Efficient focusing scheme for transverse velocity estimation using cross-correlation

The blood velocity can be estimated by cross-correlation of received RE signals, but only the velocity component along the beam direction is found. A previous paper showed that the complete velocity vector can be estimated, if received signals are focused along lines parallel to the direction of the flow. Here a weakly focused transmit field was used along with a simple delay-sum beamformer. A modified method for performing the focusing by employing a special calculation of the delays is introduced, so that a focused emission can be used. The velocity estimation was studied through extensive simulations with Field II. A 64-elements, 5 MHz linear array was used. A parabolic velocity profile with a peak velocity of 0.5 m/s was considered for different angles between the flow and the ultrasound beam and for different emit foci. At 60 degrees the relative standard deviation was 0.58 % for a transmit focus at 40 mm. For 90 degrees the new approach gave a relative standard deviation of 8.3 % with a focus at 40 mm and 8.0 % at a transmit focus of 150 mm. Pulsatile flow in the femoral artery was also simulated. A purely transverse flow profile could be obtained with a relative standard deviation of less than 10 % over the whole cardiac cycle, which is sufficient to show clinically relevant transverse color flow images.