Optical Coherence Tomography and Optical Coherence Tomography-Angiography

The fundamental principle behind OCT imaging is light interference. The basic mechanism of an OCT can be described with the help of a Michelson interferometer, as described below. The light from a low coherence (high bandwidth) source is split into two paths by a coupler, directing it along two separate arms of the interferometer, the sample arm, and the reference arm. As the light exits either arm, it is passed through lenses and mirrors to control its shape, depth of focus, intensity, etc. A reference mirror reflects the light back into the reference arm, and it returns along the same path it came on, whereas the light in the sample arm is backscattered by the sample of interest. The returning light from both arms recombines at the coupler and generates an interference pattern recorded by the detector. The sample beam has to be moved across the sample to record an A scan at each point, which can be combined to generate an OCT image or a B scan. For a given wavelength, the axial resolution is based on the bandwidth, whereas the lateral resolution is limited by the diffraction caused by the pupil and the minimum spot size of the focused probing beam.