Spectral domain OCT (SD-OCT) devices have two scan patterns to analyze the optic nerve head (ONH): volume scans and line scans.
Volume Scans
Volume scans acquire a volumetric set of data, centered at the ONH. It delineates the optic disc margin and optic disc surface contour and is segmented to obtain the retinal nerve fiber boundaries. Each device has its own scanning protocol. The Cirrus HD-OCT identifies the center of the optic disc and creates a 3.46-mm circle on this location and calculates the thickness of the retinal nerve fiber layer (RNFL). The Heidelberg Spectralis creates a cylindrical volume with a diameter of 3.4 mm through and around the ONH ( ). The Optovue RTVue’s protocol for the ONH consists of a grid pattern with circular and radial scans that acquires a 4- × 4-mm volume around the optic nerve. Because different machines use circles of different diameters around the center of the ONH, the measurement of RNFL between machines is not comparable ( ).
Retinal Nerve Fiber Layer Thickness (RNFL)
OCT devices calculate RNFL thickness as the distance between the internal limiting membrane and the outer aspect of the RNFL ( Fig. 1 ).
Ganglion Cell Complex
The ganglion cell complex (GCC) consists of the thickness of three inner retinal layers: the NFL, the ganglion cell layer, and the inner plexiform layer. The scan is centered at the fovea, and the software presents the results as a color-coded map, comparing to a normative database ( Fig. 2 ).
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