ICG angiography = Indocyanine Green Angiography. It’s an imaging test that maps blood flow in the choroid, the blood-vessel layer under your retina.
What it is
- Dye test: A green dye called indocyanine green is injected into your arm vein
- Infrared camera: Special camera uses near-infrared light at 790-805 nm to see the dye through the retinal pigment
- Shows choroid: Unlike fluorescein angiography which shows retinal vessels, ICG goes deeper and highlights choroidal circulation
Why doctors order it
- See through blood/pigment: ICG light penetrates melanin, fluid, and thin blood. Useful when fluorescein images are blocked
- Diagnose specific conditions:
- Choroidal neovascularization (CNV): Especially “occult” CNV hidden under the retina in AMD
- Polypoidal choroidal vasculopathy (PCV): Branching vessels with polyp-like aneurysms, common in African/Asian patients
- Central serous chorioretinopathy (CSCR): Shows choroidal hyperpermeability areas
- Lacquer cracks: Checks if CNV is growing through the cracks
- Inflammatory diseases: Vogt-Koyanagi-Harada, choroiditis, to see active lesions
- Tumors: Choroidal melanoma vs nevus
How the test works
- Prep: Pupils dilated. You’ll sign consent and get an IV line
- Injection: 25 mg ICG dye injected into arm. It binds to plasma proteins and stays inside blood vessels
- Imaging phases:
- Early 0-1 min: Choroidal arteries and choriocapillaris fill
- Mid 5-15 min: Venous phase and leakage patterns show
- Late 20-30 min: Staining or “washout” patterns. Polyps stay bright late
- Time: Takes 20-30 minutes total
ICG vs Fluorescein angiography
Feature ICG Fluorescein
Light used Near-infrared 790-805 nm Blue light 490 nm
What it shows Choroidal vessels Retinal vessels
Dye leakage Minimal. Stays in vessels Leaks easily from damaged vessels
See through blood Yes No
Best for PCV, CSCR, occult CNV, choroidal tumors Diabetic retinopathy, vein occlusion, macular edema
Safety & side effects
- Very safe: Severe reaction rate is about 0.05%, much lower than fluorescein
- Common effects: Temporary green discoloration of skin, green urine for 1 day
- Mild reactions: Nausea in 1-2%, sneezing, itching
- Contraindications: Iodine allergy because ICG contains iodine. Also avoid in liver disease and pregnancy unless essential
- No kidney issues: Unlike CT contrast, ICG isn’t nephrotoxic
What to expect after
- Vision: Not blurred by the dye itself, but dilation makes you light sensitive for 4-6 hours
- Drive: Bring someone. You can’t drive until dilation wears off
- Results: Retina specialist reads images same day and decides on treatment like anti-VEGF or photodynamic therapy
Bottom line: If fluorescein angiography shows the retinal circulation, ICG shows the “basement” blood supply under it. It’s the go-to test when doctors suspect PCV, CSCR, or hidden CNV, especially in highly pigmented eyes.