Optical Coherence Tomography (OCT) is a non-invasive medical imaging test used to examine the structures at the back of the eye. It provides high-resolution, cross-sectional pictures of the retina and the optic nerve using light waves. This technology allows eye care professionals to measure the distinct layers of the retina with micron-level precision, which is crucial for the early detection and management of numerous sight-threatening diseases.
How Optical Coherence Tomography Works
The mechanism behind Optical Coherence Tomography is rooted in interferometry, a physics principle involving splitting and recombining light waves. The OCT machine emits a beam of low-coherence light, typically near-infrared, which is split into two paths: the sample arm (directed into the patient’s eye) and the reference arm (traveling a fixed distance inside the machine).
As the light penetrates the eye, it reflects off different tissue layers within the retina, such as nerve fibers and photoreceptors. This reflected light recombines with the light from the reference arm, creating an interference pattern only when the beams have traveled a nearly identical optical distance.
The machine measures these interference patterns, or “light echoes,” to determine the precise depth and intensity of the reflections. Sophisticated computer software translates this depth-resolved data into a detailed, two-dimensional cross-sectional image, known as a B-scan. By rapidly scanning the light beam across a small area, the system reconstructs a complete three-dimensional map of the retina and optic nerve.
The Patient Experience: What Happens During an OCT Scan
The OCT scan is a quick, safe, and painless process, usually taking less than ten minutes. Your provider may administer dilating drops beforehand, though the scan is often performed without them. Dilation helps widen the pupil, resulting in clearer images, especially for patients with small pupils or cataracts.
To begin, you sit comfortably in front of the machine, placing your chin on the rest and your forehead against the support strap. You will be asked to focus on a target light inside the machine, typically a small green or red light.
While the machine is working, you will see a flash or a red scanning line move across your vision, but nothing touches the eye. Image capture takes only a minute or two per eye. If you received dilating drops, your vision may be blurry and sensitive to light for a few hours afterward, requiring you to arrange transportation home.
Critical Eye Conditions Revealed by OCT
OCT is a diagnostic tool that allows eye care professionals to identify signs of disease earlier than traditional methods. The precise, layer-by-layer visualization it offers is indispensable for diagnosing, monitoring, and guiding the treatment of several serious conditions.
Age-Related Macular Degeneration (AMD)
OCT is considered the gold standard for AMD assessment. It clearly shows the presence of drusen, small deposits of protein and lipids that accumulate beneath the retina and are an early sign of the condition. In the wet form of AMD, the scan precisely identifies the location and extent of fluid accumulation and swelling, which results from leaking, abnormal blood vessels under the retina.
Glaucoma
In the management of Glaucoma, OCT provides objective, quantitative measurements of structural damage that often precedes noticeable vision loss. The technology measures the thickness of the retinal nerve fiber layer (RNFL), the bundle of axons forming the optic nerve. Thinning of this layer is a strong indicator of nerve damage caused by glaucoma, allowing doctors to track disease progression over time with high accuracy.
Diabetic Retinopathy
OCT is highly effective for patients with Diabetic Retinopathy, the most common cause of vision loss in working-age adults. The test detects and quantifies diabetic macular edema (DME), which is the swelling of the macula due to fluid leakage from damaged blood vessels. By precisely measuring the central subfield thickness, OCT allows for objective assessment of the edema and is used to monitor the effectiveness of treatments such as intravitreal injections.