Eye health resource
What Is Optical Coherence Tomography (OCT)?
OCT is a quick, non-contact scan that creates detailed cross-sectional images of the retina and optic nerve. Learn how it differs from Optos retinal photography and when one or both may be useful.
Reviewed for accuracy by Justin Bazan, OD on .
Optical coherence tomography, or OCT, is a quick, non-contact imaging test that creates detailed cross-sectional images of structures inside the eye. Rather than taking a surface photograph, OCT uses reflected light to show layers of the retina and structures around the optic nerve.
Park Slope Eye uses a Topcon Maestro OCT. OCT can provide useful structural information when evaluating or monitoring conditions such as glaucoma, macular disease, and diabetic retinal changes. It does not diagnose those conditions by itself; the scan is interpreted together with your symptoms, medical history, examination findings, and any other relevant testing.
How Does OCT Work?
OCT uses light to build high-resolution cross-sectional images. A common analogy is an optical version of ultrasound: ultrasound uses sound waves, while OCT uses light.
The test can show differences between retinal layers and measure tissue thickness. Around the optic nerve, OCT can measure structures such as the retinal nerve-fiber layer that may change in glaucoma and other optic-nerve conditions.
What Is an OCT Scan Like?
You sit at the instrument, place your chin and forehead against the supports, and look at a fixation target while the machine scans the eye. Nothing touches the surface of the eye, and there is no ionizing radiation.
The scan itself is usually very quick. Dilation is not automatically required just because OCT is being performed, although dilation may still be recommended for the overall eye examination when symptoms, risk factors, pupil size, or clinical findings make it appropriate.
OCT vs. Optos: What Is the Difference?
OCT shows depth and layers; Optos shows a very wide surface view.
- OCT: creates cross-sectional images that can show retinal thickness, contour, fluid, and optic-nerve structural measurements.
- Optos ultra-widefield retinal imaging: creates a broad photograph of the retina, including much of the peripheral retina.
Neither test is simply “better.” They answer different clinical questions. A widefield photograph may show an abnormality across a broad area, while OCT may provide more detailed structural information through a smaller area of retina or optic nerve. In some cases the two tests complement each other.
For the widefield-photography side of the comparison, see What Can Retinal Imaging Detect?
What Can OCT Help Show?
Glaucoma and the Optic Nerve
Glaucoma damages the optic nerve and can cause permanent vision loss. OCT can measure and document optic-nerve and retinal nerve-fiber structures, giving the doctor another way to look for suspicious structural change and to follow change over time.
OCT is only one part of a glaucoma evaluation. Eye pressure, optic-nerve appearance, visual-field testing, corneal measurements, family history, drainage-angle findings, and other factors may also matter.
Macular Conditions
The macula is the central part of the retina used for detailed vision. OCT can show swelling, fluid, thinning, membranes, holes, and other structural changes in or around the macula that may be associated with conditions such as age-related macular degeneration, macular edema, epiretinal membrane, or a macular hole.
Diabetic Retinal Changes
Diabetes can affect retinal blood vessels. OCT is particularly useful when the doctor needs to look for or measure retinal swelling, including diabetic macular edema. A complete diabetic eye evaluation may require retinal photography, dilation, or other examination as well.
Why Would I Need Both OCT and Retinal Photography?
Because the two tests provide different types of information. Think of the retinal photograph as a broad map and OCT as a cross-sectional detail view.
For example, a photograph may document the overall appearance of the optic nerve or retina, while OCT can measure specific retinal or nerve-fiber structures. Or a photograph may reveal a visible macular abnormality that OCT can then characterize in greater structural detail.
The doctor may recommend one test, both tests, or neither depending on the reason for your visit and what the examination shows.
Does OCT Replace Dilation?
No. OCT can provide detailed structural images without dilation in many patients, but it does not provide the same examination as looking directly through a dilated pupil when dilation is clinically indicated.
For more on that distinction, see Will I Need to Have My Eyes Dilated at My Eye Exam?
OCT at Park Slope Eye
OCT may be used when there is a clinical reason to look more closely at the retina or optic nerve. Park Slope Eye also publishes its current self-pay OCT screening fee for transparency; coverage depends on the reason for testing and your insurance plan.
Learn more about comprehensive eye exams, medical eye care, or our insurance and fee information.
Originally published . Substantially updated .