Accurate ultrasonic measurement of two-dimensional displacement of heart wall motion for estimation of myocardial regional strain rate

Methods for imaging of strain rate in the heart wall are useful for quantitative evaluation of regional heart function. We developed a method which can accurately measure the heart wall motion along an ultrasonic beam based on phase changes in RF echoes. However, there are some components in the wall motion which are not along each ultrasonic beam. Therefore, the measurement of motion in the direction perpendicular (lateral) to the beam has been required in addition to that in the axial direction. However some unsolved problems remain in estimation of the distribution of lateral motion of the wall. In this study, two-dimensional displacement was estimated by 2-D cross-correlation between RF echoes. Important parameters, the sizes of a region-of-interest and search region, which determine tracking accuracy, were adaptively optimized by referring to instantaneous wall velocities, in the respective cardiac phases. The correlation coefficient between the lateral displacement estimated by the 2-D tracking with optimized parameters in longitudinal-axis view and in apical view (corresponding to lateral displacement in longitudinal-axis view) separately and accurately estimated by the 1-D phase-based method was 0.93. These results show possibility of this method for accurate measurement of two-dimensional heart motion to assess the regional myocardial strain rate.