**2. Principle and technique**

In simple terms, OCT is similar to ultrasound in that a beam of sound or light directed onto a tissue is differentially reflected from structures with different acoustic or optical properties. The time it takes for the sound or light to reflect from the

#### **Figure 1.**

*a) Principle of time domain OCT. b) Principle of spectral domain OCT. c) Principle of swept source OCT.*

#### *Optical Coherence Tomography in Diabetic Retinopathy DOI: http://dx.doi.org/10.5772/intechopen.100587*

different structures determines the dimensions of the structures. This provides an image similar to the A-scan or depth scan of ultrasound. Imaging of laterally adjacent depth scans provides a two-dimensional or B-scan image. The time delay involved when using light is in femtoseconds requiring interferometry to do the calculations [7].

The first generation OCT machine or Time-Domain OCT (TD-OCT) uses low time-coherence interferometry to obtain depth scans (**Figure 1a**). A beam splitter splits the light coming from a broadband light source, one directed to the eye and the other to the reference mirror. The position of the reference mirror is changed to mirror the depth of the various layers of tissue being scanned. Light reflected from the two sources is collected and the interferogram is analyzed to give a complete depth scan. TD-OCT involves two scans, one for depth scan and one for lateral scan, thus, resulting in a lesser number of scans acquired per second.

With the use of spectrometer and Fourier-domain technique in the next generation OCT, called Spectral-domain OCT (SD-OCT), the disadvantage of performing a depth scan was avoided. SD-OCT uses an array of photo-detectors to capture the depth scan without having to move the reference mirror (**Figure 1b**). Therefore, only a lateral scan has to be performed [7]. This increased the scan speed enormously. Further refinement of technology led to the change of the broadband nearinfrared superluminescent diode light source of wavelength 840 nm in SD-OCT to a tunable swept laser source with a center wavelength of 1050 nm [8]. In conjunction, the array of photodetectors in SD-OCT was replaced with a single photodetector [8]. This led to the evolution of Swept-source OCT (SS-OCT) (**Figure 1c**). SS-OCT provides increased scan speed and denser scans with greater resolution as more A-scan and B scans are acquired per second. The scan area is also increased along with scan depth due to the use of a longer wavelength light source which allows better penetration through retinal pigment epithelium (RPE).
