Optical Coherence Tomography (OCT) is a non-invasive imaging technique that provides detailed cross-sectional views of biological tissues. It is often described as an “optical ultrasound” because it uses light waves instead of sound waves to create images. This specialized scan allows medical professionals to visualize internal tissue structures with high resolution, making it a valuable tool for diagnosis and monitoring various conditions.
How OCT Imaging Works
OCT imaging operates on the principle of light interference. A low-coherence light source, near-infrared light, is directed towards the tissue being examined. This light beam is split, with one part going to the tissue and the other to a reference mirror.
When the light reflects off different layers within the tissue, it combines with the light from the reference mirror, creating interference patterns. These patterns are then measured to determine the properties of the backscattered light. By analyzing these reflections, the OCT system constructs a detailed cross-sectional image of the tissue. This process enables imaging at a resolution of 1 to 15 micrometers.
Key Applications of OCT Imaging
OCT imaging’s most widespread application is in ophthalmology. It provides detailed views of the retina’s layers, aiding in the early detection and management of numerous eye diseases. For instance, it helps identify signs of age-related macular degeneration (AMD), such as drusen deposits and fluid accumulation.
OCT is also used for glaucoma detection by measuring the thickness of the retinal nerve fiber layer (RNFL); thinning of this layer can indicate glaucomatous damage. It assists in diagnosing and monitoring diabetic retinopathy, a complication of diabetes that affects blood vessels in the retina. Beyond ophthalmology, OCT has emerging applications in cardiology for imaging coronary arteries and in dermatology and oncology for visualizing tissue structures.
Interpreting an OCT Image
An OCT image presents a cross-sectional “slice” of tissue, resembling a microscopic view but without any physical contact. In ophthalmology, for example, an OCT scan of the retina displays distinct layers as alternating light and dark bands. Medical professionals examine these images for abnormalities in reflectivity, thickness, continuity, and surface contour within each layer.
A healthy retina will show a symmetric foveal depression and distinct layers, including the nerve fiber layer, ganglion cell layer, and retinal pigment epithelium. Deviations from this normal appearance, such as increased thickness or areas of decreased brightness (hyporeflectivity), can indicate conditions like macular edema. Increased brightness (hyperreflectivity) might suggest lipid exudates or an epiretinal membrane.
What to Expect During an OCT Scan
Undergoing an OCT scan is a straightforward experience. No special preparation is required, and the procedure can be done immediately after a standard eye examination. Patients will sit in front of the OCT machine and rest their chin on a support, keeping their head still.
During the scan, the machine uses low-power laser light to image the eye without making contact. Patients may be asked to focus on a green target, and they might observe a red line moving across their vision as the scan progresses. The entire process is quick, taking about 5 to 10 minutes for both eyes, and is painless. In some cases, eye drops may be used to dilate the pupils, which can lead to temporary light sensitivity and blurry vision for a few hours afterward.