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. Usually, NSI requires channel data and flexible beamformers (Receive-NSI). We investigate Angle-NSI, which operates on per-angle images during coherent plane-wave compounding. Approach. A matrix formulation was used to distinguish receive-domain and angle-domain null subtraction expressions and to identify their independent computational components. A small-angle model gives a physical interpretation of the angular null. The method was evaluated using a simulated point target, experimental resolution phantom targets, 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. Six beamforming methods were compared: delay-and-sum (DAS), coherence-factor-weighted DAS (CF-DAS), minimum variance beamforming (MV), delay-multiply-and-sum (DMAS), Receive-NSI and Angle-NSI. Main results. Angle-NSI reduced the simulated -6 dB lateral width from 0.226 mm for DAS to 0.116 mm, compared with 0.009 mm for Receive-NSI, while the axial width was 0.720 mm for these three methods. The lateral narrowing was confirmed on an experimental resolution phantom. Angle-NSI had shorter reconstruction time than Receive-NSI and the lowest post-delay processing time among the nonlinear methods. In power Doppler, microbubble trace widths were reduced by 50.2% for Receive-NSI and 30.9% for Angle-NSI compared to DAS. On a phantom and in vivo, both NSI variants reduced generalized contrast-to-noise ratio relative to the conventional methods. Significance. Angle-NSI provides simple post-processing of per-angle beamformed images. It narrows the apparent lateral response at low computational cost, at the expense of B-mode detectability: this makes it a well-suited alternative for high-resolution power Doppler applications.