**8. Clinical interpretation of OCT in glaucoma diagnosis**

#### **8.1 Stratus OCT (Carl Zeiss)**

The Stratus OCT utilizes various protocols for analysis of the optic nerve head and the retinal nerve fiber layer.

#### *8.1.1 Optic nerve head analysis*

The "Fast Optic disc " pattern is used to analyze the optic nerve head. It consists of six evenly placed radial lines centered on the optic nerve head. Each of the six

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**Figure 3.**

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma*

lines consists of 128 A scans crossing an area of 4 mm in length vertically with a total scan time of 1.92 s. The optic nerve head analysis report consists of a B scan image of the optic nerve head taken horizontally and an overall gray scale image of

The parameters for individual radial scan analysis are derived from an algorithm which takes into account the location of the inner limiting membrane (ILM) and the termination of the retinal pigment epithelium (RPE layer)/Bruch's membrane. Since the RPE/Bruch's termination defines the edge of the disc margin the horizontal distance between the two points on the RPE/Bruch's termination gives the disc diameter. A second line is drawn 150 μm above and parallel to the one connecting the two points on the RPE/Bruch's termination. This is the plane that separates the rim from the cup. The cup diameter is the horizontal distance between the two points of intersection of this line with the ILM. Rim length is the difference between the cup diameter and the rim diameter. The rim area is total area above this line. The optic nerve head analysis is a conglomeration of all six radial scans arranged in a spoke pattern and interpolation of data between each of the scans created by smooth lines created around the disc and cup margins. Disc and cup areas are computed as the area within these margins, and rim area is calculated as the disc area minus cup area. The cup disc ratios (both horizontal and vertical) are calculated by the maximum distances of the disc and cup margins in the horizontal and vertical meridian, respectively. The vertical integrated rim area is an area interpolated around the discs of individual scans to define a rim volume. The horizontal integrated rim width is the mean of average nerve widths multiplied by the value

the optic disc demonstrating the disc and the cup margins (**Figure 3**).

*Optic nerve head analysis report (adapted from Carl Zeiss OCT Manual).*

*DOI: http://dx.doi.org/10.5772/intechopen.84202*

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma DOI: http://dx.doi.org/10.5772/intechopen.84202*

**Figure 3.** *Optic nerve head analysis report (adapted from Carl Zeiss OCT Manual).*

lines consists of 128 A scans crossing an area of 4 mm in length vertically with a total scan time of 1.92 s. The optic nerve head analysis report consists of a B scan image of the optic nerve head taken horizontally and an overall gray scale image of the optic disc demonstrating the disc and the cup margins (**Figure 3**).

The parameters for individual radial scan analysis are derived from an algorithm which takes into account the location of the inner limiting membrane (ILM) and the termination of the retinal pigment epithelium (RPE layer)/Bruch's membrane. Since the RPE/Bruch's termination defines the edge of the disc margin the horizontal distance between the two points on the RPE/Bruch's termination gives the disc diameter. A second line is drawn 150 μm above and parallel to the one connecting the two points on the RPE/Bruch's termination. This is the plane that separates the rim from the cup. The cup diameter is the horizontal distance between the two points of intersection of this line with the ILM. Rim length is the difference between the cup diameter and the rim diameter. The rim area is total area above this line. The optic nerve head analysis is a conglomeration of all six radial scans arranged in a spoke pattern and interpolation of data between each of the scans created by smooth lines created around the disc and cup margins. Disc and cup areas are computed as the area within these margins, and rim area is calculated as the disc area minus cup area. The cup disc ratios (both horizontal and vertical) are calculated by the maximum distances of the disc and cup margins in the horizontal and vertical meridian, respectively. The vertical integrated rim area is an area interpolated around the discs of individual scans to define a rim volume. The horizontal integrated rim width is the mean of average nerve widths multiplied by the value

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

in the Fourier transform of the frequencies of light reflected.

**6. Spectrum of OCT in the diagnosis of glaucoma**

**7. Types of OCT**

**7.1 Stratus or time domain OCT**

decreases the chance of missing lesions.

**7.3 Anterior segment OCT (AS-OCT)**

**8.1 Stratus OCT (Carl Zeiss)**

and the retinal nerve fiber layer.

*8.1.1 Optic nerve head analysis*

recombine and form the interference pattern at the beam splitter, the interference pattern is split by a grating into its frequency components, all of these components are simultaneously detected by a charge-coupled device (CCD) (**Figure 2**). SD-OCT is also known as Fourier domain OCT (FD-OCT) because the distances are encoded

The OCT can scan the optic nerve head (ONH), peripapillary retinal nerve fiber (RNFL), and the macular area (GCC—ganglion cell complex). An addition to the spectrum of posterior evaluation is the anterior segment OCT (AS-OCT) which utilizes a higher wavelength light to capture images of the anterior chamber angle.

From its inception, OCT images were acquired in a time domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30° apart. Time-domain OCT (TD-OCT) systems featured scan rates of 400 A-scans per second with an axial resolution of 8–10 μm in tissue [7]. Since the slices are 30° apart, there always is a possibility to miss pathology between the slices.

SD-OCT, on the other hand, achieves scan rates of 20,000–52,000 A-scans per second and a resolution of 5–7 μm in tissue [8, 9]. This increased scan rate and number diminishes the likelihood of motion artifact, enhances the resolution, and

Anterior segment OCT utilizes higher wavelength light (1310 nm) as compared

The Stratus OCT utilizes various protocols for analysis of the optic nerve head

The "Fast Optic disc " pattern is used to analyze the optic nerve head. It consists of six evenly placed radial lines centered on the optic nerve head. Each of the six

to 830 nm of traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration allowing clear images of the parts of the anterior segment (cornea, anterior chamber, iris, and angle). Currently, there are two commercially produced dedicated anterior segment devices, the slit lamp-OCT (SL-OCT: Heidelberg Engineering) and the Visante (Carl Zeiss Meditec, Inc.).

**8. Clinical interpretation of OCT in glaucoma diagnosis**

**7.2 Fourier domain OCT (FD-OCT) or spectral domain OCT (SD-OCT)**

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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 during a scan lead to loss of focal defects.
