Optical coherence tomography principle
![optical coherence tomography principle optical coherence tomography principle](https://image.slidesharecdn.com/optical-coherence-tomography-120421562843883-4/95/slide-10-1024.jpg)
Key to longitudinal scanning is the fact that the reference mirror can be mechanically mobilized, resulting in a shift of the reference beam, extending the reference path. Based on the amplitude of the interference signals that arise from interference of the reflected light from retinal layers of different depths and the reference light from various path lengths, a single axial scan, the so-called A-scan, is deduced. As the path length of the moving reference mirror is known, it is possible to calculate the depth of the tissue from which the fraction of reflected light arises based on the specific time delay. The reference mirror is axially translocated in order to match echo time delays from various tissue layers. Light reflected from superficial structures of the retina or uppermost retinal layers has a shorter echo time delay than light reflected from deeper retinal structures (innermost layers). This phenomenon allows accurate measurement of the echo time delay. Interference of light beams with low coherence only occurs in cases where the distance traveled by the light in both arms of the interferometer is equivalent to within the coherence length in order to allow interference by co-occurrence on the level of the photodetector. As the name of this technology suggests, TD-OCT is based on the difference in the time delay of the sample light echoes reflected from the different retinal layers as a function of their depth within the tissue (here, the retina) and the reference beam echo, a single echo that is varied by changing the position of the mobile reference mirror (i.e., the reference arm-length).
![optical coherence tomography principle optical coherence tomography principle](https://image.slidesharecdn.com/optical-coherence-tomography-120421562843883-4/95/optical-coherence-tomography-principle-and-uses-in-ophthalmology-5-728.jpg)
The basic components and setup of a TD-OCT system are detailed in Figure 2.1. Time-domain (TD) optical coherence tomography