How does it work?

Optical coherence tomography

Consultant ophthalmic surgeon Mr Shafiq Rehman on a technique that has revolutionised retinal medicine

Interview: Viel Richardson Optical coherence tomography (OTC) is a technology that allows us to image various parts of the eye using electromagnetic radiation—light—and to see its internal structures in exquisite detail. The eye is particularly well suited for this type of scan as the cornea, a natural lens, allows light to enter the eye in ways that are well understood. The scanner first projects a beam of light into an optical splitter, which divides the beam into two streams. One is a reference stream, allowing the scanner to precisely analyse the phase of the beam projected; the other stream is sent to the eye. These electromagnetic waves go through the cornea, hit the retina and bounce back. The key to OTC is that each returning wave arrives back at the scanner modified in a slightly different way depending on the properties of the structure it encountered. This means the returning waves will have slightly different phases to the ones they had when projected. The scanner records and analyses these changes in phase, in relation to the reference beam, to create an initial map of the structures the light waves encountered in the eye.

These are then fed through very sophisticated computer algorithms which interpret the information and create an artificial image, which is a reflection of the real structure of the retina. This type of scanning can be incredibly detailed, highlighting features almost down to the cellular level. Some of the machines can see structures in the eye that are only 3-4 micrometres across. To give you a sense of what that means, a red blood cell is about 7-8 micrometres in diameter.

Unlike sound waves, which can penetrate deeper into human tissue, light waves are really only transmissible when the structure is largely transparent. For example, there is a structure called the retinal pigment epithelium, a dense layer of cells that regulates the transport of nutrients and waste products to and from the retina. While it is very reflective, we can still see some details behind it. But then we have a structure called the choroid. This is intensely infiltrated by blood vessels and represents a kind of barrier. Some devices using longer wavelengths and higher energy levels can get a bit below the choroid’s surface, but not very much.

It is important to stress that OCT scans are not diagnostic in themselves, unlike those produced by some other scanning technologies. We still rely on clinical examination, talking to the patient and running other tests, but OCT is an absolutely core part of the evaluation of a patient.

If I see a patient in their seventies with deteriorating eyesight, such as blurring and distortion of images, there is a strong possibility that the patient is suffering from dry or wet macular degeneration. Dry macular degeneration is basically wear and tear. Like with any other part of the body, tissue in the eye can atrophy as we get older and lose some of its effectiveness. Wet macular degeneration is very different. Here, abnormal blood vessels grow through the retinal pigment epithelium behind the retina. These blood vessels then explode, causing bleeding and scarring which can lead to a very rapid deterioration of the patient’s vision. They can go from having normal eyesight to very poor eyesight within a matter of days. This type of macular degeneration clearly needs to be addressed as quickly as possible. OCT scans are extremely good at differentiating between those two types of macular degeneration, making it an absolutely essential tool.

Another condition this technology helps to diagnose well is a macular

Macular Part of the retina with a very high concentration of photoreceptor cells, responsible for the the fine vision required for reading. Choroid A highly vascular, pigmented tissue which covers most of the eye behind the retina. Diabetic retinopathy A complication of diabetes where high sugar levels damage blood vessels in the light-sensitive tissue at the back of the eye which make up the retina.

Light source Reference mirror

REFERENCE BEAM REFERENCE BEAM

Optical splitter

DIAGNOSTIC BEAM TO EYE

DIAGNOSTIC BEAM FROM EYE

REFERENCE BEAM DIAGNOSTIC BEAM FROM EYE

Patient

Detector

The key thing about optical coherence tomography is the way it is increasing our understanding of the pathophysiology—the physiological processes— associated with these conditions.

edoema, a very specific syndrome which is part of a complex disorder called diabetic retinopathy. With macular edoema, fluid accumulation in the retina leads to swelling of the macula, the central part of the retina. When a person looks at an object, the image falls onto the macular, so any changes in the macular structure will have a significant impact on the person’s visual acuity. OCT technology is not only good at highlighting this, but it can also measure the level of swelling, allowing us to quantify the syndrome’s progress. This allows us to develop age and genderbased normative reference databases, against which we can compare a patient’s condition. These images let us see the impact our treatment is having on those swellings and therefore how successful the treatment is being. This really helps personalise the treatment we can offer.

One of the really exciting things about OCT is that it allows us to treat conditions we could not treat before. For many years a condition called vitreomacular traction syndrome (VMT) was extraordinarily hard, if not impossible, to spot. The vitreous gel is a sort of inert structure sitting in the space between the cornea and the retina. As we get older it can partially pull away from the retina. This is known as posterior vitreous detachment (PVD) and it is extraordinarily hard to visualise, and therefore diagnose, with the naked eye. In a patient with symptoms of mild vision disturbance you can spend three or four minutes examining the area in detail through a microscope and see little or nothing suggestive of PDV. On an OCT scan, it stands out very clearly, looking like a separate line that is pulling the retina forward. This means we can diagnose VMT quickly and accurately. Fifteen years ago we could only postulate a diagnosis. We could asses the symptoms and surmise that the patient may be suffering from VMT, but we could not be absolutely sure. Now we can even chart the effectiveness of any treatment in real time. It represents remarkable progress.

Another condition we can now diagnose and treat quickly is called a macular hole. Again, it can be quite hard to see a very small macular hole as it is starting to develop, but on OCT it’s immediately obvious. You can see the altered structure and decide how to manage it very early in the process.

Each one of these is a wonderful advancement, but the key thing about optical coherence tomography is the way it is increasing our understanding of the pathophysiology—the physiological processes—associated with these conditions. We now understand much more about the mechanisms by which these conditions develop and progress. This means we are better informed about how to manage and treat them.

It’s remarkable how much OCT has revolutionised the field of treating retinal disorders. Fifteen years ago, OTC scanners did not exist; now there isn’t a department in the country that would want to work without one.

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