Editing Perfusion (section)

==Measurement==
{{main|Perfusion scanning}}

In equations, the symbol Q is sometimes used to represent perfusion when referring to [[cardiac output]]. However, this terminology can be a source of confusion since both cardiac output and the symbol Q refer to [[volumetric flow rate|flow]] (volume per unit time, for example, L/min), whereas perfusion is measured as flow per unit tissue mass (mL/(min·g)).{{citation needed|date=March 2021}}

===Microspheres===
{{main|Microspheres}}
Microspheres that are labeled with [[radioactive isotopes]] have been widely used to measure perfusion since the 1960s. Radioactively labeled particles are injected into the test subject and a [[radiation detector]] measures radioactivity in tissues of interest.<ref name=Wagner>Studies of the Circulation with Radioactive Microspheres., Wagner et al, Invest. Radiol., 1969. 4(6): pp. 374–86.</ref> Microspheres are used in [[radionuclide angiography]], a method of diagnosing heart problems.

In the 1990s, methods for using [[fluorescent]] microspheres became a common substitute for radioactive particles.<ref name=OnlineGlenny>{{cite web|title=Fluorescent Microspheres|url=http://fmrc.pulmcc.washington.edu/DOCUMENTS/FMRCMAN99.pdf|publisher=[[Fluorescent Microsphere Resource Center]]|url-status=dead|archive-url=https://web.archive.org/web/20121002175850/http://fmrc.pulmcc.washington.edu/DOCUMENTS/FMRCMAN99.pdf|archive-date=2012-10-02}}</ref>

===Nuclear medicine===
{{main|Nuclear medicine}}
Perfusion of various tissues can be readily measured [[in vivo]] with nuclear medicine methods which are mainly [[positron emission tomography]] (PET) and [[single photon emission computed tomography]] (SPECT).{{citation needed|date=January 2014}} Various radiopharmaceuticals targeted at specific organs are also available, some of the most common are:{{citation needed|date=March 2021}}
* [[Technetium-99m|<sup>99m</sup>Tc]] labelled [[HMPAO]] and ECD for brain perfusion ([[Cerebral blood flow|rCBF]]) studied with SPECT
* [[Technetium-99m|<sup>99m</sup>Tc]] labelled [[Tetrofosmin]]  and [[Technetium (99mTc) sestamibi|Sestamibi]] for [[myocardial perfusion imaging]] with SPECT
* [[Xe-133|<sup>133</sup>Xe]]-gas for absolute quantification of brain perfusion ([[Cerebral blood flow|rCBF]]) with SPECT
* [[Oxygen-15#Oxygen-15|<sup>15</sup>O]]-labeled water for brain perfusion ([[Cerebral blood flow|rCBF]]) with PET (absolute quantification is possible when measuring arterial radioactivity concentration)
* [[Rubidium-82 chloride|<sup>82</sup>Rb-chloride]] for measuring myocardial perfusion with PET (absolute quantification is possible)

===Magnetic resonance imaging===
{{main|Magnetic resonance imaging}}
Two main categories of magnetic resonance imaging (MRI) techniques can be used to measure tissue perfusion [[in vivo]].
* The first is based on the use of an injected [[contrast agent]] that changes the [[magnetic susceptibility]] of blood and thereby the MR signal which is repeatedly measured during [[Bolus (medicine)|bolus]] passage.<ref>Huettel, S. A.; Song, A. W.; McCarthy, G. (2009), Functional Magnetic Resonance Imaging (2 ed.), Massachusetts: Sinauer, {{ISBN|978-0-87893-286-3}}</ref> 
* The other category is based on [[arterial spin labelling]] (ASL), where arterial blood is [[magnet]]ically tagged before it enters into the tissue being examined and the amount of labelling that is measured and compared to a control recording obtained without spin labelling.<ref>{{Cite journal|title = Applications of arterial spin labeled MRI in the brain|journal = Journal of Magnetic Resonance Imaging|date = 2012-05-01|issn = 1522-2586|pmc = 3326188|pmid = 22246782|pages = 1026–1037|volume = 35|issue = 5|doi = 10.1002/jmri.23581|first1 = John A.|last1 = Detre|first2 = Hengyi|last2 = Rao|first3 = Danny J. J.|last3 = Wang|first4 = Yu Fen|last4 = Chen|first5 = Ze|last5 = Wang}}</ref>

===Computed tomography (CT)===
{{main|Computed tomography}}
Brain perfusion (more correctly transit times) can be estimated with contrast-enhanced computed tomography.<ref>L. Axel. "Cerebral blood flow determination by rapid-sequence computed-tomography: theoretical analysis".  ''Radiology'' 137: 679–86, December 1980</ref>

===Thermal diffusion===
{{main|Molecular diffusion}}
Perfusion can be determined by measuring the total [[Molecular diffusion|thermal diffusion]] and then separating it into [[thermal conductivity]] and perfusion components.<ref>{{cite journal  |vauthors=Vajkoczy P, Roth H, Horn P, et al |title=Continuous monitoring of regional cerebral blood flow: experimental and clinical validation of a novel thermal diffusion microprobe |journal=Journal of Neurosurgery |volume=93 |issue=2 |pages=265–74 | date=August 2000 |pmid=10930012 |doi=10.3171/jns.2000.93.2.0265|s2cid=30375395 }}</ref> [[rCBF]] is usually measured continuously in time.  It is necessary to stop the measurement periodically to cool down and reassess the [[thermal conductivity]].{{citation needed|date=March 2021}}