Spatial Angular Compounding with Affine Model Based Optical Flow for Improvement of Motion Estimation.

Tissue motion estimation is an essential step for ultrasound elastography. Our previous study has shown that the affine-model based optical flow (OF) method outperforms the normalized cross-correlation based block matching (BM) method in motion estimation. However, the quality of lateral estimation using OF is still low due to inherent limitation of ultrasound imaging. BM based spatial angular compounding (SAC) has been developed to obtain better motion estimation. In this paper, OF based SAC (OF-SAC) is proposed to further improve the performance of lateral (and axial) estimation, and it is compared with BM based SAC (BM-SAC). Plane wave as well as focused wave is transmitted in both simulations and phantom experiments on a linear array. In order to compare the performance quantitatively, the root-mean-square error (RMSE) of axial/lateral displacement and strain, and signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of axial/lateral strain are used as the evaluation criteria in the simulations. In the phantom experiments, the SNR and CNR are used to assess the quality of axial/lateral strain. The results show that for both OF and BM, SAC improves the performance of motion estimation, regardless of using plane or focused wave transmission. More importantly, OF-SAC is shown to outperform BM-SAC with lower RMSE, higher SNR, and higher CNR. In addition, preliminary in-vivo experiments on the carotid artery of a healthy human subject also prove the superiority of OF-SAC. These results suggest that OF-SAC is preferred for both axial and lateral motion estimation to BM-SAC.