Since its introduction in 1991,1
optical coherence tomography (OCT), has found its place as a widely accepted imaging technique, especially in ophthalmology and other biomedical applications. It represents an interferometric, non‐invasive optical tomographic imaging technique offering millimetre penetration with submicrometre axial and lateral resolution. The technique when first described demonstrated a 30‐µm resolution, but modern tomography such as the introduction of third‐generation commercial OCT instruments (Stratus OCT3, Carl Zeiss Meditec, Dublin, CA) in 2002 made it possible to obtain an axial image resolution of approximately 10 μm. In ophthalmology, OCT makes it possible to obtain noncontact, high‐resolution, cross‐sectional imaging of the retina. This ability of OCT to image tissue morphology in situ and in real time has been termed “optical biopsy”. Quantitative measurement or morphology of the retinal architecture can be used to assess retinal pathology, and the data are then displayed in a false‐colour topographic map. The evaluation of OCT tomograms depends on the observer to identify both differences in the relative reflectivity of different tissue layers and morphological changes in tissue structures.
Interpretation of a normal OCT is imperative before pathological disease processes can be appreciated. OCT is able to resolve three highly reflecting layers, believed to correspond to the vitreous/retina, inner/outer photoreceptor segments, and RPE/choriocapillaris interfaces.2
Figure 1 demonstrates an OCT image of the normal human macula. The nerve fibre layer (NFL) is the innermost layer of the retina, followed by the inner plexiform layer (IPL), the inner nuclear layer (INL), the outer plexiform layer (OPL), the outer nuclear layer (ONL), the photoreceptor inner and outer segments (IS/OS) and the retinal pigment epithelium (RPE). The most backscattering of light is produced from the NFL and the plexiform layers, and can be seen on the OCT image as red or white false‐colour.3
Figure 1Normal OCT scan.
It is postulated that the reflection emanating from the junction of the inner and outer segments may be caused by the sudden boundary formulated between the structures of the inner, and highly organised outer segments which contains stacks of membranous discs that are rich in the visual pigment rhodopsin.4
OCT imagery of the photoreceptor outer segment supports this interpretation, with the increased reflection being attributed to the increased thickness in the foveal region corresponding to the well‐known increase in the length of the outer cone segments in this region. The melanin‐containing RPE produces strong backscattering and is visualised clearly just below the photoreceptor outer segment. Beneath the RPE, the choriocapillaris and the choroid are visualised as the optical backscattering structure, but unfortunately vascular structures cause light to scatter and limit the penetration of light and the ability of OCT to visualise any of the deeper structures.
Uveitis, although classically associated with the development of macular oedema, can cause a spectrum of retinal morphological changes, some of which characterise specific disease processes. At present, fluorescein angiography is one of the most widely used investigations for detecting the presence of macular oedema. This ancillary test is invasive and certainly not without risk, with approximately 20% of recipients experiencing nausea, with anaphylaxis and death as rare sequelae.5
Furthermore, the information that is provided is qualitative, and its interpretation is highly subjective. Evidence now suggests that OCT is as effective at detecting macular oedema as is fluorescein angiography, but is superior in demonstrating axial distribution of fluid. Compared with flourescein angiography, the OCT sensitivity for detecting cystoid macular oedema was 96%, with a specificity of 100%.6
OCT has been shown to detect macular thickening, even before any angiographic evidence of macular oedema, produces reproducible and consistent results, and provides quantitative measurements of retinal thickness that are ideal for follow‐up and assessment of treatment response to disease.