A noninvasive method to estimate tissue stress-strain relationships using quasi-static and shear wave ultrasound elastography

Abstract

Current clinically used ultrasound elastography is obtained when tissue undergoes small deformation (<2% strain). However, small deformation may not be large enough to characterize tissues that exhibit similar elasticity properties to normal tissue at small deformation but behave differently from normal tissue at large deformation (>10% strain). Therefore, tissue elasticity properties measured at large deformation may provide better diagnostic differentiation. Combining the quasi-static and shear wave ultrasound elastography methods, we propose a noninvasive method to estimate the nonlinear elasticity property of tissue. To test this method, cubic tissue-mimicking phantoms were constructed. We progressively compressed the phantom to produce a series of stepwise stress states. At every stress state, we conducted shear wave measurements in which two acoustic radiation force pushes generated shear waves, which were measured using ultrafast imaging in order to estimate the local shear modulus. For every compression step, we tracked incremental tissue displacements between two successive frames and then accumulated these displacements for all steps to obtain the cumulative displacements and strains. Using the shear modulus and cumulative strain images we calculated the stress map under an incompressibility assumption. Finally, we obtained the local stress-strain curve that reflected the nonlinear elasticity of the tested material.