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.
HOW DOES IT WORK?
Optical coherence tomography Consultant ophthalmic surgeon Mr Shafiq Rehman on a technique that has revolutionised retinal medicine Interview: Viel Richardson
8—Prognosis
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.
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.
