Editing Cardiac output (section)

===Magnetic resonance imaging===
Velocity-encoded phase contrast Magnetic resonance imaging (MRI)<ref>{{cite book |last1=Arheden |first1=Håkan |last2=Ståhlberg |first2=Freddy |name-list-style=vanc |chapter=Blood flow measurements |chapter-url=https://lup.lub.lu.se/record/1136006 |editor1-last=de Roos |editor1-first=Albert |editor2-last=Higgins |editor2-first=Charles B |title=MRI and CT of the Cardiovascular System |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |year=2006 |pages=71–90 |isbn=978-0-7817-6271-7 |edition=2nd |access-date=23 February 2022 |archive-date=23 February 2022 |archive-url=https://web.archive.org/web/20220223063034/https://lup.lub.lu.se/search/publication/1136006 |url-status=live }}</ref> is the most accurate technique for measuring flow in large vessels in mammals. MRI flow measurements have been shown to be highly accurate compared to measurements made with a beaker and timer,<ref>{{cite journal | vauthors = Arheden H, Holmqvist C, Thilen U, Hanséus K, Björkhem G, Pahlm O, Laurin S, Ståhlberg F | title = Left-to-right cardiac shunts: comparison of measurements obtained with MR velocity mapping and with radionuclide angiography | journal = Radiology | volume = 211 | issue = 2 | pages = 453–58 | date = May 1999 | pmid = 10228528 | doi = 10.1148/radiology.211.2.r99ma43453 }}</ref> and less variable than the Fick principle<ref>{{cite journal | vauthors = Razavi R, Hill DL, Keevil SF, Miquel ME, Muthurangu V, Hegde S, Rhode K, Barnett M, van Vaals J, Hawkes DJ, Baker E | title = Cardiac catheterisation guided by MRI in children and adults with congenital heart disease | journal = Lancet | volume = 362 | issue = 9399 | pages = 1877–82 | date = December 2003 | pmid = 14667742 | doi = 10.1016/S0140-6736(03)14956-2 | s2cid = 25380774 }}</ref> and thermodilution.<ref>{{cite journal | vauthors = Kuehne T, Yilmaz S, Schulze-Neick I, Wellnhofer E, Ewert P, Nagel E, Lange P | title = Magnetic resonance imaging guided catheterisation for assessment of pulmonary vascular resistance: in vivo validation and clinical application in patients with pulmonary hypertension | journal = Heart | volume = 91 | issue = 8 | pages = 1064–69 | date = August 2005 | pmid = 16020598 | pmc = 1769055 | doi = 10.1136/hrt.2004.038265 }}</ref>

Velocity-encoded MRI is based on the detection of changes in the phase of proton [[precession]]. These changes are proportional to the velocity of the protons' movement through a magnetic field with a known gradient. When using velocity-encoded MRI, the result is two sets of images, one for each time point in the cardiac cycle. One is an anatomical image and the other is an image in which the signal intensity in each [[pixel]] is directly proportional to the through-plane velocity. The average velocity in a vessel, i.e., the [[aorta]] or the [[pulmonary artery]], is quantified by measuring the average signal intensity of the pixels in the cross-section of the vessel then multiplying by a known constant. The flow is calculated by multiplying the mean velocity by the cross-sectional area of the vessel. This flow data can be used in a flow-versus-time graph. The area under the flow-versus-time curve for one [[cardiac cycle]] is the stroke volume. The length of the cardiac cycle is known and determines heart rate; ''Q'' can be calculated using equation ({{EquationNote|1}}). MRI is typically used to quantify the flow over one cardiac cycle as the average of several heart beats. It is also possible to quantify the stroke volume in real-time on a beat-for-beat basis.<ref>{{cite journal | vauthors = Petzina R, Ugander M, Gustafsson L, Engblom H, Sjögren J, Hetzer R, Ingemansson R, Arheden H, Malmsjö M | title = Hemodynamic effects of vacuum-assisted closure therapy in cardiac surgery: assessment using magnetic resonance imaging | journal = The Journal of Thoracic and Cardiovascular Surgery | volume = 133 | issue = 5 | pages = 1154–62 | date = May 2007 | pmid = 17467423 | doi = 10.1016/j.jtcvs.2007.01.011 | doi-access = free }}</ref>

While MRI is an important research tool for accurately measuring ''Q'', it is currently not clinically used for haemodynamic monitoring in emergency or intensive care settings. {{As of|2015}}, cardiac output measurement by MRI is routinely used in clinical cardiac MRI examinations.<ref>{{cite journal | vauthors = Pennell DJ, Sechtem UP, Higgins CB, Manning WJ, Pohost GM, Rademakers FE, van Rossum AC, Shaw LJ, Yucel EK | title = Clinical indications for cardiovascular magnetic resonance (CMR): Consensus Panel report | journal = European Heart Journal | volume = 25 | issue = 21 | pages = 1940–65 | date = November 2004 | pmid = 15522474 | doi = 10.1016/j.ehj.2004.06.040 | author10 = Society for Cardiovascular Magnetic Resonance | author11 = Working Group on Cardiovascular Magnetic Resonance of the European Society of Cardiology | doi-access = free }}</ref>