Optical Coherence Tomography PIONEERING EXPERIMENT group at The Institute of Medical Physics( IMP), Medical University of Vienna and Professor James Fujimoto’ s group at Massachusetts Institute of Technology( MIT) were instrumental to the commercial development and clinical utility of OCT. Professor Fercher’ s group demonstrated the first measurement of the axial length of a living human eye using LCI in 1986 [ 2 ], enabling the development of the first commercial ocular biometry system for measuring intraocular distances, which has been critical for the management of cataract. On the other hand, Professor Fujimoto’ s team advanced the development of femtosecond lasers in medicine, leading to the first demonstration of OCT for imaging the eye in 1991 [ 1 ] and the launching of the first commercial OCT system by Zeiss in 1996. Among several prizes, the two teams were jointly awarded the 2017 Russ Prize by the United States National Academy of Engineering for their invention of OCT and its tremendous impact in clinical care, and especially in ophthalmology( Figure 1).
PRINCIPLE OF LOW COHERENCE INTERFEROMETRY AND OPTICAL COHERENCE TOMOGRAPHY
The principle of low coherence interferometry( LCI), from which OCT derives, leverages the fact that for a broadband source with multiple wavelengths, light reflecting from a given layer of the sample interferes constructively( i. e., maximum energy) with light reflecting from the reference mirror at all the wavelengths only when the location of the layer exactly matches that of the reference mirror, forming a strong peak( i. e., the sum of energies at all wavelengths). For any other layers of the sample, the mismatch with the reference mirror causes all the wavelengths to be out of phase with each other, thereby interfering destructively( i. e., cancelling out their energies). The reflectivity profile therefore has a single peak corresponding to the location of the layer. The broader the source, the more define the location of the layer, which characterizes the axial resolution of the OCT system. The same process can be repeated for individual layers of the sample by scanning the location of the reference mirror to match that of individual layers of the sample, as illustrated below.
Principle of Optical Coherence Tomography. Courtesy of Prof. D. S. Metha.
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