Optical coherence tomography (OCT) is an established medical imaging technique that uses light to capture micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue). Optical coherence tomography is based on low-coherence interferometry, typically employing near-infrared light. The use of relatively long wavelength light allows it to penetrate into the scattering medium. Confocal microscopy, another optical technique, typically penetrates less deeply into the sample but with higher resolution.
Depending on the properties of the light source (superluminescent diodes, ultrashort pulsed lasers, and supercontinuum lasers have been employed), optical coherence tomography has achieved sub- micrometer resolution (with very wide-spectrum sources emitting over a ~100 nm wavelength range).
Optical coherence tomography is one of a class of optical tomographic techniques. A relatively recent implementation of optical coherence tomography, frequency-domain optical coherence tomography, provides advantages in signal-to-noise ratio, permitting faster signal acquisition. Commercially available optical coherence tomography systems are employed in diverse applications, including art conservation and diagnostic medicine, notably in ophthalmology and optometry where it can be used to obtain detailed images from within the retina. Recently it has also begun to be used in interventional cardiology to help diagnose coronary artery disease. It has also shown promise in dermatology to improve the diagnostic process.
Starting from white-light interferometry for ''in vivo'' ocular eye measurements〔A. F. Fercher and E. Roth, "Ophthalmic laser interferometry. Proc. SPIE vol. 658, pp. 48-51. 1986.〕 imaging of biological tissue, especially of the human eye, was investigated by multiple groups worldwide. A first two-dimensional ''in vivo'' depiction of a human eye fundus along a horizontal meridian based on white light interferometric depth scans was presented at the ICO-15 SAT conference in 1990.〔A. F. Fercher, "Ophthalmic interferometry," Proceedings of the International Conference on Optics in Life Sciences, Garmisch-Partenkirchen, Germany, 12–16 August 1990. Ed. G. von Bally and S. Khanna, pp. 221-228. ISBN 0-444-89860-3.〕 Further developed in 1990 by Naohiro Tanno,〔Naohiro Tanno, Tsutomu Ichikawa, Akio Saeki: "Lightwave Reflection Measurement," Japanese Patent # 2010042 (1990) (Japanese Language)〕〔Shinji Chiba, Naohiro Tanno "Backscattering Optical Heterodyne Tomography", prepared for the 14th Laser Sensing Symposium (1991) (in Japanese)〕 then a professor at Yamagata University, and in particular since 1991 by Huang et al., optical coherence tomography (OCT) with micrometer resolution and cross-sectional imaging capabilities has become a prominent biomedical tissue-imaging technique; it is particularly suited to ophthalmic applications and other tissue imaging requiring micrometer resolution and millimeter penetration depth. First ''in vivo'' OCT images – displaying retinal structures – were published in 1993.〔A. F. Fercher, C. K. Hitzenberger, W. Drexler, G. Kamp, and H. Sattmann, " In Vivo Optical Coherence Tomography," ''Am. J. Ophthalmol'', vol. 116, no. 1, pp. 113-114. 1993.〕
OCT has also been used for various art conservation projects, where it is used to analyze different layers in a painting. OCT has interesting advantages over other medical imaging systems.
Medical ultrasonography, magnetic resonance imaging (MRI), confocal microscopy, and OCT are differently suited to morphological tissue imaging: while the first two have whole body but low resolution imaging capability (typically a fraction of a millimeter), the third one can provide images with resolutions well below 1 micrometer (i.e. sub-cellular), between 0 and 100 micrometers in depth, and the fourth can probe as deep as 500 micrometers, but with a lower (i.e. architectural) resolution (around 10 micrometers in lateral and a few micrometers in depth in ophthalmology, for instance, and 20 micrometers in lateral in endoscopy).
OCT is based on low-coherence interferometry. In conventional interferometry with long coherence length (i.e., laser interferometry), interference of light occurs over a distance of meters. In OCT, this interference is shortened to a distance of micrometers, owing to the use of broad-bandwidth light sources (i.e., sources that emit light over a broad range of frequencies). Light with broad bandwidths can be generated by using superluminescent diodes or lasers with extremely short pulses (femtosecond lasers). White light is an example of a broadband source with lower power.
Light in an OCT system is broken into two arms — a sample arm (containing the item of interest) and a reference arm (usually a mirror). The combination of reflected light from the sample arm and reference light from the reference arm gives rise to an interference pattern, but only if light from both arms have traveled the "same" optical distance ("same" meaning a difference of less than a coherence length). By scanning the mirror in the reference arm, a reflectivity profile of the sample can be obtained (this is time domain OCT). Areas of the sample that reflect back a lot of light will create greater interference than areas that don't. Any light that is outside the short coherence length will not interfere. This reflectivity profile, called an A-scan, contains information about the spatial dimensions and location of structures within the item of interest. A cross-sectional tomograph (B-scan) may be achieved by laterally combining a series of these axial depth scans (A-scan). En face imaging at an acquired depth is possible depending on the imaging engine used.
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