Extraction of choriocapillaris hemodynamic data from ICG fluorescence angiograms

There are conflicting views about the organization of the posterior pole choriocapillaris, particularly concerning blood flow through it, and there are difficulties associated with attempting to obtain such information using histologic techniques, sodium fluorescein angiography, or both. The present study uses a method of analysis based on high-speed angiograms to investigate posterior pole choroidal blood flow.

The analysis method employed is based on the premises that dye filling of the choriocapillaris is more rapid (because it is pulsatile) than dye filling of the underlying larger diameter vessels, and that fluorescence from these two overlapping layers is additive. The described analysis algorithm was applied to high-speed ICG fluorescence angiograms to emphasize information about choriocapillaris hemodynamics.

The analysis method was demonstrated in rhesus monkeys, and results indicate that the posterior pole choriocapillaris does not behave as a homogeneous structure, consisting of discrete lobular segments. In general, the cycle of dye filling of the choriocapillaris begins in the macular area and progresses radially toward the periphery in a wavelike manner, the filling cycle beginning with and being completed during one cycle of the intraocular pressure pulse.

It is possible to extract information related to choriocapillaris blood flow from high-speed ICG angiograms when a fairly well-defined dye bolus wavefront is present. Interpretation of the results suggests that the choriocapillaris lobules fill in a pulsatile manner, out of phase with each other, and may act thereby to dissipate the blood volume entering the choroid during each cardiac cycle in such a way that the retinal macular is not significantly displaced by expansion of the choroidal vascular volume. Depending upon the distribution of pressure gradients across a group of lobules, blood may flow from one lobule into or even through an adjacent one.