Liver fibrosis structure effects on viscoelasticity estimation using group shear wave speeds

Viscoelasticity measurements can improve the accuracy of liver fibrosis diagnosis for chronic hepatitis. Methods to estimate tissue viscoelasticity using frequency characteristics of the shear wave phase velocity are common but not robust. Therefore, in recent years, a technique to estimate viscoelasticity from group speeds in displacement and particle velocity has been proposed as a robust method [1]. However, we earlier reported that the liver fibrotic structure can change the frequency characteristics of phase velocity [2]. Therefore, in this study, we evaluated the influence of the liver fibrosis structure on the estimation method using group shear wave speeds. We used the previously developed liver fibrosis progression model to investigate the effect of the fibrous structure. Shear wave propagation was simulated using this model. Viscoelasticity was estimated using lookup tables that represented the relation between group speeds in displacement and particle velocity and viscoelasticity. When a shear wave reflection component is present, a directional filter is required. The directional filter affects the group speeds of the shear wave. Therefore, we established lookup tables considering the characteristics of the directional filter. Using these tables, we performed viscoelasticity estimations on liver fibrosis models and corresponding uniform models. In the uniform models, the viscoelasticity was estimated correctly. However, in the liver fibrosis models, the Young's modulus was estimated to be smaller than the actual value, and the shear viscosity coefficient was estimated to be greater than the actual value with the difference from the actual value increasing as fibrosis progressed.