Understanding optical coherence tomography (OCT)

What is optical coherence tomography?

Optical coherence tomography (OCT) is a noninvasive imaging method used to observe and gather detailed images of various parts of the eye. Depending on the OCT type, it can take images of various parts of the eye, such as the retina, optic nerve, cornea and the anterior chamber.

OCT can help detect a number of eye diseases. These include glaucoma, diabetic retinopathy, macular edema and age-related macular degeneration (AMD). Additionally, OCT can be used to monitor some neurological conditions, such as multiple sclerosis (MS).

OCT is an important component in both diagnosing and monitoring treatment efficacy over time for these conditions, among others. A special type of OCT can even provide a live image of the eye during surgery. This type is referred to as intraoperative OCT. 

What is optical coherence tomography used for?

OCT is an imaging technique that can diagnose and monitor multiple parts of the eye, such as:

• Retina – A layer of tissue in the back of the eyeball
• Anterior chamber – Space right behind the cornea
• Cornea – The outer layer of the eye
• Choroid – A middle layer of the eye
• Optic nerve – Connects the retina to the brain

Depending on the part of the eye that an eye doctor wants an image of, they may conduct:

• Anterior segment OCT – Provides a visual of the front of the eye, such as the anterior chamber and cornea.
• Posterior segment OCT – Takes an image of the back of the eye, including the retina and optic nerve.

OCT may be used for many ophthalmologic conditions, as well as some neurological conditions. Optometrists and ophthalmologists use this imaging test to get a clear view of each layer of the retina. It also helps measure each layer’s overall thickness.

OCT plays an important role in helping eye doctors determine if a treatment is working over time. It may also help determine the various stages of some conditions.

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What conditions can be detected with OCT?

OCT can aid in diagnosis of many eye conditions, including:

• Glaucoma
• Age-related macular degeneration (AMD)
• Diabetic retinopathy
• Central serous retinopathy (CSR)
• Macular hole
• Macular edema
• Macular pucker
• Vitreous traction
• Optic nerve disorders
• Geographic atrophy
• Drusen in the eye

Not only can OCT help diagnose eye conditions, but it can also monitor certain conditions that are already present. Glaucoma is one such condition that can be monitored with OCT, as the test helps eye doctors detect changes that occur in the optic nerve fibers.

There is also a newer type of OCT that eye doctors use before and during surgery. It is called intraoperative OCT. Unlike other types of OCT, an intraoperative OCT device is handheld or integrated into a microscope. It allows eye doctors to perform real-time imaging scans during various eye surgeries. This real-time feedback may improve some surgical outcomes. 

Although OCT can be used for many different eye conditions, it is not useful for those that affect the way light passes through the eye, such as cataracts. This is because OCT technology depends upon light waves for proper readings and results.

OCT and multiple sclerosis

In addition to ocular conditions, OCT is now used as a way to help monitor some neurological conditions — particularly multiple sclerosis (MS). MS is an autoimmune disease that affects the central nervous system, which includes the optic nerves.

The protective covering of nerves is called myelin, and it works to surround and insulate nerves (also known as axons). In patients with MS, the myelin can become damaged. This is because of inflammation in parts of the central nervous system.

Research has shown that retinal nerve fibers usually thin out in patients with MS. Because of this lack of protection, clinicians are able to get a more detailed look at isolated axons during OCT testing (and evaluate the retinal nerve fibers at the same time).

Although MS is usually monitored with magnetic resonance imaging (MRI), eye doctors have found that OCT technology can provide additional image details of the retinal nerves in MS patients. This imaging may provide a more precise assessment of the disease in some cases.

What to expect during an OCT test

Before capturing any image, your eye doctor may give you eye drops to dilate your pupils. This can make it easier for them to examine certain parts of your eye, such as the retina. However, the pupils are not dilated for every OCT test.

You will be seated in front of the OCT machine, where you will be able to rest your chin to help ensure there is no movement that interferes with the test. Your eyes are then scanned (without being touched), and imaging and setup are completed within a few minutes.

If your pupils were dilated, your eye doctor may give you special sunglasses. These help protect your eyes from sunlight and lessen any light sensitivity you may experience over the next few hours.

OCT angiography

Optical coherence tomography angiography (OCTA) is an imaging technique that provides more advanced photos of the choroid and retinal blood vessels. OCTA is derived from OCT technology. An OCTA scan provides three-dimensional images and approximates how blood is flowing through vessels.

OCTA can be useful in detecting and monitoring changes from diabetes and other retinal diseases over time. There may be diabetes-related changes seen on OCTA before they are visible with other techniques. 

OCTA is also used for detecting and monitoring inflammatory conditions, such as uveitis. The three-dimensional capabilities provide eye doctors with highly in-depth information about a condition. This can support more targeted treatment. 

OCTA provides detailed images of the blood flow within both the retina and the choroid (the layer of tissue between the sclera [white part of the eye] and retina).

Although OCTA provides great detail for eye doctors (and their patients), there are still some limitations. Drawbacks can include: 

• Its restricted field of view
• Inability to detect vascular leakage 
• OCTA machines may vary in effectiveness

Ensure eye health with regular eye exams

Not everyone needs specialized tests like OCT and OCTA. That said, it is important for everyone to keep up with routine comprehensive eye exams. This is the optimal way to help keep your eyes healthy. An eye exam is a chance to learn if any changes (whether mild or significant) have occurred to your eye health.

Always talk to your eye doctor if you have concerns about your vision or eyes. Even if you are not due for your annual eye exam or a problem seems minor, let your eye doctor know.

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