Motion compensated beamforming in synthetic aperture vector flow imaging

In synthetic aperture imaging the beamformed data from a number of emissions are summed to create dynamic focusing in transmit. This makes the method susceptible to motion, which is especially the case for the synthetic aperture flow estimation method, where large movements are expected. In this paper, these motion effects are considered. A number of Field II simulations of a single scatterer moving at different velocities are performed both for axial and lateral velocities from 0 to 1 m/s. Data are simulated at a pulse repetition frequency of 5 kHz. The signal-to-noise ratio (SNR) of the beamformed response from the scatterer at all velocities is compared to that of a stationary scatterer. For lateral movement, the SNR drops almost linearly with velocity to -4 dB at I m/s, while for axial movement the SNR drop is largest, when the scatterer moves a quarter of a wavelength between emissions. Here the SNR is -10 dB compared to the stationary scatterer. A 2D motion compensation method for synthetic aperture vector flow imaging is proposed, where the former vector velocity estimate is used for compensating the beamforming of new data. This method is tested on data from an experimental flow rig acquired using our RASMUS experimental ultrasound scanner and a 5.5 MHz linear array transducer. A 11.25 mu s non-linear chirp is used as excitation and the data from 128 emissions is used for estimating the flow direction and magnitude at a profile across the tube. The measurement was conducted at a How angle of 60 degrees with respect to the axial direction and a peak velocity of 0.1 m/s sampled at a pulse repetition frequency of I kHz. The mean bias across the profile was -8.4 % with respect to the peak velocity and the mean standard deviation was 12.2 % prior to compensation. When the proposed compensation was applied a mean bias of -3.6 % and a mean standard deviation of 2.8 % was seen.