*8.1.2 The retinal nerve fiber layer (RNFL) analysis*

The RNFL analysis involves a fast RNFL scan which takes approximately 1.9 s and acquires three fast circular scans 3.4 mm around the disc. This is time-efficient scan alignment and placement is required only once, each scan having an automated segmentation algorithm that detects the ILM boundary, the RNFL, and the ganglion cell body layer.

RNFL map comprises of six circular scans of 1.44, 1.69, 2.25, 2.73, and 3.40 mm radii. The scan size is 2.27 times the radius of the ONH. It helps to measure RNFL thickness with accuracy in various disc sizes. Centration of the circle around the disc is shown as an image adjacent to the scans and decentration of this circle can lead to erroneous results for RNFL thickness as closer circle position to the disc gives a thicker RNFL measurement while a position far away gives thinner readings.

The RNFL thickness is reported as overall average thickness and averages by quadrants and clock hours. The average RNFL thickness and various comparisons within the same and the other eye is also projected in a tabular form (**Figure 4**).

RNFL thickness map typically has a double hump pattern as the RNFL is thicker at the superior and inferior poles. The thickness of RNFL of a patient is compared to age matched normative data base and interpreted in different color codings. The green color encompasses the 5th to 95th percentile of the normative range for RNFL and is considered normal. The yellow color represents first to fifth percentile of the normal population and considered borderline. Anything below the first percentile

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**Figure 5.**

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma*

is shown in red and considered outside normal limits. Values greater than 95th percentile are indicated in white and depicts above normal values (**Figure 5**).

The newer generation Fourier-domain optical coherence tomography (FD-OCT) technology offers tremendous advances over the traditional time-domain (TD) technology in terms of speed and resolution. Currently, the Cirrus high-definition (HD)-OCT and RTVue 100 are commercially used to quantify peripapillary RNFL thickness in clinical practice. Studies indicate that RNFL thickness parameters measured on Cirrus OCT are reproducible and have high diagnostic sensitivity and specificity in discriminating between healthy and glaucomatous eyes [10, 11]. Furthermore, for the detection of glaucoma RNFL parameters of the RTVue-100 OCT have shown high specificity [12]. Studies indicate that ganglion cells located in the macular area are the earliest cells to be lost in glaucoma and hence this has led to utilize the less explored parameter of various

**8.2 Fourier domain OCT (FD-OCT) in glaucoma diagnosis**

*RNFL thickness average analysis report (Stratus OCT: Carl Zeiss Meditec, Inc.).*

OCT devices, the ganglion cell complex located in the macular region [13].

The ONH scan is a combination of circular scans for RNFL thickness analysis and radial scans for ONH shape analysis. Combining circular scans and radial scans into one single pattern ensures that the RNFL scan and ONH scan naturally share same center. The scan time is only 0.5 s to help minimize any effect of eye movement. Hence the scan consists of 13 circles with diameters of 1.3–4.9 mm, which is used to

*8.2.1 Optic nerve head scan pattern*

*DOI: http://dx.doi.org/10.5772/intechopen.84202*

**Figure 4.** *Retinal nerve fiber layer thickness/volume tabular output.*

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma DOI: http://dx.doi.org/10.5772/intechopen.84202*

**Figure 5.** *RNFL thickness average analysis report (Stratus OCT: Carl Zeiss Meditec, Inc.).*

is shown in red and considered outside normal limits. Values greater than 95th percentile are indicated in white and depicts above normal values (**Figure 5**).
