Basic Optic Nerve Scan Patterns and Output
▶ Volume scans: these are analogous to macular cube scans in which a volumetric set of data is acquired, centered at the optic nerve head. These may be square or rectangular cubes of data, or cylindrical cubes acquired by circumferential scanning around the optic nerve. The Cirrus HD-OCT scanning protocol acquires a 6 mm × 6 mm cube of data at the optic nerve head using a series of rapid B-scans (200 × 200). Software processing within the SD-OCT machine then identifies the center of the optic disc and creates a 3.46 mm circle centered on this location for registration purposes. The data set is then used to measure retinal nerve fiber layer thickness. The Heidelberg Spectralis volumetric scanning protocols acquire a set of three sequential circular scans, each with 256 axial scans, centered at the optic nerve head. This yields a cylindrical volume with a diameter of 3.4 mm through and around the optic nerve head.
▶ Line Scans: a single or a series of high-resolution B-scans can be obtained through the optic nerve head similar to the line scans obtained in the macula, to allow for higher resolution visualization of structure and pathology at the optic nerve head. Line scans are most frequently used in the qualitative interpretation of data and the identification of anatomic anomalies.
Volume scans obtained through the optic nerve head are processed to delineate the optic disc margin and optic disc surface contour and segmented to obtain the retinal nerve fiber layer (NFL) boundaries. Since most OCT measurements of the optic nerve head are highly sensitive to scan position, all commercially available OCT devices have motion correction software. The information obtained from the optic nerve volumetric scans is processed to obtain the following details.
