The total retinal thickness is the measurement of the distance between these two segmentation lines. Every OCT system has an inbuilt segmentation algorithm that identifies the differences in reflectance of the retinal layers and borders and then places lines over the inner and outer border of the retina. Retinal thickness is a reproducible and common quantitative measurement that is used to monitor the disease process or treatment response using OCT. Warm colors indicate thicker retinal areas and cool colors indicate thinner retinal areas. In general, all machines have a thickness map displaying the average retinal thickness values in 1 mm, 3 mm, and 6 mm diameter circles divided into sectors with color map. The analysis report varies with different OCT systems. The OCT view window consists of the analyzed OCT images with its corresponding fundus image showing the orientation of the OCT scan. The saved OCT scans are analyzed both qualitatively and quantitatively. Repeat the procedure with other scan protocols if necessary. Adjust the light entry point across the pupil to get the best signal strength. Make sure that the OCT image is upright and straight and the reference scan pattern is centered on the fovea or the area of interest. Once the fundus image on the monitor is focused, the OCT image appears on the monitor, focus is enabled by correcting refractive errors. The patient is instructed to look at the internal target at the center (for macular scanning) or an external target where appropriate. An appropriate scanning protocol is selected to scan the retinal area of interest and a live OCT window is seen. Optical Coherence Tomography Scan Acquisition ProcedureĪ minimum pupil diameter of 3 mm is required to obtain a good OCT image. These scan protocols may differ in length, density, or resolution depending on the OCT system used. When the axis coincides with the fovea, the relationship of the lesion to the fovea is documented. The radial scan consists of 6–12 line scans arranged in equal angles with common axis. Raster scan is a series of parallel line scans that can be oriented in any angle and is of higher resolution. It generates a 3D view of the image which enables the implementation of advanced, complex analysis, for example, C-scan, topographic maps, and cyst volume and gives a holistic view of the macula. A 3D scan consists of a number of horizontal line scans composing a 6 mm × 6 mm or 7 mm × 7 mm or 12 mm × 9 mm rectangular box. The commonly used scan protocols for macular scanning are three-dimensional (3D) scan, radial scan, and raster scan. Scan protocols used in the more widely used SD-OCT systems are mentioned in this section. Optical Coherence Tomography Scan Protocols Table 1 is a brief comparison of the salient features of the three types of OCT. 1 and the corresponding images are shown in Fig. The stages of evolution of various types of OCT from the original time-domain OCT to spectral domain (SD-OCT) and swept-source OCT (SSOCT) are shown in Fig.

A-Scans that are captured at adjacent retinal locations by transverse scanning mechanism are combined to produce a 2-dimensional image. The two reflected beams of light are made to produce interference patterns to obtain the echo time delay and their amplitude information that makes up an A-Scan. It uses infrared light from a super-luminescent diode that is divided into two parts: one of which is reflected from a reference mirror and the other is scattered from the biological tissue. It captures optical scattering from the tissue to decode spatial details of tissue microstructures. Optical coherence tomography (OCT) is an optical analog of ultrasound imaging that uses low coherence interferometry to produce cross-sectional images of the retina.

Optical coherence tomography basics