Direct phase-based strain estimator for ultrasound tissue elasticity imaging

Palpation has been widely used to detect hard tumorous tissues surrounded by softer normal tissues. The goal of ultrasound tissue elasticity imaging is to extract information regarding tissue stiffness that is closely related to pathology. For this tissue elasticity imaging, compression is applied first, and the amount of resulting tissue deformation or strain needs to be estimated. Traditionally, strain estimators aim to accurately derive tissue displacements between pre- and post-compression and compute strain from the displacements. However, the displacement can be as large as a thousand times of strain for typical compression levels used in ultrasound elasticity imaging. Error in displacement estimation leads to a large variance in strain, thus resulting in poor signal to noise ratio for the estimated strain. We have developed a novel strain estimator that directly estimates strain from the phase of temporal and spatial correlation instead of estimating small strain from large displacements. SNRe (signal to noise ratio of elastogram) and CNRe (contrast to noise ratio of elastogram) using the direct strain estimator are at least three times and six times larger than that using conventional displacement-based strain estimators, respectively. These results indicate that the direct strain estimator can significantly improve accuracy and lesion detectability in ultrasound elasticity imaging. In addition, the direct strain estimator is computationally efficient compared to conventional estimators, thus enabling the realtime implementation and clinical use of this new ultrasound imaging mode.