What is Optical Coherence Tomography?

Toni Henthorn

Optical coherence tomography (OCT) is a procedure used for the noninvasive examination of intraocular structures. Primarily used for analysis of the retina and optic nerve, OCT centers on the amount of light absorption or scattering that occurs when light passes through a given tissue layer. Optical coherence tomography uses a diode laser, which emits light at a wavelength of about 840 nanometers. Two beams of light, a reference beam aimed at a mirror and a detection beam aimed at the eye tissues, are compared, measured, and analyzed. OCT allows physicians to develop cross-sectional images of the anterior chamber of the eye as well as three-dimensional images of the retina.

Optical coherence tomography is primarily used for examining the retina and optic nerve.
Optical coherence tomography is primarily used for examining the retina and optic nerve.

OCT images enable physicians to detect anatomical alterations in the ocular structures that occur with glaucoma and retinal disease. Ophthalmologists can determine the overall retinal thickness as well as the thickness of individual layers of the retina to identify macular swelling, macular degeneration, and macular holes. They can easily recognize epiretinal membranes on the retinal surface. Furthermore, optical coherence tomography facilitates the evaluation of the horizontal and vertical cup to disc sizes for long-term tracking of glaucoma damage.

During optical coherence tomography scans, sometimes the pupils must be dilated.
During optical coherence tomography scans, sometimes the pupils must be dilated.

Barriers to achieving satisfactory optical coherence tomography scans include cloudiness of the cornea or lens, lack of patient cooperation, and excessive blinking. The OCT device obtains approximately 27,000 scans per second, allowing increased resolution and detail with a minimal amount of time required. Although it is possible to obtain high-quality scans through a small pupil, sometimes the pupils must be dilated. It is also useful for the patient to use artificial lubricants on the surface of the eye before the examination.

Examination with optical coherence tomography scans often provides valuable information regarding structural abnormalities. For example, subretinal tumors, such as melanomas, may raise the entire retina, producing an upward bowing of the retina visible on the OCT. An epiretinal membrane will show up as a bright line overlying the retina with wavy folds underneath the line, due to lateral traction on the retinal surface by the membrane. A full-thickness macular hole will appear as an obvious discontinuity in the retina at the macula with pockets of fluid in the adjacent retina.

Additionally, OCT scans offer useful information with respect to changes in the pattern of light reflection from a tissue. High reflectivity of light may occur with choroidal nevus, a mole-like structure deep to the retina, due to the brown pigment within it. Scar tissue will also be highly reflective. On the other hand, fluid pockets, such as cysts or detachments, will appear dark on the scan. The degree of reflectivity depends on the depth of the tissue, the composition of the tissue, and the orientation of the tissue. Horizontal structures tend to be more reflective than vertically oriented structures.

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