Editing Optical coherence tomography (section)

==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.