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Optical coherence tomography (OCT), introduced in 1991 by Dr. James Fujimoto of MIT, is an optical method for capturing high-resolution, in situ images of tissue for both research and clinical applications. Tissue can be imaged to a depth of a few millimeters through the use of a low-coherence light source and an interferometer.
In the original implementation, cross-sections of the inner eye were constructed by scanning point by point across the tissue surface and taking z-axis (depth) measurements at each point. To acquire data on the reflectance of tissue at different depths, the interferometer's reference beam was continuously adjusted with a scanning mirror — a method that is effective, but slow. Newer systems work faster by eliminating this type of mechanical movement. Instead, they exploit the spectral information contained in the reflected light. There are generally two ways to generate such information: with a broadband light source and a diffraction grating to spatially distribute the spectrum across a linear image sensor, or by adopting a tunable laser as the light source to rapidly scan a range of wavelengths. In both cases, depth data is immediately computed by applying a Fourier transform on the acquired spectrum.
Most OCT applications use detectors and light sources at 830 nm (for retinal imaging) or 1300 nm (for other tissue). In detection, sensitivity and dark current are usually not an issue, since there often is a sufficient amount of light. The more important factors are dynamic range, readout speed, and readout noise.
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