Deformation Independent Non-linearity Estimation: Studies and Implementation in Ultrasound Shear Wave Elastography

Nonlinear elasticity imaging provides additional information about tissue behavior that is potentially diagnostic and can lead to better tissue characterization. Nonlinear elastic properties of tissue become apparent upon application of large deformation to the medium. The majority of nonlinear elasticity estimation techniques rely on uni-axial compression. Here, we study the change in shear wave speed with the medium subjected to simple shear stress and also pure uniaxial compressive stress. Axial and lateral deformation of the tissue was tracked using quasi-static strain elastography. The local stress map is computed from cumulative sum of apparent shear modulus (measured by shear wave elastography) times the estimated differential strain. By fitting the change in local stress obtained to the estimated strain, nonlinear shear modulus is mapped. The rate of the change in local stress distribution in the medium differs with different deformations applied. However, the absolute value of nonlinear shear modulus obtained for different deformations applied were similar, thereby demonstrating the ability to give a quantitative measure of material non-linearity irrespective of subjected deformations.