Low-coherence light from a superluminescent diode (SLD) is directed through a beam splitter and divided into a sample beam that is focused onto the patient's retina and a reference beam that travels a calibrated delay path (Figure 2A). Because the echo time delays of light are too fast to measure directly, an optical correlation technique, known as Michelson low coherence interferometry, is used.
OCT is performed by measuring the echo delay and intensity of backscattered light from the internal tissue microstructure. OCT imaging is analogous to B-scan ultrasonography, except that OCT measures light rather than acoustic waves. 6 A summary of major milestones in the development of ophthalmic OCT is depicted in Figure 1. Stratus OCT is now accepted as a standard-of-care instrument in ophthalmology, with more than 6,000 units sold by 2006. As imaging speed increased, ergonomics improved, and standardized clinical data and clinical experience became more available, the first-generation device was succeeded by OCT2 and Stratus OCT. 5 In 1996, the first commercial OCT instrument, the Zeiss OCT, was introduced. Although OCT was originally commercialized through a startup company, Advanced Ophthalmic Devices, in 1994 the technology was transferred to Humphrey Instruments, a subsidiary of Carl Zeiss (Jena, Germany). The first in vivo studies of the human retina were published in 1993, 2,3 and these were soon followed by clinical studies performed at New England Eye Center 4 and other medical centers. 1 Because ocular media are transparent, the retina provided an ideal tissue for OCT imaging. Optical coherence tomography was developed by researchers at the Massachusetts Institute of Technology and collaborators and was first reported in Science in 1991. Now, nearly two decades since its introduction, OCT has become indispensable for research, screening, diagnosing, and monitoring diseases of the macula and optic nerve head. In ophthalmology, OCT can perform "optical biopsy" noninvasively, imaging the retina with a resolution higher than any other imaging modality other than histology. Optical coherence tomography (OCT) is a diagnostic imaging technology that provides cross-sectional images of biological tissues.
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