Laser-triggered, lipsome-encapsulated fluorescent dyes can be used as a tracer in the eye. Fluorescein is widely used in ophthalmic diagnosis, but it must be applied topically or injected intravenously, resulting in diffuse spread of dye through the retinal circulation along with high levels of background fluorescence in the choroid. Carboxyfluorescein and calcein, dyes similar to fluorescein, can be encapsulated in liposomes at high concentrations (100 mM). At such elevated concentrations, the fluorescence is quenched, but when the dye is released from the liposome, it is diluted and fluoresces intensely. Liposomes containing these fluorescent dyes can be injected intravenously and once in the circulation, can be trigged to release their contents by a localized pulse of light from a laser. The dye converts the laser into heat, which raises the temperature of the liposomal envelope, lyses the liposome, and releases the dye. The ability to control location, timing, and repetition of dye release would allow local flow patterns to be visualized without the background of the underlying choroidal circulation or adjacent vessel networks. The system will make assessment of localized perfusion defects possible. The creation of a better defined wavefront, achieved with the generation of a bolus, opens the way for quantitative measurement of flow. By controlling the time between the release of the dye from the liposomes and the time of recording, we have been able in preliminary studies to measure the velocity of blood in vitro, as well as in the retinal vessels of rabbits and monkeys. Moreover, the possibility of repeating the procedure allows refinement of alignment and focusing of the camera, thereby optimizing the quality of the photographs. Additionally, measurements can be performed before and after induced changes of physiologic conditions (i.e., oxygen or nitrogen breathing, exposure to light or pharmacological agents). The system offers the control of time, location, and repetition of delivery, which potentially will be useful in a number of therapeutic applications as well.
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