Editing Optical coherence tomography (section)

==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 }}</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 }}</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>