Overview of Optical Coherence Tomography
Optical coherence tomography (OCT) is a non-invasive way of taking detailed pictures of the body’s tissues using visible and infrared light. It’s widely used for eye imaging to help diagnose and monitor various eye conditions. This can include looking at the front and back parts of the eye. It’s a common tool doctors use for managing vitreoretinal and macular diseases, as well as conditions affecting the optic nerve head, including glaucoma, which is damage to the optic nerve often caused by high eye pressure.
The OCT technique has improved a lot since the first commercial device became available in 1996. There are now three main types of OCT: time-domain, spectral-domain, and swept-source. These types vary in how they take images, their speed, clarity, and the range of what they can image.
Time-domain OCT (TD-OCT) is the older version of this technique, working a bit like ultrasound technology. It uses an interferometer, a device that measures the delay and strength of light reflected back from different tissue depths, to create two-dimensional images. But, it has some limitations, only able to capture one point at a time, and the quality of images it provides is limited. As a result, newer techniques, offering faster speeds and better-quality images, have now largely replaced TD-OCT.
Spectral-domain OCT (SD-OCT) is a newer type of OCT that is faster, can go deeper into tissue, and provides higher quality images. It uses a spectrometer, a device that measures the spectrum of light reflected back, and allows measurement of several tissue points at the same time. This technology enables three-dimensional tissue imaging with improved resolution. Furthermore, SD-OCT has expanded its utility beyond ophthalmology and is now used in dermatology, cardiology, and gastroenterology too.
Enhanced depth imaging (EDI), a feature of the newer SD-OCT, allows doctors to see deeper ocular structures better, specifically the choroid – a layer of blood vessels and connective tissue between the retina and the white of the eye. This is particularly helpful for diagnosing and managing diseases that affect the choroid.
Swept-Source OCT (SS-OCT) is a more advanced imaging technique. It uses a laser to capture high-resolution images of the anterior segment, retina, optic nerve, and choroid. It delivers improved depth imaging compared to SD-OCT.
OCT angiography (OCTA) is a method that allows doctors to visualize the blood vessels in the retina and choroid in three dimensions. It’s especially helpful for understanding a variety of neuro-ophthalmological conditions, including multiple sclerosis, anterior ischemic neuropathy, hereditary optic neuropathy, and glaucoma.
Anatomy and Physiology of Optical Coherence Tomography
The Optical Coherence Tomography (OCT) is a tool doctors use to diagnose and manage eye disorders. OCT gives us a 3D picture of the different layers within the retina which helps in various steps of eye examination. To better explain, it provides us a high-resolution image of the retinal layers. The retina is the thin layer of tissue at the back of your eye that senses light and sends images to your brain. All the details that OCT provides are extremely helpful for the doctors in finding out what is wrong with the eye.
The image produced by OCT will show different layers of retina and each one will appear with different brightness. The level of brightness, or “reflectance,” can help the doctor identify any signs of eye disease. Some parts can appear darker in OCT images and this might be an indication of fluid accumulation in some layers of the retina. On the other hand, some parts can appear brighter, possibly because of things like blood or other substances.
Using OCT, the measurements of various parts of the retina can be taken. The Macula, an area in the center of the retina which is responsible for sharp vision, is one of those parts. Its thickness can be measured and this measurement is crucial in diagnosing many eye disorders. Other measurements such as thicknesses of retinal nerve fibre, ganglion cell layer and others are also taken.
Although the typical measurements may vary based on the specific OCT device used, there are generally agreed upon normal ranges. For instance, the normal thickness of the central macula usually comes within 200 to 250 micrometers, the retinal nerve fiber layer should be 90 to 110 micrometers, and so on.
All these measurements provide important clues about the health of the eye and any abnormalities in these measurements may need further consideration from the doctor.
Why do People Need Optical Coherence Tomography
Optical Coherence Tomography (OCT) is a handy tool for spotting potential issues related to vision. It functions as an ‘eye scanner,’ capturing detailed pictures of your eyes, which can assist doctors in working out why you may be having trouble with your sight. While it’s a great medical tool, it isn’t a replacement for the thorough examinations or medical histories that your doctor would usually gather.
Retinal Eye Disease
The OCT has been invaluable in detecting and treating many conditions affecting the macula. The macula is the part of your eye that controls detailed, sharp vision. When the macula is not working well, you might notice problems like a blurry spot in your central vision or visual distortions.
An eye scan using OCT is advisable even if your doctor sees abnormalities in your macula during an eye examination because OCT usually provides results that match more with what you’re experiencing. Conditions that OCT can help recognize include:
* Macular Edema: This involves swelling in the macula, which may occur due to diabetic retinopathy, eye infections, some systemic diseases that affect the eyes, retinal veins blockage, certain medications, or as a result of surgery.
* Macular diseases: Conditions like wet age-related macular degeneration or choroidal neovascular membrane.
* Macular holes and conditions like cellophane maculopathy and macular pucker.
* Genetic conditions affecting the macula, such as Stargardt or Best disease.
Your doctor could use OCT images to watch how your macula condition is changing over time, see how any prescribed treatments are working, or decide when surgery may be necessary based on changes in the retina’s thickness and function.
Other Retinal Issues
The eye scanner is also useful in spotting problems in the retina that can cause vision loss; conditions like retinal detachment, central serous retinopathy, or pathological myopia.
OCT technology is also valuable for assessing issues with the optic nerve, like optic neuritis, an inflammation that can cause vision loss and eye pain. Moreover, it is handy in the management and evaluation of glaucoma, a condition that affects your optical nerve. OCT can give your doctor important information about the thickness of your retinal nerve fiber layer and your ganglion cell layer, both of which get smaller in eyes affected by glaucoma. OCT is also useful as a screening tool in patients who are at a high risk of developing glaucoma.
Doctor can use an OCT to get a complete view of the front of your eye as well, to precisely measure the elements that make up this part of the eye. When it comes to diagnosing acute angle-closure glaucoma, a condition associated with fast-onset sight loss, OCT is particularly helpful. It can also be used to check and map the cornea, particularly in people with corneal opacities, scars, or dystrophies. Also, during surgery, OCT has been used in various procedures such as corneal transplants, refractive surgeries, implantation of rings for keratoconus, and trabeculectomy procedures.
When a Person Should Avoid Optical Coherence Tomography
Optical Coherence Tomography (OCT), a procedure that uses light waves to produce detailed images of the inside of the eye, doesn’t have any absolute restrictions. But, there are cases when the images may not come out clearly. If a person’s eyes have an abnormal opacity (cloudiness) in the cornea (the clear layer at the front of the eye), severe cataracts (clouding of the lens inside the eye), or blood leakage within the eye, the light waves may not pass through or reflect off the retina (the layer at the back of the eye that senses light) properly. This can lower the quality of OCT images, making the procedure less effective.
Equipment used for Optical Coherence Tomography
OCT, or Optical Coherence Tomography, is a technique that makes use of light to capture micrometer-resolution pictures of biological tissue, such as the retina in the eye. It works by shining a light, usually in the near-infrared spectrum, onto the tissue. At the same time, the light is also directed onto a reference mirror. The two reflected light rays create an interference pattern which helps generate what we call A-scans. Numerous A-scans side by side can then create a real-time, cross-sectional image of the tissue, which is referred to as a B-scan.
There are different types of OCT techniques. TD-OCT, or Time-Domain OCT, typically scans at a speed of 400 A-scans per second. On the other hand, SD-OCT, or Spectrum-Domain OCT, can scan up to 100,000 A-scans per second, which lets it capture higher-resolution images much quicker. SS-OCT, or Swept Source OCT, uses a tunable laser with a longer wavelength, and can scan at a speed of up to 4 million A-scans per second. Similarly, AS-OCT, or Anterior Segment OCT, also uses a longer wavelength that allows it to scan the entire front segment of the eye.
In the past, OCT machines were big and stationary, meaning patients had to go to a specific medical facility to get the test done. But, thanks to technological advances, we now have portable OCT devices. These machines are small, lightweight, and easy to operate. Many of them can be held in the hand and run on batteries, making them usable in a variety of settings, including rural areas, emergency departments, and operating rooms.
Moreover, OCT technology is now being utilized during surgery – a practice known as intraoperative OCT. This method offers high-resolution imaging of the area being operated on, providing much more detail than traditional imaging techniques could. Integration of the OCT device into the surgical microscope has greatly improved precision and safety during surgeries, often resulting in fewer complications and better surgical outcomes. This advanced technology is finding wide-ranging uses in surgeries related to both the front and back parts of the eye.
Who is needed to perform Optical Coherence Tomography?
An OCT, which stands for Optical Coherence Tomography, is a test for your eyes. It can be done by anyone who takes care of eyes, like an optometrist or ophthalmologist, as long as they’ve gotten the right training and have shown they know what they’re doing. This test helps the eye professionals understand how your eyes are doing, and make sure they can give you the best care possible.
Preparing for Optical Coherence Tomography
OCT, which stands for Optical Coherence Tomography, is a type of eye test that doctors use to take images of your eyes without touching them. Usually, before this test, doctors won’t use any medicine to make your pupils bigger, which is a process called dilatation. But sometimes, if you have small pupils, they might choose to do so. This helps them get better images of your eyes.
Your doctor will discuss with you the possibility of making your pupils bigger for the test. It’s important to know that if they do this, you might be sensitive to light for a short while and your vision may be blurry. Sometimes, this can make it hard to drive for a little while after the test.
Sometimes, the images taken during the OCT test can be affected by things like excessive eye movement. These are called artifacts and can distort the results. To reduce this, doctors may use devices to keep your eyes steady. This is why it’s really important to try your best to keep still during the test, which only lasts for a short while.
How is Optical Coherence Tomography performed
There are different scanning techniques your doctor can use with a tool called spectral-domain optical coherence tomography (SD-OCT), a kind of advanced medical imaging. The commonly used ones for examining the macula (a part of the eye) are the three-dimensional cube, raster, and radial scans.
The cube scan provides a detailed 3D picture of the macula by combining multiple line scans over a specified area. The size of the area can vary – it could be 6 x 6 mm, 7 x 7 mm, or 12 x 9 mm. A raster scan involves a series of parallel line scans arranged at any angle. This scan provides even more detail. You might also encounter radial scans. These are multiple line scans located at equal angles around a single point; if the point is the fovea (the center of the macula), this scan can help identify the position of any disease.
When it comes to measuring the thickness of the choroid (a layer in your eye), this should be done with SD-OCT by a trained eye care professional. Something to note is that if your pupil is dilated, this can temporarily make your choroid seem thicker and affect the measurement. Therefore, it’s crucial to measure the choroid before any dilation. The method involves setting a reference line as close as possible to the choroid without touching it for the most accurate reading.
OCTA, or optical coherence tomography angiography, is another commonly used imaging technique for examining the blood vessels in the retina and choroid. It works by picking up changes in signals over time caused by red blood cells moving against the stationary tissue in the retina. This allows your doctor to see the blood vessels in your retina and choroid in one scan, particularly useful for identifying different layers and structures within your eye. One thing to note is that OCTA might not be able to spot low-flow vascular issues, like choroidal polyps, but can distinguish between certain types of blood vessel growth and monitor progress after treatment.
Possible Complications of Optical Coherence Tomography
Understanding some of the problems related to OCT (Optical Coherence Tomography) can help you make sense of your eye test results. OCT is a non-invasive test that uses light waves to take pictures of your retina, the part of your eye responsible for vision. During the scan, the eye is often divided into layers using a computer program. But sometimes, the program can misidentify these layers which we call ‘segmentation defect’. This mistake usually happens among patients with conditions like vitreomacular traction (when the jelly-like substance in your eyes pulls on your macula) or wet age-related macular degeneration (a condition that affects central vision).
An ‘artifact’ is basically a glitch or mistake in the image. There are two common types of artifacts – motion and shadow artifacts. Motion artifacts can happen if the eye moves too much during the scan. Shadow artifacts can occur if there’s a blockage, like a cataract, that dims the light beam. Both these artifacts can mistakenly change the thickness of several layers of your retina in OCT scans. These layers are the retinal nerve fiber layer, ganglion cell layer, and inner plexiform layer – all vital parts of your eye’s anatomy.
If you’re being tested for glaucoma, these artifacts can affect the results. That’s why your doctor will try to minimize these mistakes by positioning you correctly and making sure you keep your eyes as still as possible during the scan. Regardless of the steps taken, the doctor will also double-check and make sure no artifacts were formed before interpreting your results.
The good news is that there are specific types of OCT scans, such as SS-OCT and EDI-OCT, which are less likely to produce artifacts, especially when checking for glaucoma.
What Else Should I Know About Optical Coherence Tomography?
Macular edema is a condition that causes swelling in the macula, the part of the eye that provides detailed and central vision. Different issues can cause this swelling, but diabetes is usually the most common cause. To diagnose and evaluate macular edema, doctors can use a technology called OCT (Optical Coherence Tomography). This tool has significantly improved our understanding and diagnosis of this condition.
Another eye condition that can be evaluated using OCT is age-related macular degeneration. This disease causes a person to lose their central vision. OCT can help doctors see changes in the eye that show this disease has begun, like the build-up of small yellow deposits known as “drusen.”
OCT is also beneficial in diagnosing and understanding disorders that affect the interface between the vitreous (a clear, gel-like substance that fills the back of the eye) and the retina (the light-sensitive tissue at the back of the eye). In some cases, the vitreous can stick to the retina and become detached, causing issues. OCT can show doctors how closely the vitreous is attached to the retina.
OCT is also helpful in evaluating a group of eye diseases that feature a thickening of the choroid (a layer filled with blood vessels that lies between the retina and the sclera, the eye’s white part). This group of diseases includes conditions such as central serous chorioretinopathy and polypoidal choroidal vasculopathy.
Glaucoma, a condition that can cause damage to your eye’s optic nerve, is also best evaluated using OCT. This device can detect changes even before any visual issues emerge. It provides an objective assessment and allows doctors to monitor disease progression over time.
Finally, OCT can help diagnose a variety of other eye diseases, including paracentral acute middle maculopathy (a condition related to blood flow problems in the eye), ethambutol toxicity (a vision problem caused by a particular medicine), plaquenil retinopathy (retina damage from a particular medication), crystalline retinopathy (a condition where crystal-like deposits form in the retina), and Stargardt disease (a form of inherited juvenile macular degeneration).
