2-D motion estimation using two parallel receive beams

We describe a method for estimating 2-D target motion using ultrasound. The method is based on previous ensemble tracking techniques, which required a t least four parallel receive beams and 2-D pattern matching. In contrast, the method described requires only two parallel receive beams and 1-D patte rn matching. Two 1-D searches are performed, one in each lateral direction. The direction yielding the best match indicates the lateral direction of m otion. Interpolation provides sub-pixel magnitude resolution. We compared t he two beam method with the four beam method using a translating speckle ta rget at three different parallel beam steering angles and transducer angles of 0, 45, and 90 degrees. The largest differences were found at 90 degrees , where the two beam method was generally more accurate and precise than th e four beam method and also less prone to directional errors at small trans lations. We also examined the performance of both methods in a laminar flow phantom. Results indicated that the two beam method was more accurate in m easuring the flow angle when the flow velocity was small. Computer simulati ons supported the experimental findings. The poorer performance of the four beam method was attributed to differences in correlation among the paralle l beams. Specifically, center beams 2 and 3 correlated better with each oth er than with the outer beams. Because the four beam method used a compariso n of a kernel region in beam pair 2-3 with two different beam pairs 1-2 and 3-4, the 2-to-1 and 3-to-4 components of this comparison increased the inc idence of directional errors, especially at small translations. The two bea m method used a comparison between only two beams and so was not subject to this source of error. Finally, the two beam method did not require amplitu de normalization, as was necessary for the four beam method, when the two b eams were chosen symmetric to the transmit axis. We conclude that two beam ensemble tracking can accurately estimate motion using only two parallel re ceive beams.