Samantha Mann MD, MRCOphth, Specialist Registrar, Moorfields Eye Hospital NHS Foundation Trust
Abbreviations
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What is OCT?
Optical coherence tomography (OCT) is a relatively new diagnostic modality that provides high-resolution, cross-sectional imaging of ocular tissues in vivo. It is predominantly used for posterior segment imaging to measure retinal and macula thickness and has been used to study and monitor diseases such as vitreomacular traction, epiretinal membranes, macular holes and macular oedema in diabetes, vein occlusions and uveitis. [1,2]
How does OCT work?
The technique is analogous to ultrasound, except that the use of light (wavelength 843nm) rather than sound waves enables a higher longitudinal resolution up to 10µm.[1] An interferometer is used to split the low coherence infrared light into a probe beam which is directed into the eye and a second beam aimed at a reference mirror. The reflected light from the retinal tissue and reference mirror then interact to produce an interference pattern which is detected and processed into a signal.[3] Strong reflections occur at the interface between materials of different refractive indices and from tissues with high scattering coefficients. Micro-structural ‘distances’ within the eye can be measured from the ‘echo’ time taken for the light to reflect from the various structures within the eye. This provides ultrasound-A-scan-like information at a single point.[2] A 2-dimensional B-mode tomographic image is then generated by simultaneously performing and displaying 100 longitudinal adjacent scans. Although media opacities such as vitreous haemorrhage and corneal oedema which reduce the intensity of incident light may limit the quality of images, good quality pictures are still obtainable through mild to moderate cataract.
Interpretation of the OCT image
The final image is then displayed in false colour. Dark colour (blue and black) represent regions of low relative optical reflectivity (i.e fluid within the retina) and bright colours (red and white) represent regions of high optical reflectivity. Conventionally, the image is orientated with the vitreous cavity superiorly which is non-reflective and black (see Figure 1). The normal fovea is identified by its characteristic depression and its thickness has been calculated as 147 +/- 17µm in normal eyes.[4] The posterior hyaloid is sometimes seen as a visible blue membrane. Posterior to this, at the inner retinal margin, a highly reflective red band corresponds to the nerve fibre layer. The intermediate layers of retina exhibit moderate reflectivity and the dark layer immediately posterior to this represents the photoreceptor outer segments. Beneath this, a further highly reflective red layer corresponds to the RPE and choriocapillaris and lastly another dark layer of minimal reflectivity represents the choroid and sclera.[5] Retinal blood vessels are identified by their increased back scatter and by blocking the reflections from the RPE and choriocapillaris. The larger choroidal vessels have minimally reflective dark lumens.[1]
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References
1. Hrynchak P, Simpson T. Optical coherence tomography: an introduction to the technique and its use. Optom Vis Sci. 2000;77(7):347-356. [abstract]
2. Baumal CR. Clinical applications of optical coherence tomography. Curr Opin Ophthalmol. 1999;10(3):182-188. [abstract]
3. Jaffe GJ, Caprioli J. Optical coherence tomography to detect and manage retinal disease and glaucoma. Am J Ophthalmol. 2004;137(1):156-169. [abstract]
4. Hee MR, Puliafito CA, Wong C, Duker JS, Reichel E, Rutledge B et al. Quantitative assessment of macular edema with optical coherence tomography. Arch Ophthalmol. 1995;113(8):1019-1029. [abstract]
5. Voo I, Mavrofrides EC, Puliafito CA. Clinical applications of optical coherence tomography for the diagnosis and management of macular diseases. Ophthalmol Clin North Am. 2004;17(1):21-31. [abstract]
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