This will cause obvious concern to those new to OCT interpretation however, this is a normal feature of the fovea, representing the elongation of cone photoreceptors to enable closer packing and hence provide high visual acuity. Secondly, as you move over the foveal region of an OCT scan, the outer segments of photoreceptors appear to become oedematous (Figure 2B). Firstly, because blood vessels are highly reflective, they cause a shadow to fall underneath them in the OCT scan due to blocking of the infrared OCT signal (Figure 2A).This can also occur with dense vitreous floaters, which will cast a shadow across all retinal layers of the OCT scan. When new to interpreting OCT images, a couple of normal anatomical features can cause concern. Conversely, structures including the RNFL and RPE are much more dense, therefore they appear brighter (or red in a colour image). Similarly, if fluid is present this will also appear black. As the vitreous is not very dense, it appears black. Images in greyscale utilise brighter shading in place of warmer colours. With a colour image, large reflections are depicted by warm colours (yellow to red), while smaller reflections are depicted by cooler colours (blue to green). Differentiation of the retinal layers is possible due to their varying scattering properties and differences in optical densities. Overlaying multiple scans is very beneficial in eyes where the signal strength is poor eg in eyes with cataract, as this will boost the signal and allow a higher image quality.Ī healthy macular greyscale OCT B-scan is shown in Figure 1, with the different retinal layers identified. Five-line cross scans, which take five horizontal and five vertical scans multiple times and overlap the images to create a smoother image are very useful if you require better definition of the different retinal layers, in order to determine where a defect lies. In addition to 3D macular scanning, additional macular scan protocols can provide further information on the macular region. They also include identification of persistent vitreomacular traction and epiretinal membranes which may explain a slight reduction in vision which may have gone undiagnosed without the use of OCT diagnosis of conditions such as central serous retinopathy (CSR) early detection of diabetic maculopathy and screening for macular oedema post cataract surgery. However, the benefits of macular OCT do not stop at AMD detection and monitoring. In community optometry this is also one of the biggest uses of OCT, allowing differentiation between wet and dry AMD, therefore avoiding unnecessary referrals. Macular OCT is now an invaluable tool within hospitals for monitoring wet age-related macular degeneration (AMD) and has become the most commonly used diagnostic test to aid in treatment decisions. ![]() Covering the whole of the macular region, and typically consisting of over 30,000 A-scans, this scan gives high enough resolution to view all the retinal layers, while still being able to be captured in most commercially available equipment in just a couple of seconds, meaning patient comfort is maximised. The 3D macular cube scan is probably the most popular OCT scan protocol used. Consecutive B-scans can then be aligned to produce a 3D cross-section of the retina. OCT generates cross-sectional or three-dimensional images by utilising low coherence interferometry to detect and measure the depth and magnitude of back scattered (reflected) light.2 A two-dimensional, cross-sectional retinal image is produced as the light source scans across the retina, stacking and aligning consecutive axial-scans (A-scans) side by side to produce a two-dimensional transverse-scan (B-scan).3 Eye movements are corrected by digital processing (cross-correlation scan registration) to align the A-scans, and digital smoothing techniques are used to further reduce image noise.4 The image produced resembles that of a histological section, with contrast produced by differences in the refractive index and scattering properties of the different retinal layers. Since OCT was first demonstrated in 1991,1 it has rapidly evolved as the only non-invasive diagnostic technique able to provide images of the retinal microstructure. This first article in the series will review macular OCT scans. With use of OCT growing in the primary eye care setting, this series of articles will discuss macular, disc and anterior OCT imaging in turn, breaking down and reviewing all the information you obtain from most commercially available OCTs. Optical coherence tomography (OCT) is a non-invasive, non-contact, transpup-illary imaging technique able to produce high-resolution images detailing the 3D structure of the eye, from the anterior segment to the posterior pole.
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