VisR Ultrasound With Non-Normal ARF-AoS Incidence Interrogates Both Shear and Young’s Elastic Moduli in Transversely Isotropic Materials

This study evaluates the potential for interrogating the Young’s elastic moduli in anisotropic media, including tissue, using Viscoelastic Response (VisR) ultrasound. VisR is an on-axis acoustic radiation force (ARF)-based elasticity imaging method that has been demonstrated previously for assessing the shear elastic moduli of transversely isotropic (TI) materials when the applied ARF excitation was incident normal to the axis of symmetry (AoS). It is hypothesized that by applying a range of non-normal ARF excitations and monitoring the percent change in VisR-derived relative elasticity (RE) versus ARF-AoS incidence angle, both the shear and the Young’s elastic moduli may be interrogated. The hypothesis was tested using in silico experiments, which showed that while RE measured at normal ARF-AoS incidence was related to only longitudinal shear modulus ( ${\boldsymbol{{\mu }_{L}}}$ ) alone, the percent change in RE over ARF-AoS incidence angle (defined as ${\boldsymbol{\Delta RE}}$ ) exhibited a strong linear correlation with the ratio of longitudinal shear-to-Young’s moduli ( ${\boldsymbol{{\mu }_{L}{/}{E}_{L}}}$ ), with correlation coefficients of 0.97-0.99. Additionally, the linear regression slopes of ${\boldsymbol{\Delta RE}}$ versus ARF-AoS incidence angle statistically discriminated TI materials with ${\boldsymbol{{\mu }_{L}{/}{E}_{L}}}$ ratios that were as little as 7% different. These findings were validated ex vivo in chicken pectoralis major and bovine longissimus dorsi muscles, where the rate of change in of ${\boldsymbol{\Delta RE}}$ versus ARF-AoS incidence angle distinguished the two muscles with statistical significance. The results obtained in this study suggest that the rate of change of ${\boldsymbol{\Delta RE}}$ with ARF-AoS incidence angle may serve as a novel, semi-quantitative biomarker for characterizing anisotropic tissues such as kidney, skeletal muscle, and breast.