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Indocyanine Green Angiography: Imaging the Blood Vessels Beneath the Retina
What Is ICG Angiography?
ICG angiography is a dye-based imaging procedure that creates detailed photographs of the choroidal circulation using infrared light. Understanding how this test works helps explain why it is recommended for certain eye conditions.
The choroid is a rich, multi-layered network of blood vessels positioned between the retina and the sclera (the tough white outer wall of the eye). It provides the majority of the blood supply to the outer layers of the retina, making it essential for central and peripheral vision. Many serious conditions affecting vision, including certain forms of macular degeneration and rare choroidal disorders, involve changes within this vascular layer. Seeing the choroid clearly is a critical step toward accurate diagnosis and appropriate treatment.
Indocyanine green is a water-soluble dye that, once injected into a vein, quickly binds to proteins circulating in the blood. This tight protein binding means the dye leaks very little from the choroidal vessels, which are naturally porous. As a result, the choroidal blood vessels stay highlighted long enough for the camera to capture detailed images at multiple stages of circulation.
ICG fluoresces (emits light) in the near-infrared range, at wavelengths that penetrate through the retinal pigment epithelium (the RPE, a thin layer of cells between the retina and choroid), through macular pigment, and even through fluid or deposits within the retina. This penetrating ability is what makes ICGA uniquely capable of visualizing the choroid when other imaging methods fall short.
Fluorescein angiography (FA) is a related but distinct imaging test. FA uses a different dye to highlight the blood vessels within the retina and works very well for detecting retinal conditions such as diabetic retinopathy and retinal vein occlusion. However, the fluorescein signal is largely blocked by the RPE before it can reach the choroid, which limits FA's ability to image deeper vascular structures.
ICG angiography was developed specifically to overcome this limitation. The two tests are highly complementary, and retina specialists often order both in a single session to gain the most complete view of the retina and choroid together. Each test answers different clinical questions, and using them together leads to more accurate diagnoses and better-informed treatment plans.
When Is ICG Angiography Recommended?
ICGA is a targeted tool. It is recommended when a retina specialist needs detailed information about the choroidal blood vessels that other imaging methods cannot reliably provide. Several conditions are particularly well-suited to evaluation with ICGA.
Age-related macular degeneration (AMD) is one of the most common causes of vision loss in older adults. The wet form of AMD involves the growth of abnormal new blood vessels beneath the retina, a process called choroidal neovascularization (CNV). Some types of CNV, known as occult CNV, are difficult or impossible to see on fluorescein angiography alone. ICGA can reveal these hidden vessels with much greater clarity, helping specialists identify the type, size, and exact location of abnormal growth. This information is essential for choosing the most effective treatment approach.
Polypoidal choroidal vasculopathy (PCV) is a condition in which branches of choroidal blood vessels develop bulging, polyp-like formations. These polyps can leak fluid or bleed beneath the retina, causing vision loss that resembles wet AMD but may require a different treatment approach. ICGA is considered the most reliable method for diagnosing PCV because it clearly reveals the characteristic branching vessel network and distinctive polyp structures that other imaging methods frequently miss.
Accurately distinguishing PCV from other forms of wet AMD matters because some patients with PCV benefit from a combination of photodynamic therapy and anti-VEGF injections (medications that block the growth of abnormal blood vessels), rather than anti-VEGF therapy alone. Without ICGA, this distinction is often impossible to make with confidence.
Retinal angiomatous proliferation (RAP) is a specific pattern of abnormal blood vessel growth associated with AMD in which new vessels develop within the retina and grow toward or connect with the choroidal circulation. ICGA helps retina specialists identify the origin and stage of RAP lesions, which directly influences how the condition is managed. Without accurate characterization on ICGA, treatment decisions may be based on incomplete information.
Central serous chorioretinopathy (CSCR) is a condition in which fluid accumulates beneath the retina, often causing a blister-like area of detachment in the central vision zone (the macula). The underlying cause involves abnormal behavior in the choroidal blood vessels, including increased leakiness and vascular congestion. ICGA helps map the full extent of choroidal vascular abnormalities in CSCR, often revealing a broader area of involvement than OCT or fluorescein angiography can detect. This wider view is particularly useful for planning treatment in patients with chronic or recurring CSCR.
Uveitis is the term for inflammation inside the eye, and it can involve structures at the front, middle, or back of the eye. Certain forms of uveitis, including sympathetic ophthalmia (a rare condition in which inflammation in one eye triggers inflammation in the other, sometimes following injury or surgery) and other posterior uveitides, can involve the choroid. In these cases, ICGA can detect areas of choroidal inflammation that are invisible on fluorescein angiography or OCT alone. This information helps retina specialists understand the full extent of the inflammatory process and guide appropriate therapy.
What to Expect During the Test
ICG angiography is performed in a clinic setting by trained retinal imaging technicians under the supervision of a retina specialist. The procedure is well-tolerated by most patients and is generally completed within an hour from start to finish.
A member of our clinical team will review your medical history before the procedure begins. You will be asked about any known allergies, particularly to iodine or iodine-containing contrast agents used in other types of medical imaging such as CT scans. This is important because the ICG dye preparation contains sodium iodide. You will also be asked about any history of liver disease, kidney problems, and whether you are pregnant or breastfeeding.
Dilating eye drops will be placed in your eyes before imaging begins. These drops widen the pupil to give the camera the broadest possible view inside the eye. Dilation typically takes 20 to 30 minutes to take effect and will keep your near vision blurry for several hours afterward. We recommend arranging for someone to drive you home after the appointment.
A small needle is placed in a vein in your arm, typically at the hand or the inner elbow. The ICG dye is then injected quickly through the needle. The standard dose is typically around 25 milligrams dissolved in a small volume of solution, though the dose may be adjusted based on your pupil size, level of ocular pigmentation, and the specific camera system in use.
As the dye circulates through the bloodstream into the choroidal vessels, a specialized fundus camera equipped with infrared filters captures a series of photographs. Images are taken at multiple time points, covering the early filling phase, peak circulation, and late washout of the dye. This sequence allows the retina specialist to analyze blood flow patterns throughout the choroid. The full imaging session typically takes between 20 and 30 minutes. The camera never touches your eye, and the imaging itself is completely painless.
Once imaging is complete, the needle is removed and you are free to leave. You may notice a mild greenish tint to your skin or a slight darkening of your urine for one to two days after the procedure as the dye clears from your body. These changes are entirely normal and temporary. Staying well hydrated can help your body eliminate the dye more quickly. Because dilating drops temporarily blur near vision and increase light sensitivity, wearing sunglasses and avoiding driving until your vision has fully returned to normal is advisable.
Safety and Possible Side Effects
ICG angiography has a well-established safety record and is generally considered a low-risk procedure. As with any test involving an injected substance, some patients may experience mild reactions, and individual health factors can influence the level of risk for certain individuals.
At standard doses, adverse events from ICG angiography are uncommon. When reactions do occur, they are usually mild and brief. The most commonly reported effects include a temporary drop in blood pressure, a slightly elevated heart rate, and hives (raised, itchy skin welts). These typically resolve on their own within minutes of the injection. Serious adverse events are rare and occur far less frequently than with many other contrast-based imaging procedures used in medicine.
Because the ICG dye preparation contains sodium iodide, patients with a known iodine sensitivity or allergy face a higher risk of a reaction. Shellfish allergy is sometimes associated with iodine sensitivity, though the relationship is not always straightforward. Our team always asks about iodine allergies, shellfish allergies, and any prior reactions to contrast dyes before proceeding with the test.
In rare cases, patients with iodine sensitivity have experienced more serious reactions, including difficulty breathing and anaphylaxis (a severe, whole-body allergic response). If you have any history of allergic reactions to contrast agents or a known iodine sensitivity, it is essential to inform your retina specialist before your appointment so that appropriate precautions can be put in place or an alternative imaging approach can be considered.
Patients with significant liver disease should discuss this with their retina specialist before undergoing ICGA, as the liver is the primary organ responsible for clearing the dye from the body. Patients with severe kidney disease may also face a higher risk of side effects. ICG angiography is generally avoided during pregnancy because there are insufficient studies to confirm its safety for a developing baby. Your retina specialist will carefully weigh the clinical need for the test against any individual health factors before recommending it.
How ICG Angiography Fits Into Your Retina Care
ICGA is one component of a broader diagnostic approach. It provides specific information about the choroidal circulation that other tests cannot fully capture, making it a valuable complement to, rather than a replacement for, other imaging tools used in retina care.
Optical coherence tomography (OCT) is a non-invasive test that uses light waves to create detailed cross-sectional images of the retinal layers. OCT has become a cornerstone of retina care because it is quick, painless, and provides excellent structural information. OCT angiography (OCTA) is a newer, dye-free variation that can detect blood flow within some retinal and choroidal vessels. However, neither standard OCT nor OCTA provides the depth of choroidal vascular detail that ICG angiography offers, particularly in complex or advanced cases where the deeper choroid is involved.
In practice, fluorescein angiography, ICGA, and OCT are used together to build the most complete picture of what is happening in and beneath the retina. Each test answers different clinical questions, and the combination gives your retina specialist the information needed to make precise, individualized decisions about your care.
The findings from ICGA often directly change the course of treatment. Correctly identifying whether abnormal blood vessel growth represents occult choroidal neovascularization, polypoidal choroidal vasculopathy, or retinal angiomatous proliferation is clinically important because each condition may respond differently to available therapies. Anti-VEGF medications, which block the chemical signals that trigger abnormal vessel growth, are central to treating many choroidal conditions, but the specific medication chosen, the dosing schedule, and whether additional treatments are combined may all depend on what ICGA reveals. Your retina specialist will review your full imaging results and recommend the approach best suited to your individual situation.
Imaging technology continues to evolve in ways that expand the clinical value of ICGA. Ultra-widefield ICGA systems can now capture a panoramic view of the choroidal vasculature in a single image, revealing abnormalities in the far peripheral choroid that older camera systems could not detect. Combined imaging platforms allow retina specialists to acquire OCT, scanning laser ophthalmoscopy (SLO), and ICG images simultaneously, making it possible to compare different types of findings side by side during a single visit. These developments continue to improve diagnostic accuracy and efficiency for patients with complex choroidal conditions.
Symptoms That May Need Urgent Attention
Some of the conditions for which ICGA is used can progress rapidly and cause lasting vision loss if not treated promptly. Recognizing warning symptoms and knowing when to seek immediate care is an important part of protecting your vision.
If you experience any of the following symptoms, contact a retina specialist immediately or go to an emergency room without delay. These can be signs of a serious retinal event that requires urgent evaluation and treatment.
- A sudden increase in floaters (spots, strings, or cloudy areas drifting across your vision)
- Flashes of light in one or both eyes
- A dark shadow, curtain, or veil moving across your field of vision
- Sudden loss of vision in one eye
- Straight lines appearing wavy or distorted (a symptom called metamorphopsia)
If you have been diagnosed with wet AMD, central serous chorioretinopathy, polypoidal choroidal vasculopathy, or another condition involving the choroid, keeping all scheduled follow-up appointments is essential. Changes in the choroidal blood vessels can occur gradually and without noticeable symptoms in the early stages. Regular imaging visits, which may include ICGA at specific points in your care, allow your retina specialist to detect those changes early and adjust your treatment plan before significant vision loss occurs.
Frequently Asked Questions
Patients often have practical questions about what ICG angiography involves and how it fits into their overall care. The answers below are intended to provide additional guidance that goes beyond what is covered in the sections above.
These are two related but distinct tests that use different dyes and answer different clinical questions. Fluorescein angiography is the better choice for conditions that primarily affect the retinal vasculature, such as diabetic retinopathy or retinal vein occlusion. ICG angiography is used specifically when detailed information about the choroidal vessels beneath the retina is needed. In many situations, both tests are performed during the same visit because the information they provide is complementary rather than redundant. Your retina specialist will determine which test or combination of tests is appropriate based on your specific diagnosis.
ICG dye is processed primarily by the liver and is largely cleared from the bloodstream within 24 hours of injection. The mild greenish tint some patients notice in their skin or the slight darkening of their urine during this time is harmless and resolves on its own. Drinking extra fluids in the hours following the procedure can support this clearance process. If you notice any symptoms beyond these cosmetic changes, such as a worsening skin rash or difficulty breathing, contact our office right away, as these could indicate a delayed reaction that warrants attention.
A known iodine or shellfish allergy does not automatically prevent you from having ICG angiography, but it does require a careful conversation with your retina specialist before the test is scheduled. Depending on the nature and severity of your allergy, pre-treatment with antihistamines or corticosteroids may reduce the risk of a reaction. In some cases, an alternative diagnostic approach may be recommended instead. The most important step is to disclose all known allergies during your medical history review so that your care team can make the safest possible plan for you before any dye is used.
Whether you will need repeat ICGA depends on your diagnosis and the path your treatment takes. Some patients have the test once at the time of initial diagnosis to confirm a diagnosis and characterize a lesion, and are then monitored with non-invasive OCT imaging during treatment. Others may need ICGA at specific intervals to assess how well treatment is working or to evaluate whether a condition has changed over time. Your retina specialist will explain whether follow-up ICG angiography is part of your ongoing care plan and what those visits will involve.
Most patients complete ICG angiography without any significant discomfort. Brief nausea, a feeling of warmth, or mild dizziness shortly after the injection can occasionally occur and typically resolve within minutes on their own. If you develop hives, difficulty breathing, throat tightening, or feel significantly unwell after leaving our office, seek emergency care immediately, as these can be signs of a delayed allergic reaction. For any symptoms that concern you but do not feel like an emergency, contact our office so that a member of our team can advise you on the most appropriate next steps.
Schedule a Consultation
At New England Retina Associates, our fellowship-trained retina specialists offer ICG angiography as part of a comprehensive, individualized approach to diagnosing and treating conditions affecting the back of the eye throughout Connecticut. Whether you have been referred for this test or are seeking evaluation for a new vision concern, we welcome you to schedule an appointment at any of our four offices. Our team is committed to providing the expert, attentive care you need to understand your condition and protect your vision for the long term.
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