Vector Velocity Imaging Using Cross-Correlation and Virtual Sources

Previous investigations have shown promising results in using the directional cross-correlation method to estimate velocity vectors. The velocity vector estimate provides information on both velocity direction and magnitude. The direction is estimated by beamforming signals along directions in the range [0◦; 180◦[ and identifying the direction that produces the largest correlation across emissions. An estimate of the velocity magnitude is obtained from the spatial shift between signals beamformed along the estimated direction. This paper expands these investigations to include estimations of the vector velocities of a larger region by combining the estimations along several scan lines. In combination with a Bmode image, the vector velocities are displayed as an image of the investigated region with a color indicating the magnitude, and arrows showing the direction of the flow. Using the RASMUS experimental ultrasound scanner, measurements have been carried out in a water tank using a 7 MHz transducer. A 6 mm tube contained the flow and a Danfoss, MAG 3000, magnetic flow meter measured the volume flow. The tube has a parabolic flow profile with a peak velocity of 0.29 m/s. During the experiments fixed beam-to-flow angles at {60◦, 75◦, 90◦} have been applied. The images are obtained using a pulse repetition frequency of 15 kHz, and the images contain 33 lines with 60 emissions for each line. Corresponding to the three fixed beam-to-flow angles, the angle estimates along the center scan line have a bias of {−3.9◦,−12.8◦,−18.1◦} and standard deviation of {10.0◦, 18.2◦, 32.2◦}. The estimates of the velocity magnitude have bias of {4.4%, 8.1%,−5.4%} and standard deviation of {9.7%, 14.3%, 13.4%} relative to the peak velocity. The amount of in-tube angle estimates in the range of ±10◦ from the true angle are {74%, 77%, 66%}. In-vivo measurements are carried out on a human volunteer. These measurements include the common carotid artery and the femoral bifurcation.