Optical coherence tomography of neurosensory layers

Spectral domain OCT (Spectralis OCT; Heidelberg Engineering, Heidelberg, Germany) was carried out in the central 20 degree macular area of one eye of each patient. This fourth‐generation OCT system has an optical depth resolution of 7 μm and lateral resolution of 14 μm in tissue and enables non‐invasive cross‐sectional imaging of ocular structures with high detail (Castro Lima et al. 2011). Proprietary noise reduction of the device reduces the axial A‐scan resolution to a digital resolution of 3.5 μm/pixel. The transverse digital resolution depends on the amount of registered A‐scans and can be adjusted. The device produces up to 40 000 A‐scans per second and simultaneously images the fundus with a 30 degree infrared confocal scanning laser ophthalmoscope (SLO). The built‐in real‐time eye movement tracking system recognizes features in the SLO image including blood vessels and the optic disc. This enables stabilization of the scan coordinates in relation to the retina coordinates and minimizes motion artefacts during image acquisition. Activated eye‐tracking was used to average multiple repeated OCT scans at each line of the macular cube scan protocol, whereupon single B‐scan frames were only added to the averaged OCT image, if the scanning position matched the original position in the SLO image. The eye‐tracking feature allows for a significant reduction of background noise and visualization of retinal microstructures with enhanced contrast and reduced variance (Pemp et al. 2013). A built‐in viewing software (HRA Viewing Module version 6.7.17; Heidelberg Engineering) was used to automatically segment the retinal nerve fibre layer (RNFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL) and photoreceptor layer (PRL) of all obtained B‐scans. Thus, the three‐dimensional mapping of individual retinal layers was achieved. For quantitative analysis, the central macular area was overlaid with a circular ETDRS grid consisting of three concentric circles with diameters of 1, 3 and 6 mm. By this means, the total macular volume and volumes of individual neurosensory retinal layers were obtained in the central area of 6 mm using the built‐in automated layer segmentation.