![]() The Heidelberg Spectralis system has chosen the posterior part of the third hyper-reflective band from the OCT, corresponding to the level of the RPE-Bruch’s membrane complex (RPE outer). In an OCT image, the junction is represented by a second hyper-reflective band beneath the junction of the inner and outer segments, and this second line is most clearly visible in the foveal area. For spectral domain instruments, the definitions have changed, and Cirrus HD OCT defines the outer border at the level of interdigitation between outer segments and the retinal pigment epithelium (RPE) following the anatomical concept of the outer border of the neuroretina. The segmentation is not ideal as a measurement of retinal thickness, as the outer segments, and therefore a part of the neuroretina, were excluded. This definition was probably chosen because this line is the first highly reflecting line in the neuroretina, apart from the vitreoretinal surface. In previous time domain OCTs, the Stratus OCT used the junction between inner and outer segments as the outer border. The discrepancies between the inbuilt software systems are particularly due to different definitions of the outer retinal border, which is defined anywhere between the junction between inner and outer segments to the posterior part of the retinal pigment epithelium and Bruch’s membrane complex surface. The inner border is chosen uniformly to the vitreoretinal surface, which is well defined with an abrupt change in reflectance for the nearly optically empty vitreous body to the reflecting inner limiting membrane and retinal nerve fiber layer surface of the retina. Many OCT devices are available and the instruments often differ in the software algorithms used for segmentation, leading to considerable differences in the nominal retinal thickness. The measurement of retinal thickness is considered a reliable and reproducible measurement in healthy subjects, and is used in daily clinics for a large number of retinal diseases with severe pathology, with some decrease in reproducibility. The resolution of ophthalmic OCT is typically around 5–7 μm in the axial direction and 10 μm in the lateral dimension. Optical coherence tomography (OCT) is used for the quantification of retinal thickness in a large number of retinal diseases, and the present technology is based on a spectral domain detection system, illumination at 840 to 870 nm and scanning speeds in the range of 20–50,000 A-scans/sec. The retinal thicknesses from the Cirrus and from the Spectralis differed by 14 μm with the standard software of the instruments, and by less than 3 μm when analyzed with the same custom-made software, indicating that the major differences between the two SD-OCT systems are due to differences in their built-in software algorithms. ![]() Also for the RPE_OS complex, Spectralis measured a greater thickness than did Cirrus, with a mean of 3.32 μm ( p < 0.0001) for all points. ![]() For the central 1 mm, the difference was 1.78 μm ( p = 0.0414), and for all points out to 6 mm, the Spectralis retinal thickness was also significantly larger than the Cirrus thickness ( p = 0.0052), though the mean difference was only 1.85 μm. When Cirrus and Spectralis scans were analyzed with the same software, the retinal thickness at the foveal center was 225.92 μm (SD 17.0) using the Cirrus and 228.70 μm (SD 18.4) using the Spectralis the difference of 2.78 μm was not significant ( p = 0.055). Thickness was analyzed at the fovea, the central 1-mm line and the 6-mm line. Scans from similar positions and passing the fovea were analyzed by custom-made software. Thirty-seven healthy eyes were examined within the same session with a Cirrus OCT and a Spectralis OCT, the latter using averaged B-scans. The purpose of this study was to evaluate if differences in thickness when using a spectral domain Cirrus OCT or a Heidelberg Spectralis are due to hardware differences, or if they are caused by the segmentation algorithms. Spectral-domain optical coherence tomographies (OCTs) from different companies do not give identical retinal thicknesses.
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