▶ 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.

    Occasionally, the concentric circular scanning protocol can be accompanied by the addition of radial line scans that allow for better registration of the concentric scans. The RTVue’s optic nerve head scan pattern consists of a grid pattern with circular and radial scans that acquires a 4 mm × 4 mm volume around the optic nerve. Some machines also have the ability to acquire ganglion cell complex scans as part of the glaucoma imaging protocol, that look at cubes of data centered on the macula.

▶ 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.

Retinal Nerve Fiber Layer (RNFL) Thickness

RNFL thickness is calculated by the OCT devices as the distance between the internal limiting membrane and the outer aspect of the NFL (Fig. 5.1.1). Because the RNFL varies with distance from the center of the optic nerve, most machines use a circle of a pre-defined diameter (usually between 3.4–3.46 mm) around the center of the optic nerve as the reference point at which to calculate RNFL thickness. One of the reasons that measurement of the RNFL between machines is not comparable is that different machines use circles of different diameters around the center of the optic nerve head.

RNFL thickness is then compared to age, ethnicity and disk size-matched normative databases. The results are displayed in various forms including a false color scale where green represents normal, yellow represents a ‘borderline’ RNFL thickness (less than a 5% probability of being normal), and red represents an abnormal RNFL thickness with less than a 1% probability of being normal. Results from the two eyes are also compared and any discrepancy between the two is highlighted. The RNFL thickness may be displayed as an average for the overall map, for quadrants, sectors, hemispheres or clock hours.

One of the most useful RNFL displays in clinical practice is the sinusoidal signal profile wave corresponding to the RNFL thickness profile 360° around the optic nerve, starting from the temporal region and proceeding through the superior, inferior, nasal and back to the temporal region (TSINT). This normally presents a ‘double hump’ pattern corresponding to the superior and inferior quadrants, where the RNFL is thickest. The inferior and superior quadrant RNFL thickness has been demonstrated to be the most useful in glaucoma diagnosis.

Optic Nerve Morphology

The software in various SD-OCT machines also calculates and displays areas for the optic disc, cup and rim, volumes for optic nerve head, cup and rim, cup-to-disc ratios and cup-to-disc horizontal and vertical ratios (Fig. 5.1.2).

Ganglion Cell Complex (GCC)

The ganglion cell complex consists of three inner retinal layers: the NFL (formed by axons of the ganglion cells), the ganglion cell layer (cell body of the ganglion cells) and the inner plexiform layer (dendrites of the ganglion cells). The GCC scan is a series of B-scans centered on the macula and quantifies the thickness in all of these three layers. After image processing, GCC thickness is calculated as the distance between the internal limiting membrane and the outer boundary of the inner plexiform layer. The software presents the results as a color-coded ‘map’, which compares the examined eye with a normative database and indicates deviations from the normal values (Figs 5.1.2 and 5.1.3). The GCC scan must be precisely centered at the fovea to have its results compared with a normative database, or to permit progression analysis.

The GCC thickness analysis may be displayed as an average overall thickness, averages in the superior and inferior hemiretina, superior–inferior difference in GCC thickness, the global loss volume (integration of all negative deviation values normalized by the overall map area), and the focal loss volume (integration of negative deviation values in the areas of significant focal loss).