Improved contrast in laser-diode-based photoacoustic images with short-lag spatial coherence beamforming

Pulsed laser diodes (PLDs) enable photoacoustic imaging with lower cost, increased portability, and higher frame rates compared to conventional Q-switched Nd:YAG lasers. However, the main disadvantage of the PLD is its low peak power, which necessitates averaging thousands of photoacoustic signals to achieve signal-to-noise ratios that are comparable to those produced by an Nd:YAG laser. The averaging process degrades temporal resolution with minimal improvements to image contrast. This work is the first to investigate the use of a PLD and short-lag spatial coherence (SLSC) beamforming to display high-contrast photoacoustic images with minimal to no signal averaging required. The mean contrast in single photoacoustic images of targets as deep as 5-15 mm from the surface was improved by 11-17 dB with SLSC beamforming when compared to conventional delay-and-sum (DAS) beamforming of the same data. The SLSC correlation kernel was adjusted based on laser pulse lengths to demonstrate applicability to coded excitation. Results suggest that coherence-based beamforming can overcome current limitations with real-time PLD-based photoacoustic imaging.