Double-illumination photoacoustic microscopy.

Recent developments of optical-resolution photoacoustic microscopy (OR-PAM) have improved its spatial resolution and imaging speed. However, the penetration depth of OR-PAM is still limited to ~1 mm in tissue, owing to the strong tissue scattering. Here, we have developed double-illumination PAM (DI-PAM), which illuminates the sample from both top and bottom sides simultaneously. Through phantom and in vivo experiments, we have demonstrated for thin targets that DI-PAM has a penetration depth of ~2 mm in tissue at 532 nm and a focal zone of 260 μm, both significant improvements over traditional reflection or transmission-mode OR-PAM.