Ensemble tracking for 2D vector velocity measurement: Experimental and initial clinical results

We describe a new method, called Ensemble Tracking, for estimating two-dimensional velocities with ultrasound. Compared to previous speckle tracking techniques, Ensemble Tracking measures motion over smaller times and distances, increasing maximum velocities and reducing errors due to echo decorrelation. Ensemble Tracking uses parallel receive processing, 2D pattern matching, and interpolation of the resulting tracking grid to estimate subpixel speckle translations between successive ultrasonic acquisitions. In this study, small translations of a tissue mimicking phantom were quantified at transducer angles of 0 degrees, 45 degrees, and 90 degrees. Measurements over three parallel beam spacings and all transducer angles had mean errors from -4% to +11%, when parallel beam amplitudes were normalized. Such amplitude normalization substantially improved results at 45 degrees and 90 degrees. The amplitude, spacing, and correlation between the parallel beams were quantified, and their effects on the accuracy and precision of estimates are discussed. Finally, initial clinical results demonstrate the ability to track and display blood flow in the carotid artery.