Editing Optical coherence tomography (section)

==Introduction==
[[File:HautFingerspitzeOCT.gif|thumb|300px|Optical coherence tomogram of a fingertip. It is possible to observe the sweat glands, having "corkscrew appearance"]]
Interferometric reflectometry of biological tissue, especially of the human eye using short-coherence-length light (also referred to as partially-coherent, low-coherence, or broadband, broad-spectrum, or white light) was investigated in parallel by multiple groups worldwide since 1980s. Lending ideas from [[ultrasound imaging]] and merging the time-of-flight detection with optical interferometry to detect optical delays in the pico- and femtosecond range as known from the [[autocorrelator]] in the 1960's, the technique's development was and is tightly associated with the availability of novel electronic, mechanical and photonic abilities. Stemming from single lateral point low-coherence interferometry the addition of a wide range of technologies enabled key milestones in this computational imaging technique. High-speed axial and lateral scanners, ultra-broad spectrum or ultra-fast spectrally tunable lasers or other high brightness radiation sources, increasingly sensitive detectors, like high resolution and high speed cameras or fast A/D-converters that picked up from and drove ideas in the rapidly developing photonics field, together with the increasing availability of computing power were essential for its birth and success. In 1991, [[David Huang (ophthalmologist)|David Huang]], then a student in [[James Fujimoto]] laboratory at [[Massachusetts Institute of Technology]], working with Eric Swanson at the MIT Lincoln Laboratory and colleagues at the Harvard Medical School, successfully demonstrated imaging and called the new imaging modality "optical coherence tomography".<ref>{{cite journal | vauthors = Huang D, Swanson EA, Lin CP, Schuman JS, Stinson WG, Chang W, Hee MR, Flotte T, Gregory K, Puliafito CA | display-authors = 6 | title = Optical coherence tomography | journal = Science | volume = 254 | issue = 5035 | pages = 1178–1181 | date = November 1991 | pmid = 1957169 | pmc = 4638169 | doi = 10.1126/science.1957169 | bibcode = 1991Sci...254.1178H }}</ref> Since then, OCT with micrometer axial resolution and below <ref>{{Cite journal |last1=Povazay |first1=B. |last2=Bizheva |first2=K. |last3=Unterhuber |first3=A. |last4=Hermann |first4=B. |last5=Sattmann |first5=H. |last6=Fercher |first6=A. F. |last7=Drexler |first7=W. |last8=Apolonski |first8=A. |last9=Wadsworth |first9=W. J. |last10=Knight |first10=J. C. |last11=Russell |first11=P. St J. |last12=Vetterlein |first12=M. |last13=Scherzer |first13=E. |date=2002-10-15 |title=Submicrometer axial resolution optical coherence tomography |url=https://opg.optica.org/ol/abstract.cfm?uri=ol-27-20-1800 |journal=Optics Letters |language=EN |volume=27 |issue=20 |pages=1800–1802 |doi=10.1364/OL.27.001800 |pmid=18033368 |bibcode=2002OptL...27.1800P |issn=1539-4794|url-access=subscription }}</ref> and cross-sectional imaging capabilities has become a prominent biomedical imaging technique that has continually improved in technical performance and range of applications. The improvement in image acquisition rate is particularly spectacular, starting with the original 0.8&nbsp;Hz axial scan repetition rate<ref name="Huang_1991" /> to the current commercial clinical OCT systems operating at several hundred kHz and laboratory prototypes at multiple MHz. The range of applications has expanded from ophthalmology to cardiology and other medical specialties. For their roles in the invention of OCT, Fujimoto, Huang, and Swanson received the 2023 Lasker-DeBakey Clinical Medical Research Award and the National Medal of Technology and Innovation.<ref>{{cite journal | vauthors = Davis TH | title = QnAs with James G. Fujimoto, David Huang, and Eric A. Swanson: Winners of the 2023 Lasker~DeBakey Clinical Medical Research Award | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 120 | issue = 39 | pages = e2313883120 | date = September 2023 | pmid = 37732757 | pmc = 10523481 | doi = 10.1073/pnas.2313883120 | doi-access = free | bibcode = 2023PNAS..12013883D }}</ref> These developments have been reviewed in articles written for the general<ref name="Fujimoto_2016" /> scientific<ref>{{cite journal | vauthors = Nathans J | title = Seeing is believing: The development of optical coherence tomography | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 120 | issue = 39 | pages = e2311129120 | date = September 2023 | pmid = 37732756 | pmc = 10523475 | doi = 10.1073/pnas.2311129120 | doi-access = free | bibcode = 2023PNAS..12011129N }}</ref> and medical<ref>{{cite journal | vauthors = Toth CA | title = Optical Coherence Tomography and Eye Care | journal = The New England Journal of Medicine | volume = 389 | issue = 16 | pages = 1526–1529 | date = October 2023 | pmid = 37732605 | doi = 10.1056/NEJMcibr2307733 | s2cid = 262085371 | veditors = Phimister EG }}</ref> readership.

It is particularly suited to ophthalmic applications and other tissue imaging requiring micrometer resolution and millimeter penetration depth.<ref>{{cite journal | vauthors = Zysk AM, Nguyen FT, Oldenburg AL, Marks DL, Boppart SA | title = Optical coherence tomography: a review of clinical development from bench to bedside | journal = Journal of Biomedical Optics | volume = 12 | issue = 5 | pages = 051403 | year = 2007 | pmid = 17994864 | doi = 10.1117/1.2793736 | s2cid = 20621284 | doi-access = free | bibcode = 2007JBO....12e1403Z }}</ref>  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).<ref>{{cite journal | vauthors = Drexler W, Morgner U, Ghanta RK, Kärtner FX, Schuman JS, Fujimoto JG | title = Ultrahigh-resolution ophthalmic optical coherence tomography | journal = Nature Medicine | volume = 7 | issue = 4 | pages = 502–507 | date = April 2001 | pmid = 11283681 | pmc = 1950821 | doi = 10.1038/86589 }}</ref><ref>{{cite journal | vauthors = Kaufman SC, Musch DC, Belin MW, Cohen EJ, Meisler DM, Reinhart WJ, Udell IJ, Van Meter WS | display-authors = 6 | title = Confocal microscopy: a report by the American Academy of Ophthalmology | journal = Ophthalmology | volume = 111 | issue = 2 | pages = 396–406 | date = February 2004 | pmid = 15019397 | doi = 10.1016/j.ophtha.2003.12.002 }}</ref>

OCT is based on low-coherence interferometry.<ref>{{cite journal | vauthors = Riederer SJ | title = Current technical development of magnetic resonance imaging | journal = IEEE Engineering in Medicine and Biology Magazine | volume = 19 | issue = 5 | pages = 34–41 | year = 2000 | pmid = 11016028 | doi = 10.1109/51.870229 }}</ref><ref>{{cite book|author1=M. Born |author2=E. Wolf |title=[[Principles of Optics|Principles of Optics: Electromagnetic Theory of Propagation, Interference, and Diffraction of Light]]|publisher=Cambridge University Press|year=2000|isbn=978-0-521-78449-8}}</ref><ref name="Fercher">{{cite journal | vauthors = Fercher AF, Mengedoht K, Werner W | title = Eye-length measurement by interferometry with partially coherent light | journal = Optics Letters | volume = 13 | issue = 3 | pages = 186–188 | date = March 1988 | pmid = 19742022 | doi = 10.1364/OL.13.000186 | bibcode = 1988OptL...13..186F }}</ref> 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 laser]]s). 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.<ref>{{cite journal | vauthors = Fujimoto JG, Pitris C, Boppart SA, Brezinski ME | title = Optical coherence tomography: an emerging technology for biomedical imaging and optical biopsy | journal = Neoplasia | volume = 2 | issue = 1–2 | pages = 9–25 | year = 2000 | pmid = 10933065 | pmc = 1531864 | doi = 10.1038/sj.neo.7900071 }}</ref> 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 tomogram ([[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.