在超音波陣列成像系統中，由於成像聲速和組織實際聲速之間的不匹配而導致影像品質的下降。在先前的研究中已經提出利用通道域相位梯度差值來估計最佳成像聲速，但其受限於發射聚焦深度，這是因為非聚焦區中通道信號的低空間相干性造成了最佳成像聲速的高估。為了降低最佳成像聲速的估計誤差，本研究使用了合成發射孔徑波束形成方法，使整個影像深度上皆保持其通道信號的空間相干性，具體來說，將相鄰掃描線的通道信號疊加以補償非聚焦區中的聚焦品質，然後對深度和掃描線平均之後，左右子孔徑相減的相位梯度差值為零所對應到的聲速為最佳聲速。
研究結果顯示，合成發射孔徑波束形成有效地將在模擬中的最佳聲速估計誤差從4.3 %降低到0.1 %，並在ATS仿體實驗中將最佳聲速的估計誤差從8.8 %降低到0.1 %。當我們將最佳成像聲速用於波束形成時，囊腫(Cyst)區域的對比解析度和點反射體(Wire)的橫向解析度都會是最好的。與我們先前提出的研究相比，本論文所提出的改進後的方法在高雜訊的情況下其聲速估計結果也能表現出更好的的穩定性。

In ultrasound array imaging system, degradation of image quality occurs due to mismatch between beamforming sound velocity and tissue sound velocity. Channel-domain differential phase gradient has been previously proposed to optimize the beamforming sound velocity but its efficacy is limited to transmit focal depth. This is due to the low spatial coherence of channel signal in the non-focal region which leads 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 image depth. Specifically, channel signals from adjacent scanlines are combined to remedy the focusing quality in the non-focal region and then the zero of differential phase gradient between the left and right sub-apertures after depth and scanline averaging determines the optimal sound velocity. 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 ATS phantom measurement. Both the contrast ratio of the cyst region and the lateral resolution of wire reflector both peak when the optimized sound velocity is utilized for beamforming. Compared to our previous work, the improved method also exhibits higher robustness of sound velocity estimation in the presence of strong random noises.

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