Accuracy improvement of echographic speckle tracking based on analysis of estimation error caused by acoustic pressure field

In echocardiography, blood-flow measurement is important, and several methods of measuring the velocity vector of blood flow have been proposed including echographic speckle tracking. Echographic speckle tracking is typically based on blockmatching algorithms; however, they incur high calculation cost; thus, are time-consuming. To enable real-time blood-flow vector measurement, we applied the Kanade-Lucas-Tomasi (KLT) algorithm to echographic speckle tracking, but the measurement accuracy was low in preliminary trial. This is mainly because echographic speckles deform as speckles move due to the acoustic pressure field of a transmitting beam. The objective of this study was to minimize the estimation error of KLT-based speckle tracking by analyzing error propagation. We analyzed error propagation from the acoustic pressure field to the velocity error by simplifying speckle deformation and formulated the major error factors. From this analysis, we propose a policy of determining the measurement conditions, which are region-of-interest (ROI) size, waveform, and number of ROI divisions, for minimizing estimation error. We verified the proposed policy through numerical simulations. As a result of the analysis and simulations, the gradient of the pressure field, number of ROI divisions, and moving distance of a speckle accounted for most of the estimation error. In addition, optimizing these conditions restricted the mean estimation error to less than 10%. These results indicate that the accuracy of KLT-based speckle tracking can reach a practical level by designing measurement conditions based on the proposed policy.