The three scanning protocols are:

The high scanning speed of the SD-OCT allows multiple B-scans to be obtained and thus acquires a large dataset for the macular cube. The inbuilt software, 3D-volume rendering, in Cirrus HD-OCT (Carl Zeiss Meditec Inc., CA, USA) reconstructs a three-dimensional image of the whole retinal cube. The three planes of 3D SD-OCT are the X, Y, and Z planes. The X plane corresponds to the horizontal B-scan or cross section. The Y plane is the vertically reconstructed B-scan. The Z plane or the coronal plane (C-scan) corresponds to the reconstructed en face image of the retina.

A topographical map of the individual retinal layers, internal limiting membrane (ILM) and retinal pigment epithelium (RPE) are generated by 3D segmentation analysis. The ILM segmentation map is color-coded pale blue; the RPE/choriocapillaris complex is brown or cream. These maps provide an effortless pictorial representation of the morphological alterations as well as localization of the pathology. The topographical map of the retina represents the thickness of the retina, between ILM and RPE, with a cold/warm color scale. The color coding provides a global representation of the difference in thickness of the retina in comparison to a normative database.

Masking is another feature of the SD-OCT, useful when visualizing the three-dimensional retinal cube. Masking of the tissue below a desired layer from cutting enables peeling back of the tissues above the desired layer. By applying the internal limiting membrane (ILM) mask, the tissue below the ILM is preserved from being cut, and the ILM and the tissues above this layer can be peeled back. “Niche” allows for masking in two dimensions, the X and Y planes simultaneously. These facilitate imaging of the retinal pathology at various depths.

The RPE fit feature of the Cirrus HD-OCT (Carl Zeiss Meditec Inc., CA, USA) is an important advancement to understand various retinal pathology (Lumbroso et al. 2009). It provides C-scan images adapted to retinal curvature. En face images that follow the curvature of posterior pole can be obtained. RPE fit software identifies RPE/choriocapillaris complex to display it as a curved 3D section plane. The software adapts to concavity of RPE/choriocapillaris (Wei et al. 2012). Sections with thickness varying from 2 to 20 μm can be obtained with thinner sections known as slices and thicker sections known as slabs. Blood vessels of the choroid can be seen by changing the thickness of sections. A slab of selective thickness, the slab thickness of interest, can be studied by selecting and adjusting the anterior and posterior boundaries represented by two separable same-color dashed lines. The slab image represents an average signal intensity value for each A-scan location through the selected depth of the slab.

Recently, an international panel with expertise in retinal imaging reached a consensus for OCT imaging terminology (Staurenghi et al. 2014). They suggested the terms band, layer, and zone for the layers of the retina. The term band refers to the three-dimensional structure of the retinal layers anatomically. The term zone describes those regions on OCT whose anatomical correlation is not clearly delineated. The RPE/Bruch’s complex is one of the layers ascribed as zone as they are inseparable owing to interdigitation of cellular structure or tissue.

Histologically, the retina consists of ten layers, four of them are cellular and two are neuronal junctions. The axonal layers, nerve fiber layer, and plexiform layers are capable of potent light scatter and hence are hyperreflective. The light scattering potential of nuclear layers is lower. The first layer visible on OCT images is the hyperreflective ILM line at the vitreoretinal interface. The hyperreflective retinal nerve fiber layer lies next to this layer. Outer to this is a hyporeflective ganglion cell layer. Subsequently, hyperreflective plexiform layers are imaged with an inner nuclear layer situated between them. The relatively thick hyporeflective outer nuclear layer is visible next. A thin hyperreflective line underneath the outer nuclear layer corresponds to the location of the external limiting membrane (ELM). The hyperreflective layer of photoreceptor junction of inner and outer segments, currently termed as the photoreceptor inner segment ellipsoid zone, lies beneath this layer. Due to the increased length of outer cone segments in the central fovea, this line is slightly elevated in the foveal region. The outermost layer imaged is the hyperreflective RPE-Bruch’s complex. The choriocapillaris and the choroid are visible outer to the RPE-Bruch’s zone. The fovea is recognized on a cross-sectional image by its characteristic depression, due to the thinning of the retina with the absence of the inner layers at the macula (Fig. 1.1) (Table 1.2).