Fig. 4.1
(a) A 55 μm thick OCTA C-scan (autosegmented 3 × 3 mm) using the internal limiting membrane (ILM) as a plane of reference. The scan is taken from 2.6 μm underneath the ILM to 15.7 μm below the boundary between inner plexiform layer (IPL) and inner nuclear layer (INL) at the level of the ganglion cell layer (GCL), showing the arterioles and venules originating from the superior and inferior arcades and the transverse connecting capillaries. (b) The corresponding B-scan shows the precise segmentation of the superficial capillary layer
4.2 The Deep Capillary Layer
The deep capillary plexus is located in the inner nuclear layer and external plexiform layer. Former histologic studies have shown this capillary network to consist of two layers bracketing the two borders of the inner nuclear layer. On OCTA with slab segmentation thickness of 30 μm, those two layers can be merged and considered as one deep plexus.
The deep retinal layer shows a polygonal-shaped capillary plexus, radially connected toward the center (see Fig. 4.2). It is believed that the center leads to a vertical interconnecting channel that connects the superficial and deep plexuses.
Fig. 4.2
(a) A 90 μm thick OCTA C-scan using of the deep capillary layer. The scan is taken from 15.7 μm underneath the inner plexiform layer (IPL)/inner nuclear layer (INL) to 70 μm underneath, at the level of the (INL). Some projection artifact from the superficial capillary layer can be seen (arrows). (b) The corresponding B-scan shows the precise segmentation of the deep capillary layer
4.3 The Outer Retina
Viewed on OCTA, the outer retina is avascular. Using high resolution SS-OCTA allows visualization of the different layers of the outer retina (see Fig. 4.3). The outer nuclear layer (ONL) and the Henle’s fiber layer are localized between the external limiting membrane (ELM) and the outer plexiform layer (OPL). The ONL is relatively hyporeflective, but the Henle’s layer may have variable reflectivity depending on the orientation of the light source relative to the plane of the retina at that location. The myoid zone, the next hyperreflective band in the outer retina, extends between the ELM and the ellipsoid zone and represents the myoid fraction of the inner segment of the photoreceptors. The ellipsoid zone is a hyperreflective band representing an interface between inner and outer photoreceptor segments. The outermost hyperreflective band is the interdigitation zone, anterior to the retina pigment epithelium (RPE), representing the contact cylinders composed of the apical processes of the RPE cells and tips of the cone photoreceptor outer segments.
