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Normal Optic Nerve
Summary
Spectral domain (SD) OCT devices have two scan patterns used to analyze the optic nerve head: volume scans and line scans.
Volume Scans
Volume scans acquire a volumetric set of data, centered at the optic nerve head. 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 optic nerve head ( ). The Optovue RTVue’s protocol for the optic nerve head consists of a grid pattern with circular and radial scans that acquires a 4 mm × 4 mm volume around the optic nerve. Because different machines use circles of different diameters around the center of the optic nerve head, the measurement of RNFL between machines is not comparable ( ).
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