*8.2.1 Optic nerve head scan pattern*

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

*A Practical Guide to Clinical Application of OCT in Ophthalmology*

ing a scan lead to loss of focal defects.

cell body layer.

(**Figure 4**).

gives thinner readings.

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

of circumference of the disc. The major limitation of the fast scan is the need for interpolation of data between the spaces which assumes that the six scans each time has been centered exactly at the same line. Hence, significant eye movements dur-

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

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

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

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 4.**

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

create circumpapillary nerve fiber (NFL) map and 12 radial lines with 3.7 mm length, which are used to calculate the disc margin which forms the ONH scan. All B-scans (lines) are centered at the optic disc.
