Interrogation of Young’s and Shear Elastic Moduli in Anisotropic Tissues using Viscoelastic Response (VisR) Ultrasound

This study demonstrates that Viscoelastic Response (VisR) ultrasound can be used to assess the Young’s modulus of anisotropic tissues in addition to the shear modulus. VisR, which applies on-axis acoustic radiation force (ARF) excitations to measure tissue elasticity, has been used to estimate shear moduli in transversely isotropic (TI) materials when ARF is applied perpendicular to their axis of symmetry (AoS). Here, we hypothesize that varying the ARF incidence angle relative to the AoS and monitoring percent changes in VisR-derived relative elasticity (RE) allows evaluation of both shear and Young's moduli. In silico experiments showed that RE at normal ARF-AoS incidence reflected only the longitudinal shear modulus (μ_L), while percent changes in RE with varying ARF-AoS incidence angle (∆RE) showed a strong linear relationship with the ratio of longitudinal shear to Young's moduli (μ_L/E_L). Furthermore, linear regression analysis of ∆RE versus ARF-AoS angle discriminated between TI materials with μ_L/E_L ratios differing by as little as 7%. These in silico results were translated ex vivo using chicken pectoralis major and bovine longissimus dorsi muscles, where the slope of ∆RE versus ARF-AoS incidence angle distinguished the two muscle types with statistical significance. These results indicate that the rate of change in ∆RE with varying ARF-AoS incidence angle provides a novel biomarker for evaluating anisotropic tissues, such as skeletal muscle, kidney, and breast tissue.