OCT stands for optical coherence tomography, a non-invasive imaging test that creates detailed cross-sectional pictures of the retina and other structures inside your eye. It works similarly to an ultrasound, but uses light waves instead of sound, producing images with a resolution of 10 micrometers or less. That level of detail lets your eye doctor see each individual layer of the retina, measure their thickness in microns, and detect changes long before you notice any vision loss.
How OCT Works
At its core, OCT relies on a principle called light interference. The machine splits a beam of low-coherence light into two paths. One beam bounces off a reference mirror at a known distance, while the other enters your eye and reflects off the different tissue layers inside it. When these two beams recombine, they create an interference pattern that a detector records. The pattern reveals how deep inside the tissue each reflection came from, essentially mapping the structures beneath the surface.
A single depth measurement is called an A-scan. The machine rapidly collects thousands of A-scans side by side as it sweeps the light beam across your retina, and the combined set forms a B-scan: a two-dimensional cross-sectional image that looks like a thin slice through the back of your eye. Modern machines assemble multiple B-scans into full three-dimensional maps in seconds.
What Your Doctor Can See
An OCT scan displays every retinal layer, from the inner limiting membrane at the surface down through the nerve fiber layer, ganglion cells, inner and outer nuclear layers, the photoreceptor segments, and the retinal pigment epithelium at the base. Below that, newer machines can image the choroid (the blood-vessel-rich layer that nourishes the retina) and even the outer wall of the eye.
The scan also generates a color-coded thickness map. Warm colors like red and yellow mark thicker areas, while cool colors like blue and green mark thinner ones. These maps are divided into concentric circles at 1 mm, 3 mm, and 6 mm diameters, giving your doctor a quick visual summary of how the retina compares to normal values. In a healthy adult, the central macular thickness (the innermost 1 mm circle) typically measures around 200 to 250 micrometers. The retinal nerve fiber layer averages roughly 90 to 110 micrometers, and the ganglion cell layer runs about 70 to 90 micrometers.
The fovea, the tiny pit at the center of the macula responsible for sharp central vision, appears as a slight dip on the scan because the inner retinal layers thin out there, leaving the densely packed photoreceptors exposed. Disruptions in that normal dip, or fluid accumulation around it, show up clearly on OCT even when they’re invisible during a standard eye exam.
Conditions Diagnosed With OCT
OCT is used most heavily for three categories of eye disease: macular degeneration, diabetic eye disease, and glaucoma.
In age-related macular degeneration (AMD), OCT reveals the fluid buildup, drusen deposits, and abnormal blood vessel growth that characterize the disease. It’s become so central to managing wet AMD that most retina specialists now rely on OCT rather than fluorescein angiography, an older test that requires injecting dye into a vein. OCT is faster, non-invasive, and provides objective thickness measurements that track how well treatment with anti-VEGF injections is working over time.
For diabetic retinopathy, OCT detects macular edema (swelling from fluid leakage) and measures choroidal thickness, which may reflect disease severity. Doctors use repeated scans to decide when to adjust treatment, time surgical interventions, and monitor whether fluid is accumulating or resolving.
In glaucoma, the scan focuses on the retinal nerve fiber layer and ganglion cell layer, both of which thin as the disease damages the optic nerve. RNFL thinning can be one of the earliest detectable structural changes in glaucoma, sometimes appearing before any noticeable vision loss. The machine compares your measurements against a normative database of age-matched healthy eyes, flagging areas that fall outside the normal range. RNFL thickness varies naturally around the optic nerve: it’s thickest at the top and bottom (around 130 to 135 micrometers) and thinnest on the sides (roughly 74 to 87 micrometers).
Beyond these three conditions, OCT also helps evaluate retinal detachments, central serous retinopathy, pathological myopia, and pigment epithelial detachments.
OCT vs. Fluorescein Angiography
Fluorescein angiography involves injecting a fluorescent dye into your bloodstream and photographing it as it flows through the blood vessels in your retina. It’s useful for mapping blood flow and identifying leaking vessels, but it’s invasive, time-consuming, and occasionally causes nausea or allergic reactions. OCT captures structural detail without any injection or dye, which is why clinical practice has shifted heavily toward OCT-based diagnosis and monitoring.
A newer extension called OCT angiography (OCTA) can actually visualize blood flow in the retina without dye, combining the structural detail of standard OCT with vascular information that previously required fluorescein angiography. When OCTA is available, it can help distinguish between types of abnormal blood vessel growth and identify the cause of suspicious findings on standard OCT scans.
Types of OCT Machines
The two main technologies you’ll encounter are spectral domain OCT (SD-OCT) and swept source OCT (SS-OCT). SD-OCT is the current standard in most eye clinics and provides excellent retinal imaging. SS-OCT uses a different light source that penetrates deeper into tissue, making the choroid, sclera, and deeper structures more visible. In studies of highly nearsighted eyes, SS-OCT revealed pathology along the walls of the posterior staphyloma (a bulge in the back of the eye) that SD-OCT missed entirely, and it detected structural splits in the retina and vitreous that weren’t visible on the other machine.
For routine retinal and glaucoma care, SD-OCT is more than sufficient. SS-OCT adds value when your doctor needs to see beyond the retina into the choroid and sclera, particularly in conditions like pathological myopia or when choroidal thickness is clinically relevant.
What the Test Feels Like
An OCT scan takes just a few minutes. You sit in front of the machine and place your chin on a rest, similar to a slit-lamp exam. The device scans your eye with near-infrared light, which you won’t feel. There’s no contact with your eye, no bright flash, and no discomfort. Your doctor may use dilating drops beforehand to widen your pupils, though many modern machines can capture adequate images without dilation. The entire process, from sitting down to getting up, typically takes one to two minutes of actual scanning time.
There are no known risks or side effects from the scan itself. The light used is low-intensity and poses no harm to the eye. If your pupils were dilated, you’ll have temporary light sensitivity and blurred near vision for a few hours afterward, but that’s from the drops, not the OCT.
Reading Your OCT Report
If you’re handed an OCT printout, you’ll typically see a cross-sectional image of your retina alongside a color-coded thickness map. The thickness map displays average retinal thickness values in each sector. Numbers within the normal range generally appear in green, while values flagged as borderline or abnormal may appear in yellow or red, depending on the machine’s software.
On the cross-sectional image itself, bright (hyperreflective) bands represent layers like the nerve fiber layer, the junction between inner and outer photoreceptor segments, and the retinal pigment epithelium. Dark (hyporeflective) bands represent layers like the nuclear layers and photoreceptor segments. Your doctor looks for disruptions in these bands: breaks, thickening, fluid pockets, or areas where layers have separated or collapsed. A healthy scan shows smooth, continuous bands with a gentle dip at the fovea and thickness values that fall within the normal ranges for your age and ethnicity.