New shear wave velocity estimation using arrival time differences in orthogonal directions

Shear wave (SW) elasticity imaging is based on the estimation of shear wave speed (SWS) induced by the radiation force of an ultrasound beam. Although the SW initially propagates in the direction perpendicular to the pushing beam, the SW direction changes because of its refraction occurring during traveling in an inhomogeneous medium. The refraction can cause the estimation errors of SWS when the speed is measured along the initial propagation direction. In this paper, we demonstrate that the conventional SWS reconstruction methods yield biased estimates in refractive region. To overcome the problems, also, we propose a novel method in which the differences of SW arrival time in orthogonal directions are used to calculate both the speed and the direction of SW. In the proposed method, SW velocity vectors are estimated and thus their speed can be measured along the propagation direction. Therefore, the estimation errors due to the refraction can be minimized. By the simulations and phantom experiments, we verified that the proposed method can provide more accurate and uniform SWS map than the conventional methods especially in the refractive region.