The basic principles for tissue reflectivity: Retinal layers with cellular components yield low reflectivity. In contrast, retinal layers with synaptic or fiber layers yield high reflectivity. The boundaries also produce high reflectivity. Therefore, nuclear layers generally appear hyporeflective, whereas retinal nerve fiber layer and plexiform layers appear hyperreflective. A structural spectral-domain optical coherence tomography (OCT) image from a healthy subject is shown in Figure 4.1.

Vitreous: The posterior hyaloid face with incomplete posterior vitreous detachment (PVD) may be visible on the OCT scan. A separation of the posterior hyaloid face from the neurosensory retina and optic disc on structural OCT scans can help determine whether a PVD is present or absent. Retrohyaloid space is between the posterior hyaloid face and internal limiting membrane (ILM). Premacular bursa or posterior precortical vitreous pocket is an optically empty liquid space overlying the macula caused by degenerative liquefaction of the vitreous.

Cellular layers: Retinal layers where cellular body exists, such as the ganglion cell layer, inner nuclear layer (INL), and outer nuclear layer (ONL), pose low reflectivity. The ONL may be thinner than what it appears as it may include Henle fiber layer, as discussed below.

Retinal nerve fiber layer and plexiform layers: Retinal nerve fiber layer and inner plexiform layer (IPL) typically pose high reflectivity as they are located parallel to the retinal surface and perpendicular to the beam direction of the OCT light source. The exception is the outer
plexiform layer (OPL); Henle fiber layer within OPL has variable reflectivity depending on the orientation of the light source relative to the plane of the retina. Henle fiber layer contains obliquely oriented photoreceptor axons and is almost isoreflective with the ONL in untilted B-scans. Henle fiber layer can be recognized when the OCT light source is inserted at the edge of the pupil that will induce the tilting of OCT images.

Outer retinal bands: The integrity of the outer retinal hyperreflective bands on structural OCT, including the external limiting membrane (ELM), ellipsoid zone (EZ), and interdigitation zone (IZ), reflects the health of photoreceptors. The elevation of the ELM and EZ band at the central fovea termed foveal bulge is present in healthy subjects. The foveal bulge represents the crowding and thinning of cone photoreceptors and elongation of cone outer segment. It is considered a biomarker for predicting foveal function.

Ellipsoid zone: Also referred to as the inner segment/outer segment (IS/OS) junction. EZ reflects the mitochondrial-rich band in the inner segment of photoreceptors. Loss or discontinuation of the EZ band represents defects or dysfunction in the photoreceptors, which typically has a profound negative effect on the visual function.

Interdigitation zone: Also referred to as cone outer segment tip (COST). A hyperreflective band below EZ and above retinal pigment epithelium (RPE) on Fourier-domain structural OCT. The IZ band corresponds to the contact cylinders formed by the apices of the RPE cells.

Retinal pigment epithelium: Observed as the outermost and thickest hyperreflective band on structural OCT. This hyperreflectivity is due to the presence of melanin. The high melanin scattering of the RPE can obscure adjacent layers. Under normal conditions, melanin scattering of the RPE thereby limits the visibility of Bruch membrane.