View source for Optical coherence tomography

{{Short description|Imaging technique}}
{{Technical|date=April 2020}}
[[File:Optical coherence tomography of human retina.gif|thumb|328x328px|A high-resolution spectral-domain OCT scan (3×3 mm) of a dry age-related macular degeneration eye showing geographic atrophy and drusen in macula on both cross-sectional and en face fly-through.]]

'''Optical coherence tomography''' ('''OCT''') is a high-resolution imaging technique with most of its applications in medicine and biology.  OCT uses  coherent near-infrared light to obtain micrometer-level depth resolved images of biological tissue or other [[scattering]] media.  It uses [[interferometry]] techniques to detect the amplitude and time-of-flight of reflected light.

OCT uses transverse sample scanning of the light beam to obtain two- and three-dimensional images.  Short-coherence-length light can be obtained using a [[superluminescent diode]] (SLD) with a broad [[spectral bandwidth]] or a broadly tunable laser with narrow [[linewidth]].  The first demonstration of OCT imaging (in vitro) was published by a team from MIT and Harvard Medical School in a 1991 article in the journal ''[[Science (journal)|Science]]''.<ref name="Huang_1991">{{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 (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> The article introduced the term "OCT" to credit its derivation from optical coherence-domain [[reflectometry]], in which the axial resolution is based on [[temporal coherence]].<ref>{{cite journal | vauthors = Youngquist RC, Carr S, Davies DE | title = Optical coherence-domain reflectometry: a new optical evaluation technique | journal = Optics Letters | volume = 12 | issue = 3 | pages = 158–160 | date = March 1987 | pmid = 19738824 | doi = 10.1364/ol.12.000158 | bibcode = 1987OptL...12..158Y }}</ref> The first demonstrations of in vivo OCT imaging quickly followed.<ref>{{Cite journal | vauthors = Izatt JA, Hee MR, Huang D, Fujimoto JG, Swanson EA, Lin CP, Shuman JS, Puliafito CA | veditors = Parel JM, Ren Q |date=1993-06-24 |title=Ophthalmic diagnostics using optical coherence tomography |url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/1877/0000/Ophthalmic-diagnostics-using-optical-coherence-tomography/10.1117/12.147520.full |journal=Ophthalmic Technologies III |publisher=SPIE |volume=1877 |pages=136–144 |doi=10.1117/12.147520| bibcode = 1993SPIE.1877..136I |s2cid=121094027 |url-access=subscription }}</ref><ref>{{cite journal | vauthors = Swanson EA, Izatt JA, Hee MR, Huang D, Lin CP, Schuman JS, Puliafito CA, Fujimoto JG | display-authors = 6 | title = In vivo retinal imaging by optical coherence tomography | journal = Optics Letters | volume = 18 | issue = 21 | pages = 1864–1866 | date = November 1993 | pmid = 19829430 | doi = 10.1364/ol.18.001864 | bibcode = 1993OptL...18.1864S }}</ref><ref>{{cite journal | vauthors = Fercher AF, Hitzenberger CK, Drexler W, Kamp G, Sattmann H | title = In vivo optical coherence tomography | journal = American Journal of Ophthalmology | volume = 116 | issue = 1 | pages = 113–114 | date = July 1993 | pmid = 8328536 | doi = 10.1016/s0002-9394(14)71762-3 }}</ref>

The first US patents on OCT by the MIT/Harvard group described  a [[time-domain]] OCT (TD-OCT) system.<ref name="mw_1994">{{Cite patent | country = US | number = 5321501A|title=Method and apparatus for optical imaging with means for controlling the longitudinal range of the sample|gdate=1994-06-14| inventor = Swanson EA, Huang D, Fujimoto JG, Puliafito CA  |url=https://patents.google.com/patent/US5321501A/en}}</ref><ref>{{Cite patent | country = US | number = 5459570A|title=Method and apparatus for performing optical measurements|gdate=1995-10-17| inventor = Swanson EA, Huang D, Fujimoto JG, Puliafito CA |url=https://patents.google.com/patent/US5459570/en}}</ref> These patents were licensed by Zeiss and formed the basis of the first generations of OCT products until 2006.

In the decade preceding the invention of OCT, interferometry with short-coherence-length light had been investigated for a variety of applications.<ref name="Eickhoff_1981">{{Cite journal | vauthors = Eickhoff W, Ulrich R |date= November 1981 |title=Optical frequency domain reflectometry in single-mode fiber |url=https://ui.adsabs.harvard.edu/link_gateway/1981ApPhL..39..693E/doi:10.1063/1.92872 |journal=Applied Physics Letters |volume=39 |issue=9 |pages=693–695 |doi=10.1063/1.92872 |bibcode= 1981ApPhL..39..693E |issn=0003-6951}}</ref><ref>{{Cite journal | vauthors = Gillard CW, Buholz NE |title=Progress In Absolute Distance Interferometry |url=https://www.spiedigitallibrary.org/journals/optical-engineering/volume-22/issue-3/223348/Progress-In-Absolute-Distance-Interferometry/10.1117/12.7973117.full |journal=Optical Engineering |date=1983 |volume=22 |issue=3 |pages=348–353 |doi=10.1117/12.7973117 |bibcode=1983OptEn..22..348G |issn=0091-3286|url-access=subscription }}</ref><ref>{{Cite book | vauthors = Fercher AF, Roth E | chapter = Ophthalmic Laser Interferometry | veditors = Mueller GJ |date=1986-09-15 |title=Optical Instrumentation for Biomedical Laser Applications |chapter-url=https://spie.org/Publications/Proceedings/Paper/10.1117/12.938523 |journal=SPIE Proceedings |volume=0658 |page=48 |publisher=SPIE |doi=10.1117/12.938523| bibcode = 1986SPIE..658...48F |s2cid=122883903 }}</ref><ref>{{cite journal | vauthors = Youngquist RC, Carr S, Davies DE | title = Optical coherence-domain reflectometry: a new optical evaluation technique | language = EN | journal = Optics Letters | volume = 12 | issue = 3 | pages = 158–160 | date = March 1987 | pmid = 19738824 | doi = 10.1364/OL.12.000158 | bibcode = 1987OptL...12..158Y }}</ref><ref>{{cite journal | vauthors = Takada K, Yokohama I, Chida K, Noda J | title = New measurement system for fault location in optical waveguide devices based on an interferometric technique | journal = Applied Optics | volume = 26 | issue = 9 | pages = 1603–1606 | date = May 1987 | pmid = 20454375 | doi = 10.1364/AO.26.001603 | bibcode = 1987ApOpt..26.1603T }}</ref><ref name="Kachelmyer_1989">{{Cite journal | vauthors = Kachelmyer AL | veditors = Becherer RJ |date=1989-02-18 |title=Range-Doppler Imaging Waveforms And Receiver Design |url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/0999/0000/Range-Doppler-Imaging-Waveforms-And-Receiver-Design/10.1117/12.960231.full |journal=Laser Radar III |publisher=SPIE |volume=0999 |pages=138–161 |doi=10.1117/12.960231| bibcode = 1989SPIE..999..138K |s2cid=110631959 |url-access=subscription }}</ref><ref>{{cite journal | vauthors = Fercher AF, Mengedoht K, Werner W | title = Eye-length measurement by interferometry with partially coherent light | language = EN | 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><ref>{{Cite journal | vauthors = Gilgen HH, Novak RP, Salathe RP, Hodel W, Beaud P |title=Submillimeter optical reflectometry |url=https://ieeexplore.ieee.org/document/32387 |journal=Journal of Lightwave Technology |date=1989 |volume=7 |issue=8 |pages=1225–1233 |doi=10.1109/50.32387 |bibcode=1989JLwT....7.1225G |issn=1558-2213|url-access=subscription }}</ref><ref>{{cite journal | vauthors = Huang D, Wang J, Lin CP, Puliafito CA, Fujimoto JG | title = Micron-resolution ranging of cornea anterior chamber by optical reflectometry | journal = Lasers in Surgery and Medicine | volume = 11 | issue = 5 | pages = 419–425 | date = 1991 | pmid = 1816476 | doi = 10.1002/lsm.1900110506 | s2cid = 19888483 }}</ref><ref>{{cite journal | vauthors = Hitzenberger CK | title = Optical measurement of the axial eye length by laser Doppler interferometry | journal = Investigative Ophthalmology & Visual Science | volume = 32 | issue = 3 | pages = 616–624 | date = March 1991 | pmid = 2001935 | url = https://pubmed.ncbi.nlm.nih.gov/2001935/ }}</ref><ref name="Fercher_1990">{{cite conference |date=12–16 August 1990 |title=Ophthalmic interferometry |location=Garmisch-Partenkirchen, Germany |pages=221–228 |isbn=0-444-89860-3 |vauthors=Fercher AF |book-title=Proceedings of the International Conference on Optics in Life Sciences |veditors=von Bally G, Khanna S}}</ref><ref name="Naohiro Tanno_1991">{{cite conference |author1=Shinji Chiba |author2=Naohiro Tanno |year=1991 |title=Backscattering Optical Heterodyne Tomography |conference=14th Laser Sensing Symposium |language=ja}}</ref>{{Excessive citations inline|date=February 2025}} The potential to use interferometry for imaging was proposed,<ref name="Naohiro Tanno_1991" /> and measurement of [[retina]]l elevation profile and thickness had been demonstrated.<ref name="Fercher_1990" />

The initial commercial clinical OCT systems were based on point-scanning TD-OCT technology, which primarily produced [[Cross section (geometry)|cross-sectional]] images due to the speed limitation (tens to thousands of axial scans per second). Fourier-domain OCT became available clinically 2006, enabling much greater image acquisition rate (tens of thousands to hundreds of thousands axial scans per second) without sacrificing signal strength. The higher speed allowed for three-dimensional imaging, which can be visualized in both en face and cross-sectional views. Novel contrasts such as [[angiography]], [[elastography]], and optoretinography also became possible by detecting signal change over time. Over the past three decades, the speed of commercial clinical OCT systems has increased more than 1000-fold, doubling every three years and rivaling [[Moore's law]] of computer chip performance. Development of parallel image acquisition approaches such as line-field and full-field technology may allow the performance improvement trend to continue.

OCT is most widely used in [[ophthalmology]], in which it has transformed the diagnosis and monitoring of retinal diseases, [[optic nerve]] diseases, and [[cornea]]l diseases. It has greatly improved the management of the top three causes of blindness – [[macular degeneration]], [[diabetic retinopathy]], and [[glaucoma]] – thereby preventing vision loss in many patients. By 2016 OCT was estimated to be used in more than 30 million imaging procedures per year worldwide.<ref name="Fujimoto_2016">{{cite journal | vauthors = Fujimoto J, Swanson E | title = The Development, Commercialization, and Impact of Optical Coherence Tomography | journal = Investigative Ophthalmology & Visual Science | volume = 57 | issue = 9 | pages = OCT1–OCT13 | date = July 2016 | pmid = 27409459 | pmc = 4968928 | doi = 10.1167/iovs.16-19963 }}</ref>

Intravascular OCT imaging is used in the intravascular evaluation of [[Coronary arteries|coronary artery]] plaques and to guide [[stent]] placement.<ref>{{Cite journal |last1=Tearney |first1=Guillermo J. |last2=Regar |first2=Evelyn |last3=Akasaka |first3=Takashi |last4=Adriaenssens |first4=Tom |last5=Barlis |first5=Peter |last6=Bezerra |first6=Hiram G. |last7=Bouma |first7=Brett |last8=Bruining |first8=Nico |last9=Cho |first9=Jin-man |last10=Chowdhary |first10=Saqib |last11=Costa |first11=Marco A. |last12=de Silva |first12=Ranil |last13=Dijkstra |first13=Jouke |last14=Di Mario |first14=Carlo |last15=Dudeck |first15=Darius |date=March 2012 |title=Consensus Standards for Acquisition, Measurement, and Reporting of Intravascular Optical Coherence Tomography Studies |url=https://linkinghub.elsevier.com/retrieve/pii/S0735109712001027 |journal=Journal of the American College of Cardiology |language=en |volume=59 |issue=12 |pages=1058–1072 |doi=10.1016/j.jacc.2011.09.079|pmid=22421299 |hdl=1765/64850 |hdl-access=free }}</ref> Beyond ophthalmology and cardiology, applications are also developing in other medical specialties such as [[dermatology]], [[gastroenterology]],<ref>{{Cite journal |last1=Ughi |first1=Giovanni J. |last2=Gora |first2=Michalina J. |last3=Swager |first3=Anne-Fré |last4=Soomro |first4=Amna |last5=Grant |first5=Catriona |last6=Tiernan |first6=Aubrey |last7=Rosenberg |first7=Mireille |last8=Sauk |first8=Jenny S. |last9=Nishioka |first9=Norman S. |last10=Tearney |first10=Guillermo J. |date=2016-02-01 |title=Automated segmentation and characterization of esophageal wall in vivo by tethered capsule optical coherence tomography endomicroscopy |url=https://opg.optica.org/abstract.cfm?URI=boe-7-2-409 |journal=Biomedical Optics Express |language=en |volume=7 |issue=2 |pages=409–419 |doi=10.1364/BOE.7.000409 |issn=2156-7085 |pmc=4771459 |pmid=26977350}}</ref> [[neurology]] and neurovascular imaging,<ref>{{Cite journal |last1=Ughi |first1=Giovanni J. |last2=Marosfoi |first2=Miklos G. |last3=King |first3=Robert M. |last4=Caroff |first4=Jildaz |last5=Peterson |first5=Lindsy M. |last6=Duncan |first6=Benjamin H. |last7=Langan |first7=Erin T. |last8=Collins |first8=Amanda |last9=Leporati |first9=Anita |last10=Rousselle |first10=Serge |last11=Lopes |first11=Demetrius K. |last12=Gounis |first12=Matthew J. |last13=Puri |first13=Ajit S. |date=2020-07-31 |title=A neurovascular high-frequency optical coherence tomography system enables in situ cerebrovascular volumetric microscopy |journal=Nature Communications |language=en |volume=11 |issue=1 |page=3851 |doi=10.1038/s41467-020-17702-7 |issn=2041-1723 |pmc=7395105 |pmid=32737314|bibcode=2020NatCo..11.3851U }}</ref><ref>{{Cite journal |last1=Pereira |first1=Vitor M. |last2=Lylyk |first2=Pedro |last3=Cancelliere |first3=Nicole |last4=Lylyk |first4=Pedro N. |last5=Lylyk |first5=Ivan |last6=Anagnostakou |first6=Vania |last7=Bleise |first7=Carlos |last8=Nishi |first8=Hidehisa |last9=Epshtein |first9=Mark |last10=King |first10=Robert M. |last11=Shazeeb |first11=Mohammed Salman |last12=Puri |first12=Ajit S. |last13=Liang |first13=Conrad W. |last14=Hanel |first14=Ricardo A. |last15=Spears |first15=Julian |date=2024-05-15 |title=Volumetric microscopy of cerebral arteries with a miniaturized optical coherence tomography imaging probe |url=https://www.science.org/doi/10.1126/scitranslmed.adl4497 |journal=Science Translational Medicine |language=en |volume=16 |issue=747 |pages=eadl4497 |doi=10.1126/scitranslmed.adl4497 |issn=1946-6234 |pmid=38748771|url-access=subscription }}</ref> [[oncology]], and [[dentistry]].<ref>{{cite journal | vauthors = Wijns W, Shite J, Jones MR, Lee SW, Price MJ, Fabbiocchi F, Barbato E, Akasaka T, Bezerra H, Holmes D | display-authors = 6 | title = Optical coherence tomography imaging during percutaneous coronary intervention impacts physician decision-making: ILUMIEN I study | journal = European Heart Journal | volume = 36 | issue = 47 | pages = 3346–3355 | date = December 2015 | pmid = 26242713 | pmc = 4677272 | doi = 10.1093/eurheartj/ehv367 }}</ref><ref>{{cite journal | vauthors = Fujimoto J, Huang D | title = Foreword: 25 Years of Optical Coherence Tomography | journal = Investigative Ophthalmology & Visual Science | volume = 57 | issue = 9 | pages = OCTi-OCTii | date = July 2016 | pmid = 27419359 | doi = 10.1167/iovs.16-20269 | doi-access = free | hdl = 1721.1/105905 | hdl-access = free }}</ref>

==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.

==Layperson's explanation==
{{more citations needed section|date=March 2019}}
[[File:OCT OD Retinal Thickness Map.jpg|thumb|[[#Ophthalmology|Ocular OCT]] retinal thickness map, right eye]]
[[File:Retina-OCT800.png|thumb|Time-domain OCT of the macular area of a retina at 800 nm, axial resolution 3 μm]]
[[File:SD-OCT Macula Cross-Section.png|thumb|Spectral-domain OCT macula cross-section scan.]]
Optical coherence tomography (OCT) is a technique for obtaining sub-surface images of translucent or opaque materials at a resolution equivalent to a low-power microscope. It is effectively "optical ultrasound", imaging reflections from within tissue to provide cross-sectional images.<ref name="Michelessi">{{cite journal | vauthors = Michelessi M, Lucenteforte E, Oddone F, Brazzelli M, Parravano M, Franchi S, Ng SM, Virgili G | display-authors = 6 | title = Optic nerve head and fibre layer imaging for diagnosing glaucoma | journal = The Cochrane Database of Systematic Reviews | volume = 2015 | issue = 11 | pages = CD008803 | date = November 2015 | pmid = 26618332 | pmc = 4732281 | doi = 10.1002/14651858.CD008803.pub2 }}</ref>

OCT has attracted interest among the medical community because it provides tissue morphology imagery at much higher resolution (less than 10 μm axially and less than 20 μm laterally<ref>{{cite journal | vauthors = Drexler W, Morgner U, Kärtner FX, Pitris C, Boppart SA, Li XD, Ippen EP, Fujimoto JG | display-authors = 6 | title = In vivo ultrahigh-resolution optical coherence tomography | journal = Optics Letters | volume = 24 | issue = 17 | pages = 1221–1223 | date = September 1999 | pmid = 18073990 | doi = 10.1364/bio.1999.jwa2 | publisher = OSA }}</ref> ) than other imaging modalities such as MRI or ultrasound.

The key benefits of OCT are:
* Live sub-surface images at near-microscopic resolution
* Instant, direct imaging of tissue morphology
* No preparation of the sample or subject, no contact
* No ionizing radiation

OCT delivers high resolution because it is based on light, rather than sound or radio frequency. An optical beam is directed at the tissue, and the small portion of this light that reflects directly back from sub-surface features is collected. Note that most light scatters off at large angles. In conventional imaging, this diffusely scattered light contributes background that obscures an image. However, in OCT, a technique called interferometry is used to record the optical path length of received photons, allowing rejection of most photons that scatter multiple times before detection. Thus OCT can build up clear 3D images of thick samples by rejecting background signal while collecting light directly reflected from surfaces of interest.

Within the range of noninvasive three-dimensional imaging techniques that have been introduced to the medical research community, OCT as an echo technique is similar to [[ultrasound imaging]]. Other medical imaging techniques such as computerized axial tomography, magnetic resonance imaging, or positron emission tomography do not use the echo-location principle.<ref>{{cite web |url=https://www.mastereyeassociates.com/optical-coherence-tomography-scan |title=Optical Coherence Tomography provides better resolution than an MRI and Helps Diagnose Retina & Corneal Disease and Glaucoma, Part II |publisher=mastereyeassociates |date=June 13, 2017 |website=mastereyeassociates.com |access-date=June 13, 2017}}</ref>

The technique is limited to imaging 1 to 2&nbsp;mm below the surface in biological tissue, because at greater depths the proportion of light that escapes without scattering is too small to be detected. No special preparation of a biological specimen is required, and images can be obtained "non-contact" or through a transparent window or membrane.

The laser output from the instruments used is low{{snd}}eye-safe near-infrared or visible-light<ref>{{cite journal | vauthors = Shu X, Beckmann L, Zhang H | title = Visible-light optical coherence tomography: a review | journal = Journal of Biomedical Optics | volume = 22 | issue = 12 | pages = 1–14 | date = December 2017 | pmid = 29218923 | pmc = 5745673 | doi = 10.1117/1.JBO.22.12.121707 | publisher = spiedigitallibrary | bibcode = 2017JBO....22l1707S }}</ref>{{snd}}and no damage to the sample is therefore likely.

==Theory==
The principle of OCT is white light, or low coherence, interferometry. The optical setup typically consists of an interferometer (Fig. 1, typically [[Michelson interferometer|Michelson]] type) with a low coherence, broad bandwidth light source. Light is split into and recombined from reference and sample arms, respectively.

{|
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[[File:Full-field OCT setup.png|thumb|375px|Fig. 1 Full-field OCT optical setup. Components include: super-luminescent diode (SLD), convex lens (L1), 50/50 beamsplitter (BS), camera objective (CO), CMOS-DSP camera (CAM), reference (REF), and sample (SMP). The camera functions as a two-dimensional detector array, and with the OCT technique facilitating scanning in depth, a non-invasive three-dimensional imaging device is achieved.]]
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[[File:OCT B-Scan Setup-en.svg|thumb|375px|Fig. 2 Typical optical setup of single point OCT. Scanning the light beam on the sample enables non-invasive cross-sectional imaging up to 3 mm in depth with micrometer resolution.]]
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[[File:Ss-oct.PNG|thumb|375px|Fig. 3 Spectral discrimination by swept-source OCT. Components include: swept source or tunable laser (SS), beamsplitter (BS), reference mirror (REF), sample (SMP), photodetector (PD), and digital signal processing (DSP)]]
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[[File:Fd-oct.PNG|thumb|375px|Fig. 4 Spectral discrimination by Fourier-domain OCT. Components include: low coherence source (LCS), beamsplitter (BS), reference mirror (REF), sample (SMP), diffraction grating (DG) and full-field detector (CAM) acting as a spectrometer, and digital signal processing (DSP)]]
|}

===Time domain===

:<math> F _ODT \left ( \nu \right ) =  2S_0 \left ( \nu \right ) K_r \left ( \nu \right ) K_s \left ( \nu \right )  \qquad \quad (3) </math>
In time domain OCT the path length of the reference arm is varied in time (the reference mirror is translated longitudinally). A property of low coherence interferometry is that interference, i.e. the series of dark and bright fringes, is only achieved when the path difference lies within the coherence length of the light source. This interference is called autocorrelation in a symmetric interferometer (both arms have the same reflectivity), or cross-correlation in the common case. The envelope of this modulation changes as path length difference is varied, where the peak of the envelope corresponds to path length matching.

The interference of two partially coherent light beams can be expressed in terms of the source intensity, <math>I_S</math>, as

:<math> I = k_1 I_S + k_2 I_S + 2 \sqrt { \left ( k_1 I_S \right ) \cdot \left ( k_2 I_S \right )} \cdot Re \left [\gamma \left ( \tau \right ) \right] \qquad (1) </math>

where <math>k_1 + k_2 < 1</math> represents the interferometer beam splitting ratio, and <math> \gamma ( \tau ) </math> is called the complex degree of coherence, i.e. the interference envelope and carrier dependent on reference arm scan or time delay <math> \tau </math>, and whose recovery is of interest in OCT. Due to the coherence gating effect of OCT the complex degree of coherence is represented as a Gaussian function expressed as<ref name="Fercher"/>

:<math> \gamma \left ( \tau \right ) = \exp \left [- \left ( \frac{\pi\Delta\nu\tau}{2 \sqrt{\ln 2} } \right )^2 \right] \cdot \exp \left ( -j2\pi\nu_0\tau \right ) \qquad \quad (2) </math>

where <math> \Delta\nu </math> represents the spectral width of the source in the optical frequency domain, and <math> \nu_0 </math> is the centre optical frequency of the source. In equation (2), the Gaussian envelope is amplitude modulated by an optical carrier. The peak of this envelope represents the location of the microstructure of the sample under test, with an amplitude dependent on the reflectivity of the surface. The optical carrier is due to the [[Doppler effect]] resulting from scanning one arm of the interferometer, and the frequency of this modulation is controlled by the speed of scanning. Therefore, translating one arm of the interferometer has two functions; depth scanning and a Doppler-shifted optical carrier are accomplished by pathlength variation. In OCT, the Doppler-shifted optical carrier has a frequency expressed as

:<math> f_{Dopp} = \frac { 2 \cdot \nu_0 \cdot v_s } { c } \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad (3) </math>

where <math> \nu_0 </math> is the central optical frequency of the source, <math> v_s </math> is the scanning velocity of the pathlength variation, and <math> c </math> is the speed of light.

The axial and lateral resolutions of OCT are decoupled from one another; the former being an equivalent to the coherence length of the light source and the latter being a function of the optics. The axial resolution of OCT is defined as
:{|
|-
|<math> \, {l_c} </math>
|<math>=\frac {2 \ln 2} {\pi} \cdot \frac {\lambda_0^2} {\Delta\lambda}</math>
|-
|
|<math>\approx 0.44 \cdot \frac {\lambda_0^2} {\Delta\lambda} \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad (4) </math>
|}

where <math> \lambda_0 </math> and <math> \Delta\lambda</math> are respectively the central wavelength and the spectral width of the light source.<ref name="ReferenceA">{{cite book| title= Anterior & Posterior Segment OCT: Current Technology & Future Applications, 1st edition |year=2014| vauthors = Garg A }}</ref>

===Fourier domain===
[[File:Principle-TD-FD OCT.svg|thumb|450px|interference signals in TD vs. FD-OCT]]
Fourier-domain (or Frequency-domain) OCT (FD-OCT) has speed and signal-to-noise ratio (SNR) advantages over time-domain OCT (TD-OCT) and has become the standard in the industry since 2006. The idea of using frequency modulation and coherent detection to obtain ranging information was already demonstrated in optical frequency domain reflectometry<ref name="Eickhoff_1981" /> and laser radar<ref name="Kachelmyer_1989" /> in the 1980s, though the distance resolution and range were much longer than OCT. There are two types of FD-OCT – swept-source OCT (SS-OCT) and spectral-domain OCT (SD-OCT) – both of which acquire spectral interferograms which are then [[Fourier transform]]ed to obtain an axial scan of reflectance amplitude versus depth. In SS-OCT, the spectral interferogram is acquired sequentially by tuning the wavelength of a laser light source. SD-OCT acquires spectral interferogram simultaneously in a spectrometer. An implementation of SS-OCT was described by the MIT group as early as 1994.<ref name="mw_1994" /><ref>{{cite journal | vauthors = Chinn SR, Swanson EA, Fujimoto JG | title = Optical coherence tomography using a frequency-tunable optical source | language = EN | journal = Optics Letters | volume = 22 | issue = 5 | pages = 340–342 | date = March 1997 | pmid = 18183195 | doi = 10.1364/OL.22.000340 | bibcode = 1997OptL...22..340C }}</ref>   A group based in the University of Vienna described measurement of intraocular distance using both tunable laser and spectrometer-based interferometry as early as 1995.<ref>{{Cite journal | vauthors = Fercher AF, Hitzenberger CK, Kamp G, El-Zaiat SY |date=1995-05-15 |title=Measurement of intraocular distances by backscattering spectral interferometry |url=https://dx.doi.org/10.1016/0030-4018%2895%2900119-S |journal=Optics Communications |volume=117 |issue=1 |pages=43–48 |doi=10.1016/0030-4018(95)00119-S |bibcode=1995OptCo.117...43F |issn=0030-4018|url-access=subscription }}</ref><ref>{{cite journal | vauthors = Lexer F, Hitzenberger CK, Fercher AF, Kulhavy M | title = Wavelength-tuning interferometry of intraocular distances | language = EN | journal = Applied Optics | volume = 36 | issue = 25 | pages = 6548–6553 | date = September 1997 | pmid = 18259516 | doi = 10.1364/AO.36.006548 | bibcode = 1997ApOpt..36.6548L }}</ref> SD-OCT imaging was first demonstrated both in vitro and in vivo by a collaboration between the Vienna group and a group based in the Nicholas Copernicus University in a series of articles between 2000 and 2002.<ref>{{cite journal | vauthors = Leitgeb R, Wojtkowski M, Kowalczyk A, Hitzenberger CK, Sticker M, Fercher AF | title = Spectral measurement of absorption by spectroscopic frequency-domain optical coherence tomography | language = EN | journal = Optics Letters | volume = 25 | issue = 11 | pages = 820–822 | date = June 2000 | pmid = 18064195 | doi = 10.1364/OL.25.000820 | bibcode = 2000OptL...25..820L }}</ref><ref>{{cite journal | vauthors = Wojtkowski M, Leitgeb R, Kowalczyk A, Bajraszewski T, Fercher AF | title = In vivo human retinal imaging by Fourier domain optical coherence tomography | journal = Journal of Biomedical Optics | volume = 7 | issue = 3 | pages = 457–463 | date = July 2002 | pmid = 12175297 | doi = 10.1117/1.1482379 | bibcode = 2002JBO.....7..457W | s2cid = 40844236 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Wojtkowski M, Kowalczyk A, Leitgeb R, Fercher AF | title = Full range complex spectral optical coherence tomography technique in eye imaging | language = EN | journal = Optics Letters | volume = 27 | issue = 16 | pages = 1415–1417 | date = August 2002 | pmid = 18026464 | doi = 10.1364/OL.27.001415 | bibcode = 2002OptL...27.1415W }}</ref> The SNR advantage of FD-OCT over TD-OCT was first demonstrated in eye imaging <ref>{{cite journal | vauthors = Wojtkowski M, Bajraszewski T, Targowski P, Kowalczyk A | title = Real-time in vivo imaging by high-speed spectral optical coherence tomography | journal = Optics Letters | volume = 28 | issue = 19 | pages = 1745–1747 | date = April 2003 | pmid = 14514087 | doi = 10.1364/OL.28.001745| bibcode = 2003OptL...28.1745W| url =https://opg.optica.org/ol/abstract.cfm?URI=ol-28-19-1745| url-access = subscription }}</ref> and further analyzed by multiple groups of researchers in 2003.<ref>{{cite journal | vauthors = Leitgeb R, Hitzenberger C, Fercher A | title = Performance of fourier domain vs. time domain optical coherence tomography | journal = Optics Express | volume = 11 | issue = 8 | pages = 889–894 | date = April 2003 | pmid = 19461802 | doi = 10.1364/oe.11.000889 | bibcode = 2003OExpr..11..889L | doi-access = free }}</ref><ref>{{cite journal | vauthors = Choma M, Sarunic M, Yang C, Izatt J | title = Sensitivity advantage of swept source and Fourier domain optical coherence tomography | journal = Optics Express | volume = 11 | issue = 18 | pages = 2183–2189 | date = September 2003 | pmid = 19466106 | doi = 10.1364/oe.11.002183 | bibcode = 2003OExpr..11.2183C | doi-access = free }}</ref><ref>{{cite journal | vauthors = de Boer JF, Cense B, Park BH, Pierce MC, Tearney GJ, Bouma BE | title = Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography | journal = Optics Letters | volume = 28 | issue = 21 | pages = 2067–2069 | date = November 2003 | pmid = 14587817 | doi = 10.1364/ol.28.002067 | bibcode = 2003OptL...28.2067D | url = https://research.vu.nl/en/publications/8596084e-22c8-40a3-801f-9719955645b0 }}</ref>

====Spectral-domain OCT====
Spectral-domain OCT (spatially encoded frequency domain OCT) extracts spectral information by distributing different optical frequencies onto a detector stripe (line-array CCD or CMOS) via a dispersive element (see Fig. 4). Thereby the information of the full depth scan can be acquired within a single exposure. However, the large signal-to-noise advantage of FD-OCT is reduced due to the lower dynamic range of stripe detectors with respect to single photosensitive diodes, resulting in an SNR advantage of ~10 [[decibel|dB]] at much higher speeds. This is not much of a problem when working at 1300&nbsp;nm, however, since dynamic range is not a serious problem at this wavelength range.<ref name="ReferenceA"/>

The drawbacks of this technology are found in a strong fall-off of the SNR, which is proportional to the distance from the zero delay and a [[Sinc function|sinc]]-type reduction of the depth-dependent sensitivity because of limited detection linewidth. (One pixel detects a quasi-rectangular portion of an optical frequency range instead of a single frequency, the Fourier transform leads to the sinc(z) behavior). Additionally, the dispersive elements in the spectroscopic detector usually do not distribute the light equally spaced in frequency on the detector, but mostly have an inverse dependence. Therefore, the signal has to be resampled before processing, which cannot take care of the difference in local (pixelwise) bandwidth, which results in further reduction of the signal quality. However, the fall-off is not a serious problem with the development of new generation CCD or [[photodiode]] array with a larger number of pixels.

[[Optical heterodyne detection|Synthetic array heterodyne detection]] offers another approach to this problem without the need for high dispersion.

====Swept-source OCT====
Swept-source OCT (Time-encoded frequency domain OCT) tries to combine some of the advantages of standard TD and spectral domain OCT. Here the spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in single successive frequency steps and reconstructed before Fourier transformation. By accommodation of a frequency scanning light source (i.e. frequency scanning laser) the optical setup (see Fig. 3) becomes simpler than spectral domain OCT, but the problem of scanning is essentially translated from the TD-OCT reference arm into the swept source OCT light source. Here the advantage lies in the proven high SNR detection technology, while swept laser sources achieve very small instantaneous bandwidths (linewidths) at very high frequencies (20–200&nbsp;kHz). Drawbacks are the nonlinearities in the wavelength (especially at high scanning frequencies), the broadening of the linewidth at high frequencies and a high sensitivity to movements of the scanning geometry or the sample (below the range of nanometers within successive frequency steps).

==Scanning schemes==
Focusing the light beam to a point on the surface of the sample under test, and recombining the reflected light with the reference will yield an interferogram with sample information corresponding to a single A-scan (Z axis only). Scanning of the sample can be accomplished by either scanning the light on the sample, or by moving the sample under test. A linear scan will yield a two-dimensional data set corresponding to a cross-sectional image (X-Z axes scan), whereas an area scan achieves a three-dimensional data set corresponding to a volumetric image (X-Y-Z axes scan).

===Single point===
Systems based on single point, confocal, or flying-spot time domain OCT, must scan the sample in two lateral dimensions and reconstruct a three-dimensional image using depth information obtained by coherence-gating through an axially scanning reference arm (Fig. 2). Two-dimensional lateral scanning has been electromechanically implemented by moving the sample<ref name="Fercher2">{{cite journal |vauthors=Fercher AF, Hitzenberger CK, Kamp G, El-Zaiat SY |year=1995 |title=Measurement of intraocular distances by backscattering spectral interferometry |journal=Optics Communications |volume=117 |issue=1–2 |pages=43–48 |bibcode=1995OptCo.117...43F |doi=10.1016/0030-4018(95)00119-S}}</ref> using a translation stage, and using a novel micro-electro-mechanical system scanner.<ref>{{cite journal | vauthors = Yeow JT, Yang VX, Chahwan A, Gordon ML, Qi B, Vitkin IA, Wilson BC, Goldenberg AA |doi=10.1016/j.sna.2004.06.021|title=Micromachined 2-D scanner for 3-D optical coherence tomography|year=2005|journal=Sensors and Actuators A: Physical|volume=117|pages=331–340|issue=2 |bibcode=2005SeAcA.117..331Y }}</ref>

=== Line-field OCT ===
Line-field confocal optical coherence tomography (LC-OCT) is an imaging technique based on the principle of time-domain OCT with line illumination using a broadband laser and line detection using a line-scan camera.<ref>{{cite journal | vauthors = Dubois A, Levecq O, Azimani H, Davis A, Ogien J, Siret D, Barut A | title = Line-field confocal time-domain optical coherence tomography with dynamic focusing | journal = Optics Express | volume = 26 | issue = 26 | pages = 33534–33542 | date = December 2018 | pmid = 30650800 | doi = 10.1364/OE.26.033534 | bibcode = 2018OExpr..2633534D | doi-access = free }}</ref> LC-OCT produces B-scans in real-time from multiple A-scans acquired in parallel. En face as well as three-dimensional images can also be obtained by scanning the illumination line laterally.<ref>{{cite journal | vauthors = Ogien J, Levecq O, Azimani H, Dubois A | title = Dual-mode line-field confocal optical coherence tomography for ultrahigh-resolution vertical and horizontal section imaging of human skin ''in vivo'' | language = EN | journal = Biomedical Optics Express | volume = 11 | issue = 3 | pages = 1327–1335 | date = March 2020 | pmid = 32206413 | pmc = 7075601 | doi = 10.1364/BOE.385303 }}</ref><ref>{{cite journal | vauthors = Ogien J, Daures A, Cazalas M, Perrot JL, Dubois A | title = Line-field confocal optical coherence tomography for three-dimensional skin imaging | journal = Frontiers of Optoelectronics | volume = 13 | issue = 4 | pages = 381–392 | date = December 2020 | pmid = 36641566 | pmc = 9743950 | doi = 10.1007/s12200-020-1096-x | s2cid = 234456595 }}</ref> The focus is continuously adjusted during the scan of the sample depth, using a high numerical aperture (NA) microscope objective to image with high lateral resolution. By using a supercontinuum laser as a light source, a quasi-isotropic spatial resolution of ~ 1&nbsp;μm is achieved at a central wavelength of ~ 800&nbsp;nm. On the other hand, line illumination and detection, combined with the use of a high NA microscope objective, produce a confocal gate that prevents most scattered light that does not contribute to the signal from being detected by the camera. This confocal gate, which is absent in the full-field OCT technique, gives LC-OCT an advantage in terms of detection sensitivity and penetration in highly scattering media such as skin tissues.<ref>{{cite journal | vauthors = Chen Y, Huang SW, Aguirre AD, Fujimoto JG | title = High-resolution line-scanning optical coherence microscopy | journal = Optics Letters | volume = 32 | issue = 14 | pages = 1971–1973 | date = July 2007 | pmid = 17632613 | doi = 10.1364/OL.32.001971 | bibcode = 2007OptL...32.1971C }}</ref> So far this technique has been used mainly for skin imaging in the fields of dermatology<ref>{{cite journal | vauthors = Dubois A, Levecq O, Azimani H, Siret D, Barut A, Suppa M, Del Marmol V, Malvehy J, Cinotti E, Rubegni P, Perrot JL | display-authors = 6 | title = Line-field confocal optical coherence tomography for high-resolution noninvasive imaging of skin tumors | journal = Journal of Biomedical Optics | volume = 23 | issue = 10 | pages = 1–9 | date = October 2018 | pmid = 30353716 | doi = 10.1117/1.JBO.23.10.106007 | bibcode = 2018JBO....23j6007D | doi-access = free | hdl = 2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/283413 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = Ruini C, Schuh S, Gust C, Kendziora B, Frommherz L, French LE, Hartmann D, Welzel J, Sattler E | display-authors = 6 | title = Line-field optical coherence tomography: in vivo diagnosis of basal cell carcinoma subtypes compared with histopathology | journal = Clinical and Experimental Dermatology | volume = 46 | issue = 8 | pages = 1471–1481 | date = December 2021 | pmid = 34047380 | doi = 10.1111/ced.14762 | s2cid = 235232158 | doi-access = free | hdl = 11380/1259112 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = Suppa M, Fontaine M, Dejonckheere G, Cinotti E, Yélamos O, Diet G, Tognetti L, Miyamoto M, Orte Cano C, Perez-Anker J, Panagiotou V, Trepant AL, Monnier J, Berot V, Puig S, Rubegni P, Malvehy J, Perrot JL, Del Marmol V | display-authors = 6 | title = Line-field confocal optical coherence tomography of basal cell carcinoma: a descriptive study | journal = Journal of the European Academy of Dermatology and Venereology | volume = 35 | issue = 5 | pages = 1099–1110 | date = May 2021 | pmid = 33398911 | doi = 10.1111/jdv.17078 | s2cid = 230583854 }}</ref><ref>{{cite journal | vauthors = Cinotti E, Tognetti L, Cartocci A, Lamberti A, Gherbassi S, Orte Cano C, Lenoir C, Dejonckheere G, Diet G, Fontaine M, Miyamoto M, Perez-Anker J, Solmi V, Malvehy J, Del Marmol V, Perrot JL, Rubegni P, Suppa M | display-authors = 6 | title = Line-field confocal optical coherence tomography for actinic keratosis and squamous cell carcinoma: a descriptive study | journal = Clinical and Experimental Dermatology | volume = 46 | issue = 8 | pages = 1530–1541 | date = December 2021 | pmid = 34115900 | pmc = 9293459 | doi = 10.1111/ced.14801 | s2cid = 235411841 }}</ref><ref>{{cite journal | vauthors = Lenoir C, Cinotti E, Tognetti L, Orte Cano C, Diet G, Miyamoto M, Rocq L, Trépant AL, Perez-Anker J, Puig S, Malvehy J, Rubegni P, Perrot JL, Del Marmol V, Suppa M | display-authors = 6 | title = Line-field confocal optical coherence tomography of actinic keratosis: a case series | journal = Journal of the European Academy of Dermatology and Venereology | volume = 35 | issue = 12 | pages = e900–e902 | date = December 2021 | pmid = 34310768 | doi = 10.1111/jdv.17548 | s2cid = 236452537 | hdl = 11365/1278334 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = Ruini C, Schuh S, Gust C, Kendziora B, Frommherz L, French LE, Hartmann D, Welzel J, Sattler EC | display-authors = 6 | title = Line-field confocal optical coherence tomography for the in vivo real-time diagnosis of different stages of keratinocyte skin cancer: a preliminary study | journal = Journal of the European Academy of Dermatology and Venereology | volume = 35 | issue = 12 | pages = 2388–2397 | date = December 2021 | pmid = 34415646 | doi = 10.1111/jdv.17603 | s2cid = 237241412 | doi-access = free | hdl = 11380/1259110 | hdl-access = free }}</ref> and cosmetology.<ref>{{cite journal | vauthors = Pedrazzani M, Breugnot J, Rouaud-Tinguely P, Cazalas M, Davis A, Bordes S, Dubois A, Closs B | display-authors = 6 | title = Comparison of line-field confocal optical coherence tomography images with histological sections: Validation of a new method for in vivo and non-invasive quantification of superficial dermis thickness | journal = Skin Research and Technology | volume = 26 | issue = 3 | pages = 398–404 | date = May 2020 | pmid = 31799766 | doi = 10.1111/srt.12815 | s2cid = 208622348 }}</ref>

=== Full-field OCT ===
[[File:Full-field OCT.png|thumb|350px|Schematic view of a full-field OCT]]

An imaging approach to temporal OCT was developed by Claude Boccara's team in 1998,<ref>{{cite journal | vauthors = Beaurepaire E, Boccara AC, Lebec M, Blanchot L, Saint-Jalmes H | title = Full-field optical coherence microscopy | journal = Optics Letters | volume = 23 | issue = 4 | pages = 244–246 | date = February 1998 | pmid = 18084473 | doi = 10.1364/ol.23.000244 | bibcode = 1998OptL...23..244B }}</ref> with an acquisition of the images without beam scanning. In this technique called full-field OCT (FF-OCT), unlike other OCT techniques that acquire cross-sections of the sample, the images are here "en-face" i.e. like images of classical microscopy: orthogonal to the light beam of illumination.<ref>{{cite journal | vauthors = Dubois A, Boccara C | title = [Full-field OCT] | language = fr | journal = Médecine/Sciences | volume = 22 | issue = 10 | pages = 859–864 | date = October 2006 | pmid = 17026940 | doi = 10.1051/medsci/20062210859 | doi-access = free }}</ref>

More precisely, interferometric images are created by a Michelson interferometer where the path length difference is varied by a fast electric component (usually a piezo mirror in the reference arm). These images acquired by a CCD camera are combined in post-treatment (or online) by the phase shift interferometry method, where usually 2 or 4 images per modulation period are acquired, depending on the algorithm used.<ref>{{cite journal |vauthors=Dubois A, Moneron G, Boccara C |year=2006 |title=Thermal-light full-field optical coherence tomography in the 1.2 micron wavelength region |url=https://hal.archives-ouvertes.fr/hal-00520541/file/Dubois-text-original.pdf |journal=Optics Communications |language=en |volume=266 |issue=2 |pages=738–743 |bibcode=2006OptCo.266..738D |doi=10.1016/j.optcom.2006.05.016|s2cid=120323507 }}</ref><ref>{{cite journal |vauthors=Boccara AC, Harms F, Latrive A |year=2013 |title=Full-field OCT: a non-invasive tool for diagnosis and tissue selection |journal=SPIE Newsroom |language=en |doi=10.1117/2.1201306.004933 |s2cid=123478275}}</ref> More recently, approaches that allow rapid single-shot imaging were developed to simultaneously capture multiple phase-shifted images required for reconstruction, using single camera.<ref name="Zurauskas Iyer Boppart p.">{{cite journal | vauthors = Žurauskas M, Iyer RR, Boppart SA | title = Simultaneous 4-phase-shifted full-field optical coherence microscopy | journal = Biomedical Optics Express | volume = 12 | issue = 2 | pages = 981–992 | date = February 2021 | pmid = 33680554 | pmc = 7901320 | doi = 10.1364/boe.417183 | publisher = The Optical Society | doi-access = free }}</ref> Single-shot time-domain OCM is limited only by the camera frame rate and available illumination.

The "en-face" tomographic images are thus produced by a wide-field illumination, ensured by the Linnik configuration of the Michelson interferometer where a microscope objective is used in both arms. Furthermore, while the temporal coherence of the source must remain low as in classical OCT (i.e. a broad spectrum), the spatial coherence must also be low to avoid parasitical interferences (i.e. a source with a large size).<ref>{{cite book |title=Optics in Instruments |vauthors=Boccara AC, Dubois A |year=2013 |isbn=9781118574386 |pages=101–123 |language=en |chapter=Optical Coherence Tomography |doi=10.1002/9781118574386.ch3}}</ref>

==Selected applications==
Optical coherence tomography is an established medical imaging technique and is used across several medical specialties including ophthalmology and cardiology and is widely used in basic science research applications.

=== Ophthalmology ===
Ocular (or ophthalmic) OCT is used heavily by [[Ophthalmology|ophthalmologists]] and [[optometrists]] to obtain high-resolution images of the [[retina]] and [[Anterior segment of eyeball|anterior segment]]. Owing to OCT's capability to show cross-sections of tissue layers with micrometer resolution, OCT provides a straightforward method of assessing [[Retina#Retinal Optical Coherence Tomography (OCT)|cellular organization]], [[photoreceptor integrity line|photoreceptor integrity]],<ref>{{cite web|title=The ABCs of OCT|url=https://www.reviewofoptometry.com/article/the-abcs-of-oct|website=Review of Optometry}}</ref><ref>{{cite journal | vauthors = Sherman J | title = Photoreceptor integrity line joins the nerve fiber layer as key to clinical diagnosis | journal = Optometry | volume = 80 | issue = 6 | pages = 277–278 | date = June 2009 | pmid = 19465337 | doi = 10.1016/j.optm.2008.12.006 }}</ref><ref>{{cite web|title=Outer Retinal Layers as Predictors of Vision Loss|url=https://www.reviewofophthalmology.com/article/outer-retinal-layers-as-predictors-of-vision-loss|website=Review of Ophthalmology}}</ref><ref>{{cite journal | vauthors = Cuenca N, Ortuño-Lizarán I, Pinilla I | title = Cellular Characterization of OCT and Outer Retinal Bands Using Specific Immunohistochemistry Markers and Clinical Implications | journal = Ophthalmology | volume = 125 | issue = 3 | pages = 407–422 | date = March 2018 | pmid = 29037595 | doi = 10.1016/j.ophtha.2017.09.016 | hdl-access = free | hdl = 10045/74474 }}</ref> and [[axon]]al thickness in [[glaucoma]],<ref>{{cite journal | vauthors = Grewal DS, Tanna AP | title = Diagnosis of glaucoma and detection of glaucoma progression using spectral domain optical coherence tomography | journal = Current Opinion in Ophthalmology | volume = 24 | issue = 2 | pages = 150–161 | date = March 2013 | pmid = 23328662 | doi = 10.1097/ICU.0b013e32835d9e27 | s2cid = 39039199 }}</ref> [[macular degeneration]],<ref>{{cite journal | vauthors = Keane PA, Patel PJ, Liakopoulos S, Heussen FM, Sadda SR, Tufail A | title = Evaluation of age-related macular degeneration with optical coherence tomography | journal = Survey of Ophthalmology | volume = 57 | issue = 5 | pages = 389–414 | date = September 2012 | pmid = 22898648 | doi = 10.1016/j.survophthal.2012.01.006 }}</ref> [[macular edema|diabetic macular edema]],<ref name="Virgili">{{cite journal | vauthors = Virgili G, Menchini F, Casazza G, Hogg R, Das RR, Wang X, Michelessi M | title = Optical coherence tomography (OCT) for detection of macular oedema in patients with diabetic retinopathy | journal = The Cochrane Database of Systematic Reviews | volume = 1 | pages = CD008081 | date = January 2015 | issue = 4 | pmid = 25564068 | pmc = 4438571 | doi = 10.1002/14651858.CD008081.pub3 }}</ref> [[multiple sclerosis]],<ref>{{cite journal | vauthors = Dörr J, Wernecke KD, Bock M, Gaede G, Wuerfel JT, Pfueller CF, Bellmann-Strobl J, Freing A, Brandt AU, Friedemann P | display-authors = 6 | title = Association of retinal and macular damage with brain atrophy in multiple sclerosis | journal = PLOS ONE | volume = 6 | issue = 4 | pages = e18132 | date = April 2011 | pmid = 21494659 | pmc = 3072966 | doi = 10.1371/journal.pone.0018132 | doi-access = free | bibcode = 2011PLoSO...618132D }} {{Open access}}</ref> optic neuritis,<ref>{{cite journal | vauthors = Petzold A, Fraser CL, Abegg M, Alroughani R, Alshowaeir D, Alvarenga R, Andris C, Asgari N, Barnett Y, Battistella R, Behbehani R, Berger T, Bikbov MM, Biotti D, Biousse V, Boschi A, Brazdil M, Brezhnev A, Calabresi PA, Cordonnier M, Costello F, Cruz FM, Cunha LP, Daoudi S, Deschamps R, de Seze J, Diem R, Etemadifar M, Flores-Rivera J, Fonseca P, Frederiksen J, Frohman E, Frohman T, Tilikete CF, Fujihara K, Gálvez A, Gouider R, Gracia F, Grigoriadis N, Guajardo JM, Habek M, Hawlina M, Martínez-Lapiscina EH, Hooker J, Hor JY, Howlett W, Huang-Link Y, Idrissova Z, Illes Z, Jancic J, Jindahra P, Karussis D, Kerty E, Kim HJ, Lagrèze W, Leocani L, Levin N, Liskova P, Liu Y, Maiga Y, Marignier R, McGuigan C, Meira D, Merle H, Monteiro ML, Moodley A, Moura F, Muñoz S, Mustafa S, Nakashima I, Noval S, Oehninger C, Ogun O, Omoti A, Pandit L, Paul F, Rebolleda G, Reddel S, Rejdak K, Rejdak R, Rodriguez-Morales AJ, Rougier MB, Sa MJ, Sanchez-Dalmau B, Saylor D, Shatriah I, Siva A, Stiebel-Kalish H, Szatmary G, Ta L, Tenembaum S, Tran H, Trufanov Y, van Pesch V, Wang AG, Wattjes MP, Willoughby E, Zakaria M, Zvornicanin J, Balcer L, Plant GT | display-authors = 6 | title = Diagnosis and classification of optic neuritis | journal = The Lancet. Neurology | volume = 21 | issue = 12 | pages = 1120–1134 | date = December 2022 | pmid = 36179757 | doi = 10.1016/s1474-4422(22)00200-9 | s2cid = 252564095 | url = https://discovery.ucl.ac.uk/id/eprint/10156457/ | hdl = 2078.1/276419 | hdl-access = free }}</ref> and other [[eye disease]]s or systemic pathologies which have ocular signs.<ref>{{Cite journal| vauthors = Aik Kah T |date=2018|title=CuRRL Syndrome: A Case Series|url=https://actascientific.com/ASOP/pdf/ASOP-01-0016.pdf|journal=Acta Scientific Ophthalmology|volume=1|pages=9–13}}</ref> Additionally, ophthalmologists leverage OCT to assess the vascular health of the retina via a technique called OCT angiography (OCTA).<ref>{{cite journal | vauthors = Kashani AH, Chen CL, Gahm JK, Zheng F, Richter GM, Rosenfeld PJ, Shi Y, Wang RK | display-authors = 6 | title = Optical coherence tomography angiography: A comprehensive review of current methods and clinical applications | journal = Progress in Retinal and Eye Research | volume = 60 | pages = 66–100 | date = September 2017 | pmid = 28760677 | pmc = 5600872 | doi = 10.1016/j.preteyeres.2017.07.002 }}</ref> In [[Eye surgery|ophthalmological surgery]], especially retinal surgery, an OCT can be mounted on the microscope. Such a system is called an ''intraoperative OCT'' (iOCT) and provides support during the surgery with clinical benefits.<ref>{{cite journal | vauthors = Ehlers JP, Tao YK, Srivastava SK | title = The value of intraoperative optical coherence tomography imaging in vitreoretinal surgery | journal = Current Opinion in Ophthalmology | volume = 25 | issue = 3 | pages = 221–227 | date = May 2014 | pmid = 24614147 | pmc = 4119822 | doi = 10.1097/ICU.0000000000000044 }}</ref><ref>{{cite journal | vauthors = Pfau M, Michels S, Binder S, Becker MD | title = Clinical Experience With the First Commercially Available Intraoperative Optical Coherence Tomography System | journal = Ophthalmic Surgery, Lasers & Imaging Retina | volume = 46 | issue = 10 | pages = 1001–1008 | date = 2015 | pmid = 26599241 | doi = 10.3928/23258160-20151027-03 }}</ref><ref>{{cite journal | vauthors = Neuhann R, Neuhann T, Hörster R, Cursiefen C, Guell J, Siebelmann S | title = Laser-integrated real-time OCT in anterior segment procedures | journal = Journal of Cataract and Refractive Surgery | volume = 47 | issue = 12 | pages = e88–e92 | date = December 2021 | pmid = 34393183 | doi = 10.1097/j.jcrs.0000000000000773 | doi-access = free }}</ref> Polarization-sensitive OCT was recently applied in the human retina to determine optical polarization properties of vessel walls near the optic nerve.<ref>{{cite journal | vauthors = Neuhann R, Neuhann T, Hörster R, Cursiefen C, Guell J, Siebelmann S | title = Laser-integrated real-time OCT in anterior segment procedures | journal = Journal of Cataract and Refractive Surgery | volume = 47 | issue = 12 | pages = e88–e92 | date = December 2021 | pmid = 34393183 | pmc = 8367251 | doi = 10.1364/BOE.426079 | doi-access = free }}</ref>

Retinal imaging with PS-OCT demonstrated how the thickness and birefringence of blood vessel wall tissue of healthy subjects could be quantified, in vivo.<ref>{{Cite journal |last1=Afsharan |first1=Hadi |last2=Hackmann |first2=Michael J. |last3=Wang |first3=Qiang |last4=Navaeipour |first4=Farzaneh |last5=Jayasree |first5=Stephy Vijaya Kumar |last6=Zawadzki |first6=Robert J. |last7=Silva |first7=Dilusha |last8=Joo |first8=Chulmin |last9=Cense |first9=Barry |date=2021-07-01 |title=Polarization properties of retinal blood vessel walls measured with polarization sensitive optical coherence tomography |url=https://opg.optica.org/abstract.cfm?URI=boe-12-7-4340 |journal=Biomedical Optics Express |language=en |volume=12 |issue=7 |pages=4340–4362 |doi=10.1364/BOE.426079 |issn=2156-7085 |pmc=8367251 |pmid=34457418}}</ref> PS-OCT was subsequently applied to patients with diabetes and age-matched healthy subjects, and showed an almost 100% increase in vessel wall birefringence due to diabetes, without a significant change in vessel wall thickness.<ref name=":0">{{Cite journal |last1=Afsharan |first1=Hadi |last2=Silva |first2=Dilusha |last3=Joo |first3=Chulmin |last4=Cense |first4=Barry |date=August 2023 |title=Non-Invasive Retinal Blood Vessel Wall Measurements with Polarization-Sensitive Optical Coherence Tomography for Diabetes Assessment: A Quantitative Study |journal=Biomolecules |language=en |volume=13 |issue=8 |pages=1230 |doi=10.3390/biom13081230 |issn=2218-273X |pmc=10452597 |pmid=37627295 |doi-access=free}}</ref> In patients with hypertension however, the retinal vessel wall thickness increased by 60% while the vessel wall birefringence dropped by 20%, on average.<ref name=":1">{{Cite journal |last1=Afsharan |first1=Hadi |last2=Anilkumar |first2=Vidyalakshmi |last3=Silva |first3=Dilusha |last4=Dwivedi |first4=Girish |last5=Joo |first5=Chulmin |last6=Cense |first6=Barry |date=2024-01-01 |title=Hypertension-associated changes in retinal blood vessel walls measured in vivo with polarization-sensitive optical coherence tomography |journal=Optics and Lasers in Engineering |volume=172 |pages=107838 |bibcode=2024OptLE.17207838A |doi=10.1016/j.optlaseng.2023.107838 |issn=0143-8166 |doi-access=free}}</ref> The large differences measured in healthy subjects and patients suggest that retinal measurements with PS-OCT could be used as a screening tool for hypertension and diabetes.<ref name=":0" /><ref name=":1" /><ref>{{Cite web |date=2023-09-20 |title=Basic eye test could diagnose diabetes and hypertension |url=https://www.perthnow.com.au/news/health/basic-eye-test-could-diagnose-diabetes-and-hypertension-c-11962397 |access-date=2024-01-24 |website=PerthNow |language=en}}</ref>

OCT can used to measure the thickness of the [[Retinal nerve fiber layer|Retinal nerve fiber layer (RNFL)]].<ref>{{cite web | url=https://eyewiki.org/Optic_Nerve_and_Retinal_Nerve_Fiber_Imaging | title=Optic Nerve and Retinal Nerve Fiber Imaging - EyeWiki }}</ref>

=== Cardiology ===
{{main|Intravascular optical coherence tomography}}
In the settings of cardiology, OCT is used to image [[coronary arteries]] to visualize vessel wall lumen morphology and microstructure at a resolution ~10 times higher than other existing modalities such as [[intravascular ultrasound]]s, and x-ray angiography ([[intracoronary optical coherence tomography]]). For this type of application, 1&nbsp;mm in diameter or smaller fiber-optics catheters are used to access artery lumen through semi-invasive interventions such as [[percutaneous coronary intervention]]s.

The first demonstration of endoscopic OCT was reported in 1997, by researchers in Fujimoto's laboratory at Massachusetts Institute of Technology.<ref>{{cite journal | vauthors = Tearney GJ, Brezinski ME, Bouma BE, Boppart SA, Pitris C, Southern JF, Fujimoto JG | title = In vivo endoscopic optical biopsy with optical coherence tomography | journal = Science | volume = 276 | issue = 5321 | pages = 2037–2039 | date = June 1997 | pmid = 9197265 | doi = 10.1126/science.276.5321.2037 | s2cid = 43035300 }}</ref> The first TD-OCT imaging catheter and system was commercialized by [[LightLab Imaging, Inc.]], a company based in Massachusetts in 2006. The first FD-OCT imaging study was reported by [[Massachusetts General Hospital]] in 2008.<ref>{{cite journal | vauthors = Tearney GJ, Waxman S, Shishkov M, Vakoc BJ, Suter MJ, Freilich MI, Desjardins AE, Oh WY, Bartlett LA, Rosenberg M, Bouma BE | display-authors = 6 | title = Three-dimensional coronary artery microscopy by intracoronary optical frequency domain imaging | journal = JACC. Cardiovascular Imaging | volume = 1 | issue = 6 | pages = 752–761 | date = November 2008 | pmid = 19356512 | pmc = 2852244 | doi = 10.1016/j.jcmg.2008.06.007 }}</ref>  Intracoronary FD-OCT was first introduced in the market in 2009 by LightLab Imaging, Inc.<ref>{{cite press release |url=http://www.prnewswire.com/news-releases/lightlab-imaging-returns-to-europcr-2010-with-strong-and-growing-worldwide-acceptance-of-c7-xr-oct-imaging-system-94607959.html |title=LightLab launches FD-OCT in Europe |access-date=9 September 2016}}</ref> followed by [[Terumo]] Corporation in 2012 and by Gentuity LLC in 2020.<ref>{{cite journal | vauthors = Bezerra HG, Quimby DL, Matar F, Mohanty BD, Bassily E, Ughi GJ | title = High-Frequency Optical Coherence Tomography (HF-OCT) for Preintervention Coronary Imaging: A First-in-Human Study | journal = JACC. Cardiovascular Imaging | volume = 16 | issue = 7 | pages = 982–984 | date = July 2023 | pmid = 37407126 | doi = 10.1016/j.jcmg.2023.01.013 | s2cid = 258115402 }}</ref> The higher acquisition speed of FD-OCT enabled the widespread adoption of this imaging technology for coronary artery imaging. It is estimated that over 100,000 FD-OCT coronary imaging cases are performed yearly, and that the market is increasing by approximately 20% every year.<ref>{{cite web |url=http://www.bioopticsworld.com/articles/print/volume-9/issue-6/optical-coherence-tomography-beyond-better-clinical-care-oct-s-economic-impact.html |title=Optical Coherence Tomography: Beyond better clinical care: OCT's economic impact | vauthors = Swanson E |date=13 June 2016 |website=BioOptics World |access-date=9 September 2016}}</ref>

Other developments of intracoronary OCT included the combination with other optical imaging modalities for multi-modality imaging. Intravascular OCT has been combined with near-infrared [[intravascular fluorescence|fluorescence molecular imaging]] (NIRF) to enhance its capability to detect molecular/functional and tissue morphological information simultaneously.<ref>{{cite journal |display-authors=6 |vauthors=Ughi GJ, Wang H, Gerbaud E, Gardecki JA, Fard AM, Hamidi E, Vacas-Jacques P, Rosenberg M, Jaffer FA, Tearney GJ |date=November 2016 |title=Clinical Characterization of Coronary Atherosclerosis With Dual-Modality OCT and Near-Infrared Autofluorescence Imaging |journal=JACC. Cardiovascular Imaging |volume=9 |issue=11 |pages=1304–1314 |doi=10.1016/j.jcmg.2015.11.020 |pmc=5010789 |pmid=26971006}}</ref><ref>{{cite journal |display-authors=6 |vauthors=Hara T, Ughi GJ, McCarthy JR, Erdem SS, Mauskapf A, Lyon SC, Fard AM, Edelman ER, Tearney GJ, Jaffer FA |date=February 2017 |title=Intravascular fibrin molecular imaging improves the detection of unhealed stents assessed by optical coherence tomography in vivo |journal=European Heart Journal |volume=38 |issue=6 |pages=447–455 |doi=10.1093/eurheartj/ehv677 |pmc=5837565 |pmid=26685129}}</ref> In a similar way, combination with near-infrared spectroscopy (NIRS) has been implemented.<ref>{{cite journal |vauthors=Fard AM, Vacas-Jacques P, Hamidi E, Wang H, Carruth RW, Gardecki JA, Tearney GJ |date=December 2013 |title=Optical coherence tomography--near infrared spectroscopy system and catheter for intravascular imaging |journal=Optics Express |volume=21 |issue=25 |pages=30849–30858 |bibcode=2013OExpr..2130849F |doi=10.1364/OE.21.030849 |pmc=3926541 |pmid=24514658}}</ref>

=== Neurovascular ===
{{main|Intravascular optical coherence tomography}}
Endoscopic/intravascular OCT has been further developed for use in neurovascular applications including imaging for guiding endovascular treatment of ischemic stroke and brain aneurysms.<ref>{{Cite journal |last1=Gounis |first1=Matthew J. |last2=Ughi |first2=Giovanni J. |last3=Marosfoi |first3=Miklos |last4=Lopes |first4=Demetrius K. |last5=Fiorella |first5=David |last6=Bezerra |first6=Hiram G. |last7=Liang |first7=Conrad W. |last8=Puri |first8=Ajit S. |date=January 2019 |title=Intravascular Optical Coherence Tomography for Neurointerventional Surgery |journal=Stroke |volume=50 |issue=1 |pages=218–223 |doi=10.1161/STROKEAHA.118.022315|pmid=30580737 |pmc=6541539 }}</ref><ref>{{cite journal | vauthors = Chen CJ, Kumar JS, Chen SH, Ding D, Buell TJ, Sur S, Ironside N, Luther E, Ragosta M, Park MS, Kalani MY, Liu KC, Starke RM | display-authors = 6 | title = Optical Coherence Tomography: Future Applications in Cerebrovascular Imaging | journal = Stroke | volume = 49 | issue = 4 | pages = 1044–1050 | date = April 2018 | pmid = 29491139 | doi = 10.1161/STROKEAHA.117.019818 | doi-access = free }}</ref>

Initial clinical investigations with existing coronary OCT catheters have been limited to proximal intracranial anatomy of patient with limited tortuosity, as coronary OCT technology was not designed for the tortuous cerebrovasculature encountered in the brain.  However, despite these limitations, it showed the potential of OCT for the imaging of neurovascular disease.<ref>{{cite journal | vauthors = Xu X, Li M, Liu R, Yin Q, Shi X, Wang F, Gao J, Xu G, Ye R, Liu X | display-authors = 6 | title = Optical coherence tomography evaluation of vertebrobasilar artery stenosis: case series and literature review | journal = Journal of NeuroInterventional Surgery | volume = 12 | issue = 8 | pages = 809–813 | date = August 2020 | pmid = 32066569 | doi = 10.1136/neurintsurg-2019-015660 | s2cid = 211159079 }}</ref> An intravascular OCT imaging catheter design tailored for use in tortuous neurovascular anatomy has been proposed in 2020.<ref>{{cite journal | vauthors = Ughi GJ, Marosfoi MG, King RM, Caroff J, Peterson LM, Duncan BH, Langan ET, Collins A, Leporati A, Rousselle S, Lopes DK, Gounis MJ, Puri AS | display-authors = 6 | title = A neurovascular high-frequency optical coherence tomography system enables in situ cerebrovascular volumetric microscopy | journal = Nature Communications | volume = 11 | issue = 1 | pages = 3851 | date = July 2020 | pmid = 32737314 | pmc = 7395105 | doi = 10.1038/s41467-020-17702-7 | bibcode = 2020NatCo..11.3851U }}</ref>  A first-in-human study using endovascular neuro OCT (''n''OCT) has been reported in 2024.<ref>{{Cite journal |last1=Pereira |first1=Vitor M. |last2=Lylyk |first2=Pedro |last3=Cancelliere |first3=Nicole |last4=Lylyk |first4=Pedro N. |last5=Lylyk |first5=Ivan |last6=Anagnostakou |first6=Vania |last7=Bleise |first7=Carlos |last8=Nishi |first8=Hidehisa |last9=Epshtein |first9=Mark |last10=King |first10=Robert M. |last11=Shazeeb |first11=Mohammed Salman |last12=Puri |first12=Ajit S. |last13=Liang |first13=Conrad W. |last14=Hanel |first14=Ricardo A. |last15=Spears |first15=Julian |date=2024-05-15 |title=Volumetric microscopy of cerebral arteries with a miniaturized optical coherence tomography imaging probe |url=https://www.science.org/doi/10.1126/scitranslmed.adl4497 |journal=Science Translational Medicine |language=en |volume=16 |issue=747 |pages=eadl4497 |doi=10.1126/scitranslmed.adl4497 |pmid=38748771 |issn=1946-6234|url-access=subscription }}</ref><ref>{{Cite journal |last1=Siddiqui |first1=Adnan H |last2=Andersson |first2=Tommy |date=2024-09-26 |title=Shining light on neurovascular disease |journal=Interventional Neuroradiology |language=en |pages=15910199241285962 |doi=10.1177/15910199241285962 |issn=1591-0199 |pmc=11559757 |pmid=39324217}}</ref><ref>{{Cite web |title="Snake-like" Probe Images Arteries from Within - IEEE Spectrum |url=https://spectrum.ieee.org/fiber-optic-probe |access-date=2024-05-17 |website=[[IEEE]] |language=en}}</ref>

=== Oncology ===
Endoscopic OCT has been applied to the detection and diagnosis of [[cancer]] and [[precancerous lesions]], such as [[Barrett's esophagus]] and esophageal [[dysplasia]].<ref>{{cite web |url=http://www.bioopticsworld.com/articles/print/volume-6/issue-3/features/optical-coherence-tomography-gastroenterology--advanced-oct--nex.html |title=Next-gen OCT for the esophagus |date=1 May 2013 |website=BioOptics World |access-date=9 September 2016}}</ref><ref>{{cite journal | vauthors = Gora MJ, Sauk JS, Carruth RW, Gallagher KA, Suter MJ, Nishioka NS, Kava LE, Rosenberg M, Bouma BE, Tearney GJ | display-authors = 6 | title = Tethered capsule endomicroscopy enables less invasive imaging of gastrointestinal tract microstructure | journal = Nature Medicine | volume = 19 | issue = 2 | pages = 238–240 | date = February 2013 | pmid = 23314056 | pmc = 3567218 | doi = 10.1038/nm.3052 }}</ref><ref>{{cite journal | vauthors = Ughi GJ, Gora MJ, Swager AF, Soomro A, Grant C, Tiernan A, Rosenberg M, Sauk JS, Nishioka NS, Tearney GJ | display-authors = 6 | title = Automated segmentation and characterization of esophageal wall in vivo by tethered capsule optical coherence tomography endomicroscopy | journal = Biomedical Optics Express | volume = 7 | issue = 2 | pages = 409–419 | date = February 2016 | pmid = 26977350 | pmc = 4771459 | doi = 10.1364/BOE.7.000409 }}</ref>

=== Dermatology ===
The first use of OCT in dermatology dates back to 1997.<ref>{{cite journal | vauthors = Welzel J, Lankenau E, Birngruber R, Engelhardt R | title = Optical coherence tomography of the human skin | journal = Journal of the American Academy of Dermatology | volume = 37 | issue = 6 | pages = 958–963 | date = December 1997 | pmid = 9418764 | doi = 10.1016/S0190-9622(97)70072-0 | s2cid = 20078741 | url = https://nbn-resolving.org/urn:nbn:de:bvb:384-opus4-906829 }}</ref> Since then, OCT has been applied to the diagnosis of various skin lesions including carcinomas.<ref>{{cite journal | vauthors = Boone MA, Norrenberg S, Jemec GB, Del Marmol V | title = Imaging of basal cell carcinoma by high-definition optical coherence tomography: histomorphological correlation. A pilot study | journal = The British Journal of Dermatology | volume = 167 | issue = 4 | pages = 856–864 | date = October 2012 | pmid = 22862425 | doi = 10.1111/j.1365-2133.2012.11194.x | s2cid = 24965088 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Coleman AJ, Richardson TJ, Orchard G, Uddin A, Choi MJ, Lacy KE | title = Histological correlates of optical coherence tomography in non-melanoma skin cancer | journal = Skin Research and Technology | volume = 19 | issue = 1 | pages = 10–19 | date = February 2013 | pmid = 22738357 | doi = 10.1111/j.1600-0846.2012.00626.x | s2cid = 26084419 }}</ref><ref>{{cite journal | vauthors = Ulrich M, von Braunmuehl T, Kurzen H, Dirschka T, Kellner C, Sattler E, Berking C, Welzel J, Reinhold U | display-authors = 6 | title = The sensitivity and specificity of optical coherence tomography for the assisted diagnosis of nonpigmented basal cell carcinoma: an observational study | journal = The British Journal of Dermatology | volume = 173 | issue = 2 | pages = 428–435 | date = August 2015 | pmid = 25904111 | doi = 10.1111/bjd.13853 | doi-access = free }}</ref> However, the diagnosis of melanoma using conventional OCT is difficult, especially due to insufficient imaging resolution.<ref>{{cite journal | vauthors = Levine A, Wang K, Markowitz O | title = Optical Coherence Tomography in the Diagnosis of Skin Cancer | journal = Dermatologic Clinics | volume = 35 | issue = 4 | pages = 465–488 | date = October 2017 | pmid = 28886803 | doi = 10.1016/j.det.2017.06.008 }}</ref> Emerging high-resolution OCT techniques such as LC-OCT have the potential to improve the clinical diagnostic process, allowing for the early detection of malignant skin tumors – including melanoma – and a reduction in the number of surgical excisions of benign lesions.<ref name="Dubois 2018">{{cite journal | vauthors = Dubois A, Levecq O, Azimani H, Siret D, Barut A, Suppa M, Del Marmol V, Malvehy J, Cinotti E, Rubegni P, Perrot JL | display-authors = 6 | title = Line-field confocal optical coherence tomography for high-resolution noninvasive imaging of skin tumors | journal = Journal of Biomedical Optics | volume = 23 | issue = 10 | pages = 1–9 | date = October 2018 | pmid = 30353716 | doi = 10.1117/1.JBO.23.10.106007 | s2cid = 53023955 | bibcode = 2018JBO....23j6007D | doi-access = free | hdl = 2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/283413 | hdl-access = free }}  [[File:CC-BY icon.svg|50px]] This article contains quotations from this source, which is available under the [https://creativecommons.org/licenses/by/3.0/  Creative Commons Attribution 3.0 Unported (CC BY 3.0)] license.</ref> Other promising areas of application include the imaging of lesions where excisions are hazardous or impossible and the guidance of surgical interventions through identification of tumor margins.

===Dentistry===

Researchers in Tokyo medical and Dental University were able to detect enamel white spot lesions around and beneath the orthodontic brackets using swept source OCT.<ref>{{cite journal | vauthors = Velusamy P, Shimada Y, Kanno Z, Ono T, Tagami J | title = Optical evaluation of enamel white spot lesions around orthodontic brackets using swept-source optical coherence tomography (SS-OCT): An in vitro study | journal = Dental Materials Journal | volume = 38 | issue = 1 | pages = 22–27 | date = February 2019 | pmid = 30158348 | doi = 10.4012/dmj.2017-262 | doi-access = free }}</ref>

=== Research applications ===
Researchers have used OCT to produce detailed images of mice brains, through a "window" made of zirconia that has been modified to be transparent and implanted in the skull.<ref name=Nanomedicine201308>{{cite journal | vauthors = Damestani Y, Reynolds CL, Szu J, Hsu MS, Kodera Y, Binder DK, Park BH, Garay JE, Rao MP, Aguilar G | display-authors = 6 | title = Transparent nanocrystalline yttria-stabilized-zirconia calvarium prosthesis | journal = Nanomedicine | volume = 9 | issue = 8 | pages = 1135–1138 | date = November 2013 | pmid = 23969102 | doi = 10.1016/j.nano.2013.08.002 | s2cid = 14212180 | url = https://www.escholarship.org/uc/item/0th8v0p9 }}</ref><ref name="Mohan 2013">{{cite web | vauthors = Mohan G | title=A window to the brain? It's here, says UC Riverside team | website=Los Angeles Times | date=September 4, 2013 | url=https://www.latimes.com/science/sciencenow/la-sci-sn-window-brain-20130903-story.html}}</ref> Optical coherence tomography is also applicable and increasingly used in [[industrial engineering|industrial applications]], such as [[nondestructive testing]] (NDT), material thickness measurements,<ref>{{cite patent |country=US |number=7116429 |status=patent |title=Determining thickness of slabs of materials |gdate=2006-10-03 |fdate=2003-01-18 |pridate=2003-01-18 | inventor = Walecki WJ, Van P  }}.</ref> and in particular thin silicon wafers<ref>{{cite journal | vauthors = Walecki WJ, Szondy F | veditors = Novak EL, Wolfgang O, Gorecki C |title=Integrated quantum efficiency, reflectance, topography and stress metrology for solar cell manufacturing |journal=Proc. SPIE |volume=7064 |page=70640A |date=2008 |doi=10.1117/12.797541 |series=Interferometry XIV: Applications |bibcode=2008SPIE.7064E..0AW |s2cid=120257179 }}</ref><ref>{{cite journal | vauthors = Walecki WJ, Lai K, Pravdivtsev A, Souchkov V, Van P, Azfar T, Wong T, Lau SH, Koo A | veditors = Tanner DM, Ramesham R |title=Low-coherence interferometric absolute distance gauge for study of MEMS structures |journal=Proc. SPIE |volume=5716 |page=182 |date=2005 |doi=10.1117/12.590013 |series=Reliability, Packaging, Testing, and Characterization of MEMS/MOEMS IV |bibcode=2005SPIE.5716..182W |s2cid=110785119 }}</ref> and compound semiconductor wafers thickness measurements<ref>{{cite journal | vauthors = Walecki WJ, Lai K, Souchkov V, Van P, Lau SH, Koo A |date=2005 |title=Novel noncontact thickness metrology for backend manufacturing of wide bandgap light emitting devices |journal=Physica Status Solidi C |volume=2 |issue=3 |pages=984–989 |doi=10.1002/pssc.200460606 |bibcode=2005PSSCR...2..984W }}</ref><ref>{{cite journal | vauthors = Walecki W, Wei F, Van P, Lai K, Lee T, Lau SH, Koo A | veditors = Tanner DM, Rajeshuni R |title=Novel low coherence metrology for nondestructive characterization of high-aspect-ratio microfabricated and micromachined structures |journal=Proc. SPIE |volume=5343 |page=55 |date=2004 |doi=10.1117/12.530749|series=Reliability, Testing, and Characterization of MEMS/MOEMS III |s2cid=123249666 }}</ref> surface roughness characterization, surface and cross-section imaging<ref>{{cite report | vauthors = Guss G, Bass I, Hackel R, Demos SG |title= High-resolution 3-D imaging of surface damage sites in fused silica with Optical Coherence Tomography |publisher= [[Lawrence Livermore National Laboratory]] |id= UCRL-PROC-236270 |date= November 6, 2007 |url= https://e-reports-ext.llnl.gov/pdf/354371.pdf |access-date= December 14, 2010 |archive-url= https://web.archive.org/web/20170211222735/https://e-reports-ext.llnl.gov/pdf/354371.pdf |archive-date= February 11, 2017 |url-status= dead }}</ref><ref>{{cite conference | vauthors = Walecki W, Wei F, Van P, Lai K, Lee T, Lau SH, Koo A |url=http://www.gaas.org/Digests/2004/2004Papers/8.2.pdf |title=Interferometric Metrology for Thin and Ultra-Thin Compound Semiconductor Structures Mounted on Insulating Carriers |conference=CS Mantech Conference |date=2004 }}</ref> and volume loss measurements.<ref name=":2">{{Cite journal |last1=Zvagelsky |first1=Roman |last2=Mayer |first2=Frederik |last3=Beutel |first3=Dominik |last4=Rockstuhl |first4=Carsten |last5=Gomard |first5=Guillaume |last6=Wegener |first6=Martin |date=2022-12-12 |title=Towards in-situ diagnostics of multi-photon 3D laser printing using optical coherence tomography |url=https://www.light-am.com/en/article/doi/10.37188/lam.2022.039 |journal=Light: Advanced Manufacturing |volume=3 |issue=3 |pages=466–480 |doi=10.37188/lam.2022.039 |issn=2689-9620|url-access=subscription |doi-access=free }}</ref> OCT systems with feedback can be used to control manufacturing processes. With high speed data acquisition,<ref>{{cite journal | vauthors = Walecki WJ, Pravdivtsev A, Santos II M, Koo A |title=High-speed high-accuracy fiber optic low-coherence interferometry for in situ grinding and etching process monitoring |journal=Proc. SPIE |volume=6293 |page=62930D |date=August 2006 |doi=10.1117/12.675592 |series=Interferometry XIII: Applications |bibcode=2006SPIE.6293E..0DW |s2cid=121209439 }}</ref> and sub-micron resolution, OCT is adaptable to perform both inline and off-line.<ref>See, for example: {{cite web |url=http://www.zebraoptical.com/InterferometricProbe.html |title=ZebraOptical Optoprofiler: Interferometric Probe }}</ref> Due to the high volume of produced pills, an interesting field of application is in the pharmaceutical industry to control the coating of tablets.<ref>{{cite patent |country=EP |number=2799842 |status=application |title=A device and a method for monitoring a property of a coating of a solid dosage form during a coating process forming the coating of the solid dosage form |pubdate=2014-11-05 |gdate= |fdate=2014-04-29 |pridate=2013-04-30 |invent1=Markl, Daniel |invent2=Hannesschläger, Günther |invent3=Leitner, Michael |invent4=Stephan Sacher, Daniel Koller, Johannes Khinast}}; {{cite patent |country=GB |number=2513581 |status=application }}; {{cite patent |country=US |number=20140322429 A1 |status=application |url=https://www.google.com/patents/US20140322429 }}.</ref> Fiber-based OCT systems are particularly adaptable to industrial environments.<ref>{{cite journal |vauthors=Walecki WJ, Szondy F, Wang A |veditors=Xiao H, Fan S |url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol7322/73220K.pdf |title=Fiber optics low-coherence IR interferometry for defense sensors manufacturing |journal=Proc. SPIE |volume=7322 |page=73220K |date=30 April 2009 |doi=10.1117/12.818381 |series=Photonic Microdevices/Microstructures for Sensing |bibcode=2009SPIE.7322E..0KW |s2cid=120168355 |access-date=25 May 2011 |archive-date=20 August 2011 |archive-url=https://web.archive.org/web/20110820004435/http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol7322/73220K.pdf |url-status=dead }}</ref> These can access and scan interiors of hard-to-reach spaces,<ref>{{Cite journal | vauthors = Dufour M, Lamouche G, Gauthier B, Padioleau C, Monchalin JP |title=Inspection of hard-to-reach industrial parts using small diameter probes |journal=SPIE Newsroom |date=13 December 2006 |url=http://spie.org/documents/newsroom/imported/467/2006100467.pdf |doi=10.1117/2.1200610.0467 |s2cid=120476700 |access-date=December 15, 2010}}</ref> and are able to operate in hostile environments—whether radioactive, cryogenic, or very hot.<ref>{{Cite journal | vauthors = Dufour ML, Lamouche G, Detalle V, Gauthier B, Sammut P | title = Low-Coherence Interferometry, an Advanced Technique for Optical Metrology in Industry |url = http://www.ndt.net/abstract/wcndt2004/671.htm| doi = 10.1784/insi.47.4.216.63149 | journal = [[Insight: Non-Destructive Testing and Condition Monitoring]] | issn = 1354-2575| volume = 47 | issue = 4 | pages = 216–219 |date=April 2005 | citeseerx = 10.1.1.159.5249 | s2cid = 15657288 }}</ref> Novel optical biomedical diagnostic and imaging technologies are currently being developed to solve problems in biology and medicine.<ref>{{Cite journal |doi=10.1117/2.3201406.03 |title=Developing new optical imaging techniques for clinical use |journal=SPIE Newsroom |date=11 June 2014 | vauthors = Boppart S |url=http://www.spie.org/newsroom/boppart-video |url-access=subscription }}</ref> As of 2014, attempts have been made to use optical coherence tomography to identify root canals in teeth, specifically canal in the maxillary molar, however, there is no difference with the current methods of dental operatory microscope.<ref>{{cite journal | vauthors = Al-Azri K, Melita LN, Strange AP, Festy F, Al-Jawad M, Cook R, Parekh S, Bozec L | display-authors = 6 | title = Optical coherence tomography use in the diagnosis of enamel defects | journal = Journal of Biomedical Optics | volume = 21 | issue = 3 | pages = 36004 | date = March 2016 | pmid = 26968386 | doi = 10.1117/1.jbo.21.3.036004 | doi-access = free | bibcode = 2016JBO....21c6004A }}</ref><ref>{{cite journal | vauthors = Iino Y, Ebihara A, Yoshioka T, Kawamura J, Watanabe S, Hanada T, Nakano K, Sumi Y, Suda H | display-authors = 6 | title = Detection of a second mesiobuccal canal in maxillary molars by swept-source optical coherence tomography | journal = Journal of Endodontics | volume = 40 | issue = 11 | pages = 1865–1868 | date = November 2014 | pmid = 25266471 | doi = 10.1016/j.joen.2014.07.012 }}</ref>{{primary source inline|reason=Investigational study in which the authors collected the imaging data. |date=October 2016}} Research conducted in 2015 was successful in utilizing a smartphone as an OCT platform, although much work remains to be done before such a platform would be commercially viable.<ref>{{Cite journal | vauthors = Subhash HM, Hogan JN, Leahy MJ |date=May 2015 |title=Multiple-reference optical coherence tomography for smartphone applications |journal=SPIE Newsroom |url=http://spie.org/x113407.xml |doi=10.1117/2.1201503.005807 |url-access=subscription }}</ref> [[Photonic integrated circuits]] may be a promising option to miniaturized OCT. Similarly to [[integrated circuits]] silicon-based fabrication techniques can be used to produce miniaturized photonic systems. First in vivo human retinal imaging has been reported recently.<ref>{{cite journal | vauthors = Rank EA, Sentosa R, Harper DJ, Salas M, Gaugutz A, Seyringer D, Nevlacsil S, Maese-Novo A, Eggeling M, Muellner P, Hainberger R, Sagmeister M, Kraft J, Leitgeb RA, Drexler W | display-authors = 6 | title = Toward optical coherence tomography on a chip: in vivo three-dimensional human retinal imaging using photonic integrated circuit-based arrayed waveguide gratings | journal = Light: Science & Applications | volume = 10 | issue = 1 | pages = 6 | date = January 2021 | pmid = 33402664 | pmc = 7785745 | doi = 10.1038/s41377-020-00450-0 | bibcode = 2021LSA....10....6R }}</ref> In [[3D microfabrication]], OCT enables non-destructive testing and real-time inspection during additive manufacturing. Its high-resolution imaging detects defects, characterizes material properties and ensures the integrity of internal geometries without damaging the part.<ref name=":2" />

== See also ==
{{div col|colwidth=21em}}
* [[Angle-resolved low-coherence interferometry]]
* [[Ballistic photon]]
* [[Confocal microscopy]]
* [[Dual-axis optical coherence tomography]]
* [[Interferometry]]
* [[Intracoronary optical coherence tomography]]
* [[Leica Microsystems]]
* [[Medical imaging]]
* [[Novacam Technologies]]
* [[Optical heterodyne detection]]
* [[Optical projection tomography]]
* [[Spectroscopic optical coherence tomography]]
* [[Terahertz tomography]]
* [[Tomography]]
{{div col end}}

== References ==
{{reflist}}

{{Medical imaging}}
{{Eye procedures}}
{{Lasers}}

[[Category:Optical coherence tomography| ]]
[[Category:Eye procedures]]
[[Category:Laser medicine]]
[[Category:Medical equipment]]
[[Category:Optical imaging]]