Measurement of circulation time in the retinal vasculature using selective angiography

Retinal circulation times (RCT) and mean circulation time (MCT) in the retinal vasculature have been used to determine the effect of ocular disease on retinal circulation. Both RCT and MCT are usually determined by measuring dye dilution curves during a traditional fluorescein angiogram. The authors present a variation of the dye dilution technique. The primary difference is that a fluorescent dye (carboxyfluorescein or calcein) was encapsulated in liposomes at a very high concentration. The dye was then released at a specific site in a retinal artery by the application of a laser pulse delivered through the pupil of the eye. The heat pulse produced by the laser lysed the liposomes in the targeted vessel, causing a stream of dye to be released. The time it took the dye front to pass through the artery and return through the vein (RCT) was then measured. To clarify the situation, two times were actually measured, the shortest path the dye traveled from the exposure site to a corresponding site on the vein (short RCT) and the longest path through the periphery to venous filling (long RCT). The measurements were made in both cynomolgus monkeys and squirrel monkeys. The local dye release achieved with this method was found to produce a more distinct dye front than traditional angiography while also eliminating the problem of recirculation. Choroidal background fluorescence, a factor that often obscures the dye front in traditional angiography, was minimal. Furthermore, it was possible to make individual measurements on each of the four arteries and to repeat the measurements several times.