Optical Coherence Tomography

Optical coherence tomography (OCT) is a noninvasive imaging technology used to obtain high-resolution cross-sectional images of the retina. OCT is similar to ultrasound testing, except that imaging is performed by measuring light rather than sound. OCT measures the retinal nerve fiber layer thickness in glaucoma and other diseases of the optic nerve.

Imaging the retina
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The OCT During an Eye Exam

Optical coherence tomography is a way for optometrists and ophthalmologists to image the back of the eye including the macula, optic nerve, retina, and choroid. During an eye examination, optometrists and ophthalmologist can view the back of the eye and its anatomy. However, sometimes doctors need more detail or need to inspect details right below the surface, which is difficult to view with standard techniques. Some describe it as an “optical ultrasound” because it images reflections between tissues to provide cross-sectional images.

The detail that can be visualized with an OCT is at such high resolution that doctors can see much finer details than had ever been seen before in a living human eye. The resolution of OCT is finer than 10 microns (10 millionths of a meter), which is better than MRI or ultrasound. As a result, instead of looking into the eye and guessing that there might be retinal swelling simply by the way it looks, an OCT not only gives much greater detail than other methods, but it can actually show exactly what layer of the retina is accumulating fluid causing edema or swelling. It can be used to track the healing or resolution of that swelling.

Optical coherence tomography works by using interferometry, which makes it possible to image tissue with near-infrared light rather than with gamma rays or ultrasound. Interferometry works by shining a beam of light into the eye, which is reflected by tissues at different depths. Images are built based on these reflections. An OCT images to approximately two to three millimeters below the surface of the tissue. Images are obtained clearly through a transparent window, such as the cornea. The light that is emitted into the eye is safe, so no damage will occur.

What Happens During an OCT?

An OCT may very well be the easiest medical imaging test you will ever take. The test takes only a few minutes to perform. With most OCT machines, you simply place your head in a headrest. The technician will calibrate the instrument. You will be instructed to look at a light target inside the machine. The technician will then receive the image. If your pupils are very small or if the doctor wants an image of a very specific area, your pupils will be dilated with medicated eye drops. Most people do not require dilation.

An OCT is used in the evaluation of many eye conditions, including:

3 Sources
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  1. Aumann S, Donner S, Fischer J, Müller F. Optical coherence tomography (OCT): principle and technical realization. In: Bille JF, ed. High Resolution Imaging in Microscopy and Ophthalmology. Cham, Switzerland. Springer International Publishing; 2019:59-85. doi:10.1007/978-3-030-16638-0_3

  2. Chopra R, Wagner SK, Keane PA. Optical coherence tomography in the 2020s—outside the eye clinic. Eye. 2021;35(1):236-243. doi:10.1038/s41433-020-01263-6

  3. American Academy of Ophthalmology. What is optical coherence tomography?

By Troy Bedinghaus, OD
Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.