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Conventional angiography exploits the different properties of dyes during various phases of the angiogram to evaluate the integrity of retinal and choroidal vasculature, and of the condition of the retinal pigment epithelium (RPE). After an intravenous injection of a bolus of dye, fluorescence can be detected within the choroid, followed by the retina arterial and venous circuit within 5–30 s (transit-phase images). After complete filling of the retinal vein, the dye begins to be recirculated (midphase images). After 5 min (and up to 20–30 min) of recirculation, late-phase images are acquired (Fig. 7). The key differences between fluorescein and indocyanine green are summarized in Table 1 (Fig. 8).

Fig. 5. Ultrawide-field photograph of an eye with acute retinal necrosis due to herpes zoster. The overall image is cloudy due to moderate vitritis. However, peripheral retinal necrosis can be seen as areas of retinal whitening (arrowheads). Attenuation and sclerosis of the peripheral retinal arterioles can also be seen, signifying arteritis (arrows).

Fig. 6. Ultrawide-field photograph of an eye with extensive exudation resulting from retinal angioma in the inferior retina (arrows).

Fig. 7. Fluorescein angiography of an eye with superotemporal retinal vein occlusion. Choroidal flush and presence of a cilioretinal artery (white arrow) can be seen in the 8-s frame (a). This is followed by filling of the central retinal artery 2 s later (b). Laminar flow within the retina veins except in the superotemporal branch can be seen in the 14-s frame (arteriovenous phase; c), confirming the diagnosis. In the late frame taken at 5 min (d), nonperfusion of the superotemporal retina and disruption of the foveal avascular zone can be seen. Leakage from an area of neovascularization can be seen (black arrow).

Fluorescence from the dye should appear promptly within the choroidal and retinal vasculature in healthy eyes and not leak. However, hyperfluorescence may appear due to a window defect as a result of RPE atrophy, staining (such as in drusen or optic disc), pooling within a serous retinal detachment, or leakage through diseased vasculature. Conversely, hypofluorescence may result from masking by overlying tissue or perfusion defects. Viewing of stereo-pairs of images acquired at slightly different angles allows appreciation of depth of various lesions and is particularly important in diagnosing certain conditions, such as retinal angiomatous proliferation (RAP) and polypoidal CNV. In addition to still images, videoangiography allows for assessment of the dynamic features, including the filling pattern, speed, and pulsations within the vessels.

Fig. 8. Type 2 choroidal neovascularization (CNV) imaged on fluorescein (FA) and indocyanine green (ICGA) angiography. The CNV appears as hyperfluorescent network with leakage in the FA (a) but not in the ICGA (b). Feeder and draining vessels (arrows) are visible on the ICGA but not on the FA.

Table 1. Key differences between fluorescein and indocyanine green

	Fluorescein	Indocyanine green
Molecule	Small (molecular weight, 376 Da)80% bound	Large (molecular weight, 775 Da)98% plasma protein bound
Emission range	530 nm (green)Masked by RPE, thick blood and fibrotic tissue	790–805 nm (near infrared)Penetrates through RPE
Leakage	Leaks readily through abnormal vasculature	Minimal leak
Main utility	Assessing retinal vasculature and lesions above the RPE, e.g.– Diabetic retinopathy– Retinal vascular diseases– Choroidal neovascularization	Assessing choroidal vasculature, e.g.– Polypoidal choroidal vasculopathy– Choroidal inflammation– Choroidal tumors