Multi-line transmit (MLT) imaging enables the acquisition of high frame rate (HFR) data in ultrasound imaging, especially in echocardiography where capturing rapid events associated with heart motion can provide valuable information for disease diagnosis. MLT beams are generated by simultaneously transmitting multiple focused beams in different spatial directions within a single pulse-echo event. The main drawback of MLT imaging is the generation of crosstalk artifacts due to the interferences between multiple beams and targets, which will in turn decorrelate the backscattered echoes and will reduce the spatial coherence significantly, leading to poor image quality. In this study, we have investigated the effects of synthetic focusing on the overall coherence of the received signals. We have shown that achieving more accurate focusing due to the implementation of synthetic aperture beamformers could be less susceptible to the artifacts introduced with MLT, and this will in turn improve the coherence of the backscattered signals, resulting in an improved image quality. Simulation, phantom, and in-vivo experiments have been conducted to demonstrate that spatial coherence enhances as a result of synthetic focusing in MLT imaging (especially away from the transmit focus). Furthermore, we have implemented synthetic aperture methods together with coherence-based techniques to investigate their synergistic performance in further suppressing the incoherent backscattered echoes and improving target detectability. The results demonstrate that this provides considerable benefits in rejecting MLT crosstalk artifacts compared to the conventional dynamic receive focusing.
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