OCT, or optical coherence tomography, is an imaging scan that produces detailed cross-sectional pictures of the structures inside your eye. It works somewhat like an ultrasound, but uses light instead of sound, achieving resolution fine enough to distinguish individual layers of the retina as thin as 5 to 6 microns. The scan is painless, takes about one to two minutes, and has become one of the most commonly performed tests in eye care.
How OCT Creates an Image
An OCT machine uses a beam of near-infrared light directed into the eye and split into two paths, one toward your retina and one toward a reference mirror. When the reflected light from both paths recombines, the waves interfere with each other in a pattern that reveals exactly how deep each reflection came from and how strong it was. This principle, called low-coherence interferometry, allows the device to map tissue depth with extraordinary precision.
Early OCT systems (called time-domain OCT) measured one depth point at a time by physically moving the reference mirror. Modern systems use a technique called Fourier-domain OCT, which captures reflections from all depths simultaneously. This works either by spreading the returning light across a spectrometer (spectral-domain OCT) or by rapidly sweeping a laser through a range of wavelengths (swept-source OCT). The jump to Fourier-domain detection didn’t just make scans faster. It fundamentally reduced the noise floor of the measurement, producing dramatically clearer images at higher speeds.
Spectral-Domain vs. Swept-Source OCT
Most eye clinics today use one of these two Fourier-domain technologies. Spectral-domain OCT (SD-OCT) typically captures around 68,000 depth scans per second with an axial resolution of about 5 microns. Swept-source OCT (SS-OCT) is faster, reaching 100,000 scans per second, with a slightly coarser resolution of about 6.3 microns. The practical difference: swept-source OCT penetrates deeper into tissue and performs better through cataracts or other media opacities, making it particularly useful for imaging the choroid (the blood vessel layer behind the retina) and for patients with cloudy lenses.
What the Scan Shows
A standard retinal OCT produces a cross-sectional image that looks like a thin slice through the back of your eye, with each layer of the retina visible as a distinct band. Ten separate layers can be identified on a high-quality scan: the nerve fiber layer at the top (closest to the surface of the eye), followed by the ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, outer limiting membrane, the photoreceptor inner and outer segments, and the pigment epithelium at the bottom.
This level of detail matters because many eye diseases damage specific layers. Glaucoma thins the nerve fiber layer. Age-related macular degeneration disrupts the pigment epithelium and photoreceptor layers. Diabetic retinopathy causes fluid to accumulate between layers. By measuring the thickness and integrity of each layer over time, your eye doctor can detect disease progression that would be invisible during a standard exam.
Conditions OCT Helps Diagnose and Monitor
OCT is most widely used for three conditions: glaucoma, age-related macular degeneration (AMD), and diabetic eye disease.
In glaucoma, the scan measures the thickness of the retinal nerve fiber layer around the optic nerve. Thinning in this layer is one of the earliest detectable signs of glaucoma damage, often appearing before any noticeable vision loss. Repeated scans over months or years can show whether nerve fiber loss is progressing and whether treatment is working.
In AMD, OCT reveals specific features that predict how the disease will behave. Fluid trapped within the retinal layers (intraretinal fluid), fluid beneath the retina (subretinal fluid), deposits called drusen, and detachments of the pigment epithelium are all clearly visible. Research published in Ophthalmology found that two features on baseline OCT, an intact external limiting membrane and an intact ellipsoid zone, predicted significantly better vision at 12 months in patients with the “wet” form of AMD. Conversely, the presence of intraretinal fluid, particularly at the center of the fovea, predicted worse outcomes. These specific markers help doctors decide how aggressively to treat and give patients a clearer picture of their prognosis.
In diabetic retinopathy, OCT detects macular edema (swelling at the center of the retina) with precision, measuring the exact amount of fluid and thickening present. This guides treatment decisions and tracks how well injections or laser therapy are reducing the swelling.
OCT Angiography
A newer extension of OCT technology, called OCT angiography (OCTA), maps blood vessels in the retina without injecting any dye. It works by rapidly scanning the same location multiple times and comparing the results. Stationary tissue produces identical signals each time, while moving blood cells create slightly different signals from one scan to the next. The software isolates those differences and translates them into a detailed vascular map.
The result is a depth-resolved image of the retinal blood supply that approaches the quality of histological tissue slides. OCTA is particularly useful for detecting areas of poor blood flow in diabetic eye disease and for identifying the abnormal new blood vessels that characterize wet AMD. Because it requires no injection, it’s faster and carries none of the rare but real risks associated with fluorescein dye.
Imaging the Front of the Eye
OCT isn’t limited to the retina. Anterior segment OCT (AS-OCT) images the front structures of the eye, including the cornea, iris, and the drainage angle where fluid exits the eye. This is valuable for measuring corneal thickness before refractive surgery, assessing the drainage angle in narrow-angle glaucoma, and monitoring surgical outcomes after procedures like trabeculectomy. Anterior segment OCTA can even map blood vessel patterns in conditions like pterygium (a growth on the surface of the eye) or evaluate blood flow in surgical filtering blebs.
What to Expect During an OCT Scan
The scan itself is entirely noninvasive. You sit in front of the machine and rest your chin on a support, similar to the setup for other eye tests. A technician aligns the device, and you’ll be asked to look at a target light while the scan captures images over about one to two minutes. You won’t feel anything, and the light used is safe.
Your eye doctor may or may not dilate your pupils beforehand. Many modern OCT machines can capture quality images through an undilated pupil, but dilation can improve image quality in some cases, particularly for patients with small pupils or cataracts. No special preparation is needed otherwise.
Regarding cost, Medicare reimburses approximately $40 for a retinal OCT scan. Your out-of-pocket cost depends on your insurance plan and whether the scan is considered diagnostically necessary. Most insurers cover OCT when it’s ordered to diagnose or monitor a specific condition rather than as a routine screening.