Adaptive Gaussian-weighted nonlinear beamforming with coherence factor enhancement for reflection-mode PLD-based photoacoustic imaging

This paper presents a nonlinear beamforming algorithm, the adaptive Filtered Delay Adaptive-Gaussian-Weighted Multiply-and-Sum (F-D aGw MAS), and its coherence factor-augmented variant (F-D aGw MAS + CF), developed to enhance image quality in pulsed laser diode (PLD)-based reflection-mode photoacoustic imaging (PAI). Experimental validation was performed using chicken breast phantoms containing four Indian ink-filled capillary tubes (0.5 mm external diameter) positioned between 4 mm and 22 mm depths. Imaging was conducted at laser pulse energies of 18 µJ, 21 µJ, and 27 µJ. Quantitative analysis showed that F-D aGw MAS consistently outperformed conventional Delay-and-Sum (DAS) and Filtered Delay Multiply-and-Sum (F-DMAS) beamformers across all energy levels. At 27 µJ, F-D aGw MAS achieved 43–67% improvement in spatial resolution relative to DAS and 6–24% improvement compared to F-DMAS. The contrast ratio (CR) and signal-to-noise ratio (SNR) improved by 11–207% and 7–68% over DAS, and by up to 6% and 25% over F-DMAS, respectively. Incorporating coherence factor weighting further enhanced performance, with F-D aGw MAS + CF providing up to 60% better spatial resolution and 1–27% higher CR and SNR than DAS + CF. In addition, in vivo photoacoustic imaging of a healthy human wrist demonstrated clear visualization of cross-sectional view of a vascular structure at depths of approximately 6–8 mm, with improved contrast and reduced noise using the proposed method. These improvements result from the adaptive Gaussian weighting and coherence integration that strengthen coherent components and suppress off-axis noise. The robustness of F-D aGw MAS and F-D aGw MAS + CF across energy levels demonstrates their strong potential for high-contrast, high-resolution PLD-based PAI.