11/12/2023 0 Comments Normal retina thickness![]() These results suggest that patients with MH have unique foveal morphologic features that predispose them to MH development. Multiple regression analyses showed that a thinner fovea and a deeper foveal depression were associated significantly with the presence of MH ( P =. The foveal depression was significantly greater in the MH group (0.063) than in the retinal vein occlusion group (0.059) or in the healthy group (0.058 P =. There were no significant differences in the thickness of the fovea and central fovea among the eyes with epiretinal membrane (254 and 203 μm) or retinal vein occlusion (251 and 202 μm) or in the healthy group (254 and 201 μm). The fovea (1 mm) and central fovea were significantly thinner in the MH group (243 and 192 μm) than in the other groups ( P <. The foveal depression was quantified as the foveal pit depth divided by the foveal pit diameter. The average regional retinal thicknesses of the Early Treatment Diabetic Retinopathy Study sectors determined by spectral-domain optical coherence tomography were compared among 160 patients with MH, 175 patients with epiretinal membrane, 145 patients with retinal vein occlusion, and 186 healthy subjects. Of the 849 subjects studied, 183 eyes were excluded because they had an abnormal vitreofoveal interface that might have affected the foveal thickness. This map layout was first used by the ETDRS 89 and later by the Age-Related Eye Disease Study.To compare the morphologic parameters of ophthalmoscopically and tomographically normal foveae of the fellow eyes of patients with a unilateral macular hole (MH), other unilateral retinal diseases, and healthy eyes. Although thickness measurements are calculated for every A-scan line, these are usually combined to form a nine-region thickness map centred on the fovea, as shown in Figure 29. Retinal thickness measurements are calculated from the cross-sectional images using software to segment the inner and outer limiting boundaries of the retina, although there is disagreement about which boundary best represents the lower limit of the retina. The introduction of OCT improved the axial resolution of cross-sections by two orders of magnitude, allowing structures to be seen in vivo that were formerly visible only by histological examination ( Figure 28). Prior to OCT, retinal cross-sections were possible only using ultrasound imaging. Three such scanners, the Zeiss Cirrus OCT, Topcon 3D OCT-1000 and Heidelberg Spectralis, were used in this study. The latest generation of OCT scanners are based on spectral domain techniques, which dispense with the moving mirror and are thereby able to increase the acquisition rate by up to two orders of magnitude. ![]() Although faster than its predecessors it still only acquires 400 A-scans/second, which limits the number of cross-sections that can be acquired before eye movement is a problem. 88 In 2001 Zeiss released their most recent time domain OCT system, the Stratus OCT. It used a super-luminescent diode light source and time domain interferometry based on a moving mirror and a Michelson interferometer. The first commercial OCT scanner was launched in 1996 by Carl Zeiss Meditec. depth profile) and ‘B-scan’ (a collection of A-scans giving a two-dimensional cross-section). Nevertheless, much OCT terminology has been borrowed from ultrasound imaging, such as the ‘A-scan’ (a signal vs. However, unlike ultrasound, OCT is a non-invasive, non-contact technique. 1, 87 It is often referred to as an optical analogue of ultrasound imaging, measuring backscattered light rather than sound. Optical coherence tomography 86 is a popular, rapid and non-invasive technique for cross-sectional retinal imaging that has proved convenient for longitudinal studies and as a trial outcome measure. However, it has relatively poor resolution (approximately 200 µm axially and laterally) and is an invasive technique, requiring contact with the eye. One advantage of ultrasound imaging is that it is not affected by optical opacities, such as cataract or vitreous haemorrhage. These are single axial profiles recording the strength of echoes from different tissue depths. The two-dimensional cross-sectional ultrasound image is composed of a series of A-scans. Therefore, there is strong contrast between MO and normal retinal tissue. Fluid-filled structures, such as the vitreous, or cysts, neither reflect nor scatter ultrasound. Interfaces between materials with different acoustic properties generate strong echoes, whereas materials that scatter the ultrasound beam return weaker echoes. Frequencies as high as 50 MHz have been used for high-resolution imaging of the anterior segment, but these have insufficient penetration for retinal imaging. Ultrasound frequencies between 10 MHz and 20 MHz are the most useful for retinal imaging.
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