In medical ultrasound imaging system, degradation of image quality occurs due to mismatch between beamforming sound velocity and propagating sound velocity in tissue. Channel-domain differential phase gradient has been previously utilized to optimize the beamforming sound velocity but its efficacy is limited to transmit focal depth. Specifically, low spatial coherence of channel signal in the non-focal region could lead to over-estimation of beamforming sound velocity. In order to alleviate the estimation bias of beamforming sound velocity, synthetic transmit aperture beamforming is proposed in this study to maintain the spatial coherence of channel data over the entire imaging depth. By combining channel signals from adjacent scanlines to remedy the focusing quality in the non-focal region, the zero of differential phase gradient between the left and right sub-apertures can accurately determine the optimal sound velocity for beamforming. Results indicate that the synthetic transmit aperture beamforming effectively reduces the estimation bias of beamforming sound velocity from 4.3% to 0.1% in the simulations and from 8.8% to 0.1% in phantom measurement. With the optimized sound velocity, the lateral resolution improves by 14.5%. Compared to our previous work, the improved method also exhibits higher robustness of sound velocity estimation in the presence of random noises. The variation of sound velocity estimation decreases from 59.1 m/s with the previous method to 16.9 m/s with the improved method in our simulation when the channel SNR is −25 dB.
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