Multi-wavelength photoacoustic microscopy for detection of retinal vein occlusion during laser photocoagulation in rabbits

Photoacoustic microscopy (PAM) can be an effective imaging modality to visualize retinal vein occlusion during laserinduced photocoagulation on major retinal veins. Developments in the vessels could be observed before and after laser irradiation due to the change of the optical absorption spectrum of the target vessels. However, the suitable wavelength to achieve high contrast PAM images of occluded vessels is unclear. This study evaluates the effect of wavelength on PAM imaging to evaluate the photocoagulation lesions on the rabbit to optimize the wavelength for imaging. Retinal vein occlusion (RVO) was created using a 532 nm millisecond pulse duration green light with concurrent intravenous administration of Rose Bengal (5 mg/kg) in New Zealand rabbits. Imaging was acquired by the PAM system at various wavelengths ranging from 520 nm to 590 nm. In addition, the thermal lesion was also confirmed using optical coherence tomography (OCT). A group of 20 retinal veins was irradiated for 0.5 s at a laser fluence of 850 W/cm2 (power = 150 mW, beam diameter = 75 μm). Twenty shots of the laser were applied to each major vein. PAM results showed that the thermal lesion was obviously visualized and exhibited lower contrast in comparison with untreated vessels posttreatment. Photoacoustic spectroscopy exhibited that the highest PA contrast of vessels treated with Rose Bengal laserinduced RVO occurred at a wavelength of 563, 570, and 578 nm, which was higher than the PA amplitude at lower and longer wavelengths respectively. The use of multi-wavelength PAM can provide a better method for visualization and evaluation of retinal vein occlusions.