3D High-quality Power Doppler Using Coherence-based Null Subtraction Imaging

Three-dimensional (3D) power Doppler (PD) imaging enables visualization of volumetric microvasculature networks. However, 3D 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 and broad main lobes, hindering high-quality PD imaging. In this study, we aim to improve the 3D 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 coherence factor (NSI-SACF). Specifically, NSI-SACF integrates the high-resolution capability of NSI and the high-contrast performance of spatial and angular coherence factor (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 sidelobe level 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 dB 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.