3-D High-Quality Power Doppler Using Coherence-Based Null Subtraction Imaging.

Three-dimensional power Doppler (PD) imaging enables visualization of volumetric microvasculature networks. However, 3-D ultrafast PD imaging with matrix arrays suffers from limited resolution due to the restricted aperture size and the use of unfocused plane waves, while delay-and-sum (DAS) exhibits high sidelobe levels (SLLs) and broad main lobes, hindering high-quality PD imaging. In this study, we aim to improve the 3-D null subtraction imaging (NSI) and propose coherence-based NSI beamformers, including NSI based on coherence factor (NSI-CF) and NSI based on spatial and angular CF (NSI-SACF). Specifically, NSI-SACF integrates the high-resolution capability of NSI and the high-contrast performance of SACF. The proposed beamformers are validated through simulations, phantom experiments, and in vivo experiments. Qualitative and quantitative evaluations confirm the superiority of NSI-SACF. In the simulations, NSI-SACF achieves a full-width at half-maximum (FWHM) of 0.059 mm compared with 0.391 mm for DAS and 0.225 mm for SACF. NSI-SACF also reduces the SLL by 75.5 dB and 33.6 dB compared with DAS and SACF, respectively, while effectively suppressing grating lobes. In the in vivo experiments, NSI-SACF improves the contrast-to-noise ratio (CNR) of blood flow by 20.03 and 8.65 dB over DAS and SACF, respectively. Furthermore, the FWHM of the vessel is reduced to 0.098 mm with NSI-SACF, compared with 0.326 mm for DAS and 0.159 mm for SACF. In summary, the proposed NSI-SACF beamformer effectively enhances the PD imaging by simultaneously improving spatial resolution and contrast, demonstrating the potential for clinical applications.