ICG (Indocyanine Green) angiography is a tool used by retinal specialists to primarily visualize choroidal blood vessels. The dye is used either stand alone or as an adjunct to fluorescein angiography (FA). It is important to understand the difference between ICG and fluorescein.
Unlike fluorescein, ICG is a large molecule dye and has a 98% binding to serum, which prevents it from leaking from normally porous choroidal vessels. ICG molecules are excited by a higher wavelength of light than fluorescein. The emission of excited light is at 835 nm, which essentially eliminates the absorption of light in the retinal pigment epithelium, allowing, in turn, the visualization of choroidal vessels.
ICG comes in powder form, and requires reconstitution with a dilutant; typically an aqueous solvent. The dosage is dependent on the amount of dilutant used to reconstitute, and tends to vary from practice to practice. Once reconstituted, the dye is only stable for 10 hours.
Choroidal blood vessels are one of the fastest vascular flow complexes in the body. Imaging dye as it fills these vessels can be a challenge. Not only is it fast, but there is no “normative” vessel pattern, as there is with retinal vessels; so we typically can’t image an arterial, venous or transit phase.
We can’t think of ICG as we do with fluorescein in regards to sub retinal neovascularization. In FA, we look for leakage patterns to determine the type or extent of a neovascular lesion. ICG allows us to image the SOURCE of the lesion. With ICG, one can differentiate between a choroidal vessel source (CNV) or a retinal vessel source (RAP lesion). With the elimination of laser treatment for wet AMD, the days of looking for a late “hotspot” on ICG are over. Feeder vessels are also identifiable on ICG angiography, and diagnosing the source of the lesion allows the physician a better prognosis on extent or length of treatment.
ICG and FA can be performed simultaneously or separately. Mixing a usual dosage of fluorescein with a usual dosage of ICG allows for one injection and the imager to switch back and forth between FA and ICG filters to image both retinal and choroidal vascular layers. Separating the dyes by a stopcock or valve is another option. After the initial early phase of both angiograms, the process is the same. Here is the protocols I’ve used:
Draw up the usual dosage of fluorescein in one syringe, and the usual dosage of ICG in another, larger syringe. Inject the fluorescein into the ICG syringe.
Connect the syringe to the IV tubing and inject as usual. At this point the imager has only one choice for early images: ICG or FA, as both dyes reach the eye at the same time. Once early images are captured, switch filters and RE FOCUS on the proper layer. Remember that choroidal and retinal vessels are at different depths from each other.
Continue the angiograms, switching filters, flash and software screens.
Draw up the usual dosage of fluorescein in one syringe, and the usual dosage of ICG in another syringe. Attach the syringes to a 3-way stopcock and attach stopcock to the IV tubing. Inject one of the dyes (It doesn’t matter which one you inject first), and capture the early images. Switch filters, flash and software, and inject the other syringe.
Continue going back and forth per your practice protocol.
My 2 cents on timing:
As we are looking for the SOURCE of sub retinal lesions, ICG is most useful in the early phase of the angiogram. Fluorescein is also helpful in the early phase, prior to the start of the leakage of dye. Mid to late phase of the fluorescein will reveal leakage. Therefore, both ICG and fluorescein late images are taken at 5 minutes.