Shear Wave Speed Ratio for evaluation of nonlinearity of soft tissues

Imaging of tissue nonlinear elastic properties has shown potentials in providing additional information for disease diagnosis alongside traditional linear elastic properties. However, accurate local quantification of nonlinear elasticity requires precise imaging and registration of tissue strain at multiple incremental strain levels. Furthermore, it requires imaging of local stress which is measured from cumulative sum of shear wave speed squared times the differential strain. To solve this problem, here, we report measurements of shear wave speed ratio to evaluate elastic nonlinearity of tissues. In this study, we evaluate the ratio of shear wave speed (SWS) image at incremental global local strain levels to the SWS image at undeformed state of the medium. The shear wave speed ratio imaging at global strain does not require local tracking of tissue motion. On the other hand, shear wave speed ratio imaging at local strain requires local strain estimation, however, it does not require estimation of cumulative stress. Thus errors and noise associated with traditional quantitative nonlinear elasticity imaging are more compared to shear wave speed ratio imaging.