Limited view correction in low-optical-NA photoacoustic microscopy

Photoacoustic microscope (PAM) with a low optical NA suffers from limited view along the optical axis, due to the coherent cancellation of acoustic pressure waves after being excited with a smoothly focused beam. Using larger-NA (NA>0.3) objectives can readily overcome the limited-view problem, while the consequences are the shallow working distance and time-consuming depth scanning for large-volume imaging. Instead, we report an off-axis oblique detection strategy that is compatible with low-optical-NA PAM for turning up the optical-axis structures. Comprehensive photoacoustic modeling and ex vivo phantom imaging experiments are conducted to validate the efficacy of correcting the limited view. Proof-of-concept experiment results show that the visibility of optical-axis structures can be greatly enhanced (>18 dB) by making the detection angle off the optical axis larger than 45 degrees, strongly demonstrating off-axis oblique detection is a simple and cost-effective alternative method to solve the limited-view problems in low-optical-NA PAMs.