and understanding shear wave generation by the acoustic radiation force, and its propagation through different media. The simulation settings were based on a shear wave elastography prototype using a Philips iU22 scanner with a C5-1 curvilinear probe. The modeling process was divided into two steps. In the first step, the acoustic field of the ultrasound probe was calculated and the output acoustic radiation stress (ARS) result in the 3D volume was saved. In the second step, the ARS data was applied as a boundary condition to generate the shear wave. The shear wave displacement time profiles in the region of interest were recorded at the end of the second step. The simulation was performed for different media, including uniform tissues with various shear moduli and viscosities, as well as uniform tissue background with an embedded stiffer inclusion. Clear differences were observed on the shear wave displacement time profiles, as the displacement peak was attenuated and widened by the higher shear modulus and viscosity. The simulation results were also cross-checked with elasticity reconstruction algorithms based on wave equation (WE), Voigt model (VM) and time-to-peak (TTP) methods. For a medium similar to normal liver tissue with 2KPa shear modulus, all three reconstruction methods reported shear modulus approximately the same as input value when the viscosity was negligible (WE: 2.05KPa, VM: 2.06KPa, TTP: 2.12KPa). With increased viscosity in the medium (2KPa, 2PaS), TTP seemed to under-estimate shear modulus in the near-field (WE: 2.41KPa, VM: 1.98KPa & 2.11PaS, TTP: 1.38KPa). For a uniform medium with an embedded spherical inclusion, all three methods successfully detected the inclusion and reconstructed stiffness maps. The results suggested that the finite element modeling could provide valuable insight in simulating and understanding shear wave generation and propagation. It could also be an important tool to evaluate and analyze stiffness reconstruction algorithms for shear wave elastography.