**1. Introduction**

The new generations of optical coherence tomography (OCT) can reproduce the description of optic disc (including its margin, cup, and rim areas) and makes a quantitative analysis of its surrounding structures, including peripapillary retinal nerve fiber layer (RNFL) thickness in different sectors and quadrants.

OCT is a good diagnostic tool for acquired and congenital optic nerve head diseases. In addition, it can be used for monitoring peripapillary RNFL thickness in some conditions including glaucoma and chronic papilledema. OCT is not only useful in the analysis of the optic nerve disorders but also in some of the central nervous system diseases that impact the optic nerve, too.

About the principles of the optic nerve head OCT, in brief, at first it determines the margin of optic disc automatically, even in patients with peripapillary atrophy. The optic disc size between 1.3–2.5 mm<sup>2</sup> is considered standard, and outside of this range, all measurements represent in gray. Then, the device's software identifies the two most refractive zones of the retina, including the RNFL as the inner boundary and the pigment epithelium as the outer boundary of the peripapillary area, and the thickness of the peripapillary nerve fibers is measured at 3.4 mm from the center of the optic. In optic nerve head OCT printouts, strong reflection of peripapillary layers demonstrates that layers are perpendicular to the light of the imaging device. In contrast, areas with low reflection represent parallel layers with the light (**Figure 1**). The RNFL OCT scan maps represent A-scan data of the RNFL thickness from the center of the optic disc. Furthermore, ganglion cell-inner plexiform layer (GC-IPL) can be scanned, too. In these maps, the RNFL

#### **Figure 1.**

*White arrow: high reflective layers (perpendicular to the light of the OCT device). Red arrow: low reflective layers (parallel to the light).*

#### **Figure 2.**

*Peripapillary OCT: green circle: quality score, red arrow: IPL segmentation line, blue arrow: outer boundary of RNFL.*

thickness measurements are compared against age-matched normative values. An area where the RNFL thickness is <5% compared to the normal values is shown in yellow and < 1% is shown in red. Before interpretation of the RNFL thickness in these maps, it is necessary to evaluate their accuracy by controlling IPL and RNFL segmentation lines and also quality score of the images (**Figure 2**) [1].

Analysis of the results of optic nerve head OCT needs careful interpretation due to the imaging technique complexity, different aspects of optic nerve head diseases, and potential artifacts. Furthermore, glaucomatous optic neuropathy can mimic neuroophthalmology disorders, making challenges in the proper diagnosis.

Some factors may affect the resolution and reliability of optic nerve head OCT images. First, the signal strength (as a quality expresser) of the images must be assessed. For example, it must be >6/10 with the Cirrus™ HD-OCT (Carl Zeiss) device or > 50 with the Optovue (RTVue) device. A poor signal leads to poor quality and misidentification of peripapillary RNFL and their measurements. Second, motion artifacts due to eye movements may cause poor quality, but new devices have programs to resolve this common problem. Third, some ocular conditions may lead to false detection and measurement of peripapillary RNFL. These conditions are including significant cataracts, dense posterior capsular opacity, or increased axial length in pathologic myopia that may influence the measurements [2].
