OCT is short for Optical Coherence Tomography. The concept is quite simple. This technology involves directing light through the pupil into the eye. Light is split and then recombined, allowing comparison between the light that was sent to a reference mirror and the light that has reflected back from the eye. The time taken to return from the eye depends on the reflective properties of the tissue (delay).
When two structures that are one above the other demonstrate large differences in refractive index, a greater volume of light is reflected at their interface. Changes in optical density are identified and the software allows illustration in a colour coded or black and white image.
Large reflections are depicted by warm colours (red through yellow) and mild reflections are depicted by cool colours (green through blue). Some practitioners prefer to view black and white images. The more reflective a structure the whiter it is.
OCT is mainly used to discriminate the cross-sectional features of the
- optic disc
- retinal layers
- variation in retinal and RNFL thickness
- resolution of 5 μm
But some versions allow imaging of the anterior segment.
For useful images, a large pupil and clear optical media are required.
There are no moving parts so modern machines can acquire 30 000 per second. The user can focus in on an exact area of interest
3D retinal images can be created on top of which fundus images can be overlaid.
Improved clinical diagnosis can be achieved by separating retinal layers, allowing for exact visualization of the pathological changes occurring in each individual layer.
OCT allows detection of retinal pathology not visible during ophthalmoscopy in the absence of clinical signs and/or symptoms. More detailed insight into the cause of clinical symptoms and signs that can be detected ophthalmoscopically but cannot accurately diagnose based upon clinical presentation alone.
OCT may even present an opportunity for intervention before the emerging pathology becomes clinically significant
A big advantage of modern machines is the use of 1050 nm wavelength. This can pass through the retinal pigment epithelium (RPE-pigment within cells = melanin). Shorter wavelengths are scattered by the RPE and absorbed by melanin within the cells. As the 1050 nm wavelength passes through the RPE it can reach the choroid and even the sclera. Clear choroid imaging helps in the diagnosis of difficult to diagnose conditions.