Editing Magnetic resonance imaging (section)

== Contrast agents ==
{{Main|MRI contrast agent}}
MRI for imaging anatomical structures or blood flow do not require contrast agents since the varying properties of the tissues or blood provide natural contrasts. However, for more specific types of imaging, [[Exogeny|exogenous]] contrast agents may be given [[Intravenous therapy|intravenously]], [[Oral administration|orally]], or [[Joint injection|intra-articularly]].<ref name ="McRobbie"/> Most contrast agents are either paramagnetic (e.g.: gadolinium, manganese, europium), and are used to shorten T1 in the tissue they accumulate in, or super-paramagnetic (SPIONs), and are used to shorten T2 and T2* in healthy tissue reducing its signal intensity (negative contrast agents). The most commonly used intravenous contrast agents are based on [[chelate]]s of [[gadolinium]], which is highly paramagnetic.<ref>{{cite book | vauthors=Rinck PA | title=Magnetic Resonance in Medicine | chapter=Chapter 13: Contrast Agents | chapter-url=http://www.magnetic-resonance.org/ch/13-01.html |date=2014 }}</ref> In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. [[Anaphylaxis|Anaphylactoid reactions]] are rare, occurring in approx. 0.03–0.1%.<ref>{{cite journal | vauthors = Murphy KJ, Brunberg JA, Cohan RH | title = Adverse reactions to gadolinium contrast media: a review of 36 cases | journal = AJR. American Journal of Roentgenology | volume = 167 | issue = 4 | pages = 847–9 | date = October 1996 | pmid = 8819369 | doi = 10.2214/ajr.167.4.8819369 | doi-access = free }}</ref> Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo [[Radiocontrast|contrast-enhanced CT]].<ref>{{cite web|url=http://www.guideline.gov/summary/summary.aspx?doc_id=8283|title=ACR guideline|website=guideline.gov|date=2005|access-date=2006-11-22|archive-url=https://web.archive.org/web/20060929182051/http://www.guideline.gov/summary/summary.aspx?doc_id=8283|archive-date=2006-09-29|url-status=dead}}</ref>

Gadolinium-based contrast reagents are typically [[polydentate|octadentate]] complexes of [[gadolinium|gadolinium(III)]]. The complex is [[Stability constants of complexes|very stable]] (log K > 20) so that, in use, the concentration of the un-complexed Gd<sup>3+</sup> ions should be below the toxicity limit. The 9th place in the metal ion's [[coordination sphere]] is occupied by a water molecule which exchanges rapidly with water molecules in the reagent molecule's immediate environment, affecting the magnetic resonance [[relaxation time]].<ref>{{cite book|first1=Sergey |last1=Shugaev |first2=Peter |last2=Caravan|chapter=Metal Ions in Bio-imaging Techniques: A Short Overview |pages= 1–37 |title=Metal Ions in Bio-Imaging Techniques|year=2021 |editor-first1= Astrid |editor-last1=Sigel|editor-first2=Eva |editor-last2=Freisinger |editor-first3=Roland K.O. |editor-last3=Sigel|publisher= Walter de Gruyter|location= Berlin|chapter-url=https://www.deGruyter.com/document/doi/10.1515/9783110685701-007|doi= 10.1515/9783110685701-007|isbn=978-3-11-068570-1 }}</ref>

In December 2017, the [[Food and Drug Administration]] (FDA) in the [[United States]] announced in a drug safety communication that new warnings were to be included on all gadolinium-based contrast agents (GBCAs). The FDA also called for increased patient education and requiring gadolinium contrast vendors to conduct additional animal and clinical studies to assess the safety of these agents.<ref>{{cite web|url=https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-fda-warns-gadolinium-based-contrast-agents-gbcas-are-retained-body|title= FDA Drug Safety Communication: FDA warns that gadolinium-based contrast agents (GBCAs) are retained in the body; requires new class warnings|date=2018-05-16|website=USA [[FDA]]}}</ref>
Although gadolinium agents have proved useful for patients with kidney impairment, in patients with severe [[kidney failure]] requiring dialysis there is a risk of a rare but serious illness, [[nephrogenic systemic fibrosis]], which may be linked to the use of certain gadolinium-containing agents. The most frequently linked is [[gadodiamide]], but other agents have been linked too.<ref>{{cite journal | vauthors = Thomsen HS, Morcos SK, Dawson P | title = Is there a causal relation between the administration of gadolinium based contrast media and the development of nephrogenic systemic fibrosis (NSF)? | journal = Clinical Radiology | volume = 61 | issue = 11 | pages = 905–6 | date = November 2006 | pmid = 17018301 | doi = 10.1016/j.crad.2006.09.003 }}</ref> Although a causal link has not been definitively established, current guidelines in the [[United States]] are that dialysis patients should only receive gadolinium agents where essential and that [[Kidney dialysis|dialysis]] should be performed as soon as possible after the scan to remove the agent from the body promptly.<ref>{{cite web|title=FDA Drug Safety Communication: New warnings for using gadolinium-based contrast agents in patients with kidney dysfunction|url=https://www.fda.gov/Drugs/DrugSafety/ucm223966.htm|website=Information on Gadolinium-Based Contrast Agents|publisher=U.S. Food and Drug Administration|access-date=12 March 2011|date=23 December 2010}}</ref><ref>{{cite web|url=https://www.fda.gov/cder/drug/advisory/gadolinium_agents.htm |title=FDA Public Health Advisory: Gadolinium-containing Contrast Agents for Magnetic Resonance Imaging |website=fda.gov |url-status=dead |archive-url=https://web.archive.org/web/20060928204027/https://www.fda.gov/cder/drug/advisory/gadolinium_agents.htm |archive-date=2006-09-28 }}</ref>

In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released.<ref>{{cite journal | title=Gadolinium-containing contrast agents: new advice to minimise the risk of nephrogenic systemic fibrosis | journal=Drug Safety Update | date=January 2010 | volume=3 | issue=6 | page=3 | url=https://www.gov.uk/drug-safety-update/gadolinium-containing-contrast-agents-new-advice-to-minimise-the-risk-of-nephrogenic-systemic-fibrosis }}</ref><ref>{{cite web|url=http://www.ismrm.org/special/EMEA2.pdf |title=MRI Questions and Answers |access-date=2010-08-02 | publisher=International Society for Magnetic Resonance in Medicine | location=Concord, CA }}</ref> In 2008, a new contrast agent named [[Gadoxetic acid|gadoxetate]], brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: This has the theoretical benefit of a dual excretion path.<ref>{{cite web |url=http://radiology.rsna.org/content/246/1/11.full?searchid=1&HITS=10&hits=10&sortspec=relevance&resourcetype=HWCIT&maxtoshow=&RESULTFORMAT=&author1=kanal&FIRSTINDEX=0 |archive-url=https://archive.today/20120719004253/http://radiology.rsna.org/content/246/1/11.full?searchid=1&HITS=10&hits=10&sortspec=relevance&resourcetype=HWCIT&maxtoshow=&RESULTFORMAT=&author1=kanal&FIRSTINDEX=0 |url-status=dead |archive-date=2012-07-19 |title=Response to the FDA's May 23, 2007, Nephrogenic Systemic Fibrosis Update1 — Radiology |publisher=Radiological Society of North America |date=2007-09-12 |access-date=2010-08-02 }}</ref>