In vivo analysis of choroidal circulation by continuous laser-targeted angiography in the rat

PURPOSE. To obtain high-quality angiograms of the rat choriocapillaris with continuous laser-targeted angiography (LTA), for the purpose of assessing the choroidal circulation system in vivo by studying the patterns of the images. METHODS. A slit lamp was modified to incorporate two kinds of lasers (argon and diode). Carboxyfluorescein was encapsulated in heat-sensitive liposomes and injected intravenously. Encapsulated carboxyfluorescein was released locally by applying a continuous heat beam provided by diode laser (810 nm) with various powers. Video angiograms were generated with excitation illumination provided by argon laser (488 and 514 nm) to observe highly selective images of the choriocapillaris. RESULTS. Three distinct phases (filling, plateau, and draining) were observed in fluorescent images of choriocapillaris by applying the diode laser continuously. In the plateau phase, a lobe-shaped area of choriocapillaris peripheral to the laser site was illuminated, and this finite area did not change in size with continuous laser application to the same spot. When laser power was increased, a larger area of choriocapillaris was illuminated in the plateau phase. The filling and draining phases demonstrated the flow patterns in choriocapillaris lobules, which filled from a central spot and drained along a peripheral ring. CONCLUSIONS. This study showed that the rat choriocapillaris is divided into independent functional units and that the choroidal circulation is segmental under normal conditions. The results implied that in LTA, the diode laser warms up a choroidal artery and the released fluorescein flows downstream to an area of choriocapillaris fed by the same artery. LTA appeared to be a powerful method to analyze choroidal circulation in vivo.