Angle-domain null subtraction imaging from beamformed plane-wave data
Null subtraction imaging (NSI) narrows the apparent lateral response by combining fields produced with a zero-mean apodization and two DC-offset variants. Conventional NSI applies these weights across receive elements and therefore requires access to channel data and flexible beamformers, while increasing the computation time. We investigate an angle-domain analogue, termed angular NSI, that instead operates on the per-angle images produced during coherent plane-wave compounding. A matrix formulation was used to distinguish receive-domain and angle-domain null subtraction expressions and to identify their independent computational components. The method was evaluated using a simulated point target, an open microbubble (MB) traces dataset and two in vivo carotid acquisitions. Angle sampling, noise, phase jitter and target position were swept in simulation. A synchronized GPU benchmark compared the complete reconstructions. Conventional receive-domain NSI and standard delay-and-sum (DAS) imaging were references. Angle-domain NSI reduced the simulated -6dB lateral width from 0.226 mm for DAS to 0.116 mm, compared to 0.009mm for conventional receive-domain NSI, while the axial width was 0.721 mm for all methods. Median reconstruction times on an RTX A2000 GPU were 23.92, 30.19 and 24.74 ms for DAS, conventional NSI and angle-domain NSI. In power Doppler, the apparent width of the MB traces were reduced by 50.2% for conventional NSI and 30.9% for angular NSI compared to DAS. In vivo, both NSI variants reduced generalized contrast-to-noise ratio relative to DAS. Angle-domain NSI can be simply implemented as a post-processing step for all plane-wave sequences after beamforming. The technique is compatible with most of the usual beamformer implementations. It effectively reduces apparent lateral width, and its computation time is close to DAS and faster than conventional NSI.