III.6 Retinal Thickness Definition
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## III.6 Retinal Thickness Definition

Using OCT technology, there are typically 2 major highly reflective bands generally visible; inner and outer highly reflective bands (IHRB and OHRB).

The inner band corresponds to the inner portion of the retina, which consists of ILM (internal limiting membrane), RNFL (retinal nerve fiber layer), GCL (ganglion cell layer), IPL (inner plexiform layer), INL (inner nuclear layer), and OPL (outer plexiform layer). In terms of the reflectivity, they present a high-low-high-low-high pattern, in general. Presumably RNFL, IPL, and OPL are the highly reflective layers and GCL and INL are of low reflectivity. ILM itself may or may not be visible in OCT images (depending on the scanning beam incidence angle), but for convenience it is used to label the vitreo-retinal interface.

The outer band is considered as the RPE (retinal pigment epithelium) /Choroid complex that consist of portion of photoreceptor, RPE, Bruch's membrane, and portion of choroid. Within the RPE/Choroid complex, there are 3 highly reflective interfaces identifiable, presumably corresponding to IS/OS (photoreceptor inner/out segment junction), RPE, and Bruch's membrane.

Clinically 3 retinal thickness measurements are generally acknowledged and utilized; RNFL thickness, GCC (ganglion cell complex) thickness, and total retinal thickness.

RNFL thickness is defined as the distance between ILM and outer interface of the inner most highly reflective layer presumably RNFL.

GCC thickness is defined as the distance between ILM and the outer interface of the second inner highly reflective layer presumably the outer border of inner plexiform layer (IPL).

Total retinal thickness definition varies among OCT manufacturers. The classic definition is the distance between ILM and the first highly reflective interface (presumably IS-OS) in the OHRB (Total retinal thickness (ILM to IS-OS) ). A second definition is the distance between ILM and the second highly reflective interface (presumably RPE) in the OHRB (Total retinal thickness (ILM to RPE) ). A third definition is the distance between ILM and the third highly reflective interface (presumably Bruch's membrane) in the OHRB (Total retinal thickness (ILM to BM) ).

Figure III.6-1. Observable Layer Structures

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