Optical Coherence Tomography (OTC) is an imaging technique that uses the interference of light to capture tomographic images of the eye.
An OCT image (tomographic image)
Light has wave properties, and interference is when two waves overlap to result in a wave with greater or lower amplitude. Interference is more likely when waves are similarly shaped and less likely when they are not. The likelihood of this interference is referred to as coherence.
The OCT-HS100, a System Well Regarded for its Ease of Use and High Image Quality
OCT systems use the interference of light. Various wavelengths of light are sent from the light source and a device called an optical coupler, which combines and splits multiple optical fibers, is used to send some of the light to a mirror and some of it to the retina at the back of the eye. The light reflected by the mirror is used as a reference, and the strength of the interference between this light and the light reflected by each layer of the retina is measured and processed to create an image. In this way, OCT can be used to output images of the 3D structure of the retina.
How OCT works
With a depth resolution of 3 μm*, Canon’s OCT achieves a higher resolution than typical OCT. Additionally, averaging† up to 200 tomographic images makes it possible to obtain extremely high-resolution tomographic images, which allow for the observation of various layer structures, including the boundary lines between the ellipsoid zone‡ and interdigitation zone§, which are near the retinal pigment epithelium¶ (RPE).
Multi-Layered Structure Viewable with the OCT-HS100
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Canon has also developed OCT angiography (OCTA), which uses OCT tomographic images to visualize blood vessels. OCTA generates images that indicate how the OCT signal strength changes over time in order to render high-resolution images of blood flow. OCTA technology has the potential to further increase the range of clinical applications of OCT, which is already widely used for ophthalmic medical treatment. Because OCTA examinations do not require the use of contrast agents, which some people have strong allergic reactions to, they make it possible to examine the blood flow without placing undue stress on the patient’s body.
For OCTA, multiple OCT tomographic images are captured of the same location over a period of time. When comparing these images, changes in signal strength due to the movement of red blood cells can be observed. As red blood cells only move where there are blood vessels, the locations of these changes in signal strength can be used to determine the locations of blood vessels, which are then emphasized for display. This makes it possible to obtain fundus blood vessel images that are very much like fluorescein angiography fundus photos.
How Fundus-Blood-Vessel Imaging by Using OCT Works
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Averaging is a method of reducing noise by summing and dividing multiple images. Canon has applied this technology not only to OCT tomographic images but also to OCTA images. Using averaging on multiple images to reduce background noise and then correcting for distortion makes it possible to obtain high-contrast OCTA images with improved blood vessel connection. The use of OCTA averaging to stabilize image quality can also result in more reliable numbers when quantifying such characteristics as area density.
How OCTA Averaging Works