On the Performance Investigation of Diverging-Wave-Based Vector Doppler Imaging with Linear Arrays: In-silico Results

Ultrafast vector Doppler imaging (VDI) is a promising non-invasive tool for investigating complex blood flow dynamics. Recent studies showed that the plane wave (PW) based VDI at large depths is prone to strong underestimation due to the transmit beam divergence in the elevation $(y-z)$ plane in the presence of spatial velocity gradients. However, studies on the influence of beam divergence in the azimuth $(x-z)$ plane on flow imaging for deep vessels are limited in the literature. In this regard, we investigate the performance of DW-based vector velocity measurements with multifold objectives. We evaluate the influence of the location of virtual focus (VF) and the imaging depth for DW-based VDI using linear arrays with and without elevation focus. Numerical simulations using Field II for typical parabolic and flat flow profiles with DW transmit resulted in a slightly higher bias ($0.5 \%-1 \%$ higher) for deeper vessels, particularly in the absence of spatial velocity gradients. However, in the presence of beam divergence, we observed that the inferences reported for PW-based VDI in the literature shall be extended to DW-based VDI as the underestimation was notably worse for parabolic flow profiles at large imaging depths. Moreover, this warrants further experimental studies with different field divergences controlled by VF for large depths.