Editing Magnetic resonance imaging (section)

==Mechanism==

===Construction and physics===
{{Main |Physics of magnetic resonance imaging}}
[[File:Mri scanner schematic labelled.svg|class=skin-invert-image|thumb|upright=1.5 |Schematic of a cylindrical superconducting MR scanner. Top: cross section of the cylinder with primary coil, gradient coils and RF transmit coils. Bottom: longitudinal section of the cylinder and table, showing the same coils and the RF receive coil.]]

In most medical applications, [[hydrogen]] nuclei, which consist solely of a [[proton]], that are in tissues create a signal that is processed to form an image of the body in terms of the density of those nuclei in a specific region. Given that the protons are affected by fields from other atoms to which they are bonded, it is possible to separate responses from hydrogen in specific compounds. To perform a study, the person is positioned within an [[Physics of magnetic resonance imaging#MRI scanner|MRI scanner]] that forms a strong [[magnetic field]] around the area to be imaged. First, energy from an [[oscillation|oscillating]] magnetic field is temporarily applied to the patient at the appropriate [[resonance]] frequency. Scanning with X and Y gradient coils causes a selected region of the patient to experience the exact magnetic field required for the energy to be absorbed. The atoms are [[Excited state|excited]] by a [[Radio frequency|RF]] pulse and the resultant signal is measured by a [[Radiofrequency coil|receiving coil]]. The RF signal may be processed to deduce position information by looking at the changes in RF level and phase caused by varying the local magnetic field using [[Physics of magnetic resonance imaging#Gradients|gradient coils]]. As these coils are rapidly switched during the excitation and response to perform a moving line scan, they create the characteristic repetitive noise of an MRI scan as the windings move slightly due to [[magnetostriction]]. The contrast between different tissues is determined by the rate at which excited atoms return to the [[Thermodynamic equilibrium|equilibrium state]]. [[Exogeny|Exogenous]] [[contrast agent]]s may be given to the person to make the image clearer.<ref name ="McRobbie">{{cite book | vauthors = McRobbie DW | title=MRI from picture to proton | date=2007 | publisher=Cambridge University Press | location=Cambridge, UK; New York | isbn=978-0-521-68384-5}}</ref>

The major components of an MRI scanner are the main [[magnet]], which polarizes the sample, the [[Shim (magnetism)|shim coils]] for correcting shifts in the [[homogeneity (physics)|homogeneity]] of the main magnetic field, the gradient system which is used to localize the region to be scanned and the RF system, which excites the sample and detects the resulting NMR signal. The whole system is controlled by one or more computers.

[[File:Glebefields Health Centre - 2020-03-22 - Andy Mabbett - 03.jpg|thumb|left|A mobile MRI unit]]
{{Listen| filename = MRI - 2020-03-22 - Andy Mabbett.ogg |title = Audio recording |type = speech |description = A short extract of a 20-minute scanning session, recorded outside the above unit }}

MRI requires a magnetic field that is both strong and [[Homogeneity (physics)|uniform]] to a few parts per million across the scan volume. The field strength of the magnet is measured in [[tesla (unit)|teslas]] – and while the majority of systems operate at 1.5 T, commercial systems are available between 0.2 and 7 T. '''3T MRI''' systems, also called 3 Tesla MRIs, have stronger magnets than 1.5 systems and are considered better for images of organs and soft tissue.<ref>{{Cite web |title=National Cancer Institute |url=https://www.cancer.gov/publications/dictionaries/cancer-terms/def/3t-mri |access-date=2024-09-16 |website=Cancer.gov|language=en}}</ref> Whole-body MRI systems for research applications operate in e.g. 9.4T,<ref>{{Cite web |title=Tesla Engineering Ltd - Magnet Division - MRI Supercon |url=http://www.tesla.co.uk/magnet/phone/tesla-engineering-magnet-division-superconducting-magnets-mobile/tesla-engineering-magnet-division-mri-magnets-mobile.html |access-date=2022-08-16 |website=www.tesla.co.uk |language=en}}</ref><ref>{{Cite web |last=Qiuliang |first=Wang |date=January 2022 |title=Successful Development of a 9.4T/800mm Whole-body MRI Superconducting Magnet at IEE CAS |url=https://snf.ieeecsc.org/sites/ieeecsc.org/files/documents/snf/abstracts/HP149-9.4T%20whole-body%20MRI%20superconducting%20magnet.pdf |url-status=live |website=snf.ieeecsc.org |archive-url=https://web.archive.org/web/20230322153825/https://snf.ieeecsc.org/sites/ieeecsc.org/files/documents/snf/abstracts/HP149-9.4T%20whole-body%20MRI%20superconducting%20magnet.pdf |archive-date=Mar 22, 2023}}</ref> 10.5T,<ref>{{Cite journal |last=Nowogrodzki |first=Anna |date=2018-10-31 |title=The world's strongest MRI machines are pushing human imaging to new limits |url=https://www.nature.com/articles/d41586-018-07182-7 |journal=Nature |language=en |volume=563 |issue=7729 |pages=24–26 |doi=10.1038/d41586-018-07182-7|pmid=30382222 |bibcode=2018Natur.563...24N |s2cid=53153608 }}</ref> 11.7T.<ref>{{Cite web |last=CEA |date=2021-10-07 |title=The most powerful MRI scanner in the world delivers its first images! |url=https://www.cea.fr/english/Pages/News/premieres-images-irm-iseult-2021.aspx |access-date=2022-08-16 |website=CEA/English Portal |language=en}}</ref> Even higher field whole-body MRI systems e.g. 14 T and beyond are in conceptual proposal<ref>{{Cite journal |last1=Budinger |first1=Thomas F. |last2=Bird |first2=Mark D. |date=2018-03-01 |title=MRI and MRS of the human brain at magnetic fields of 14T to 20T: Technical feasibility, safety, and neuroscience horizons |journal=NeuroImage |series=Neuroimaging with Ultra-high Field MRI: Present and Future |language=en |volume=168 |pages=509–531 |doi=10.1016/j.neuroimage.2017.01.067 |pmid=28179167 |s2cid=4054160 |issn=1053-8119|doi-access=free }}</ref> or in engineering design.<ref>{{Cite journal |last1=Li |first1=Yi |last2=Roell |first2=Stefan |date=2021-12-01 |title=Key designs of a short-bore and cryogen-free high temperature superconducting magnet system for 14 T whole-body MRI |url=https://iopscience.iop.org/article/10.1088/1361-6668/ac2ec8 |journal=Superconductor Science and Technology |volume=34 |issue=12 |pages=125005 |doi=10.1088/1361-6668/ac2ec8 |bibcode=2021SuScT..34l5005L |s2cid=242194782 |issn=0953-2048}}</ref> Most clinical magnets are [[superconductivity|superconducting]] magnets, which require [[liquid helium]] to keep them at low temperatures. Lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for [[Claustrophobia|claustrophobic]] patients.<ref>{{cite journal | vauthors = Sasaki M, Ehara S, Nakasato T, Tamakawa Y, Kuboya Y, Sugisawa M, Sato T | title = MR of the shoulder with a 0.2-T permanent-magnet unit | journal = AJR. American Journal of Roentgenology | volume = 154 | issue = 4 | pages = 777–8 | date = April 1990 | pmid = 2107675 | doi = 10.2214/ajr.154.4.2107675 }}</ref> Lower field strengths are also used in a [[Portable magnetic resonance imaging|portable MRI]] scanner approved by the FDA in 2020.<ref>{{cite news |title=Guildford company gets FDA approval for bedside MRI |url=https://www.ctinsider.com/business/nhregister/article/Guilford-company-gets-FDA-approval-for-bedside-MRI-15050860.php |access-date=15 April 2020 |work=[[New Haven Register]] |date=12 February 2020 |archive-date=3 April 2020 |archive-url=https://web.archive.org/web/20200403211038/https://www.ctinsider.com/business/nhregister/article/Guilford-company-gets-FDA-approval-for-bedside-MRI-15050860.php |url-status=dead }}</ref> Recently, MRI has been demonstrated also at ultra-low fields, i.e., in the microtesla-to-millitesla range, where sufficient signal quality is made possible by prepolarization (on the order of 10–100 mT) and by measuring the [[Larmor precession]] fields at about 100 microtesla with highly sensitive superconducting quantum interference devices ([[SQUID]]s).<ref>{{cite journal | vauthors = McDermott R, Lee S, ten Haken B, Trabesinger AH, Pines A, Clarke J | title = Microtesla MRI with a superconducting quantum interference device | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 21 | pages = 7857–61 | date = May 2004 | pmid = 15141077 | pmc = 419521 | doi = 10.1073/pnas.0402382101 | bibcode = 2004PNAS..101.7857M | doi-access = free }}</ref><ref>{{cite journal | vauthors = Zotev VS, Matlashov AN, Volegov PL, Urbaitis AV, Espy MA, Kraus RH |doi=10.1088/0953-2048/20/11/S13 |title=SQUID-based instrumentation for ultralow-field MRI |journal=Superconductor Science and Technology |volume=20 |issue=11 |pages=S367–73 |year=2007  |s2cid=119160258 |arxiv=0705.0661 |bibcode=2007SuScT..20S.367Z }}</ref><ref>{{cite journal | vauthors = Vesanen PT, Nieminen JO, Zevenhoven KC, Dabek J, Parkkonen LT, Zhdanov AV, Luomahaara J, Hassel J, Penttilä J, Simola J, Ahonen AI, Mäkelä JP, Ilmoniemi RJ | display-authors = 6 | title = Hybrid ultra-low-field MRI and magnetoencephalography system based on a commercial whole-head neuromagnetometer | journal = Magnetic Resonance in Medicine | volume = 69 | issue = 6 | pages = 1795–804 | date = June 2013 | pmid = 22807201 | doi = 10.1002/mrm.24413 | s2cid = 40026232 | doi-access = free }}</ref>

===T1 and T2===
{{Further|Relaxation (NMR)}}
[[File:TR TE.jpg|class=skin-invert-image|thumb|upright=1.2|Effects of TR and TE on MR signal]]
[[File:T1t2PD.jpg|thumb|upright=1.2|Examples of T1-weighted, T2-weighted and [[MRI sequence#Proton density|PD-weighted]] MRI scans]]
[[File:Spin Orientations During Relaxation.jpg|class=skin-invert-image|thumb|upright=1.2|Diagram of changing magnetization and spin orientations throughout spin-lattice relaxation experiment]]

Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T<sub>1</sub> ([[Spin–lattice relaxation|spin-lattice]]; that is, magnetization in the same direction as the static magnetic field) and T<sub>2</sub> ([[Spin-spin relaxation time|spin-spin]]; transverse to the static magnetic field).
To create a T<sub>1</sub>-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the [[repetition time]] (TR). This image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions, and in general, obtaining morphological information, as well as for [[MRI contrast agent|post-contrast]] imaging.
{{anchor|T2-weighted_MRI}}
To create a T<sub>2</sub>-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the [[echo time]] (TE). This image weighting is useful for detecting [[edema]] and inflammation, revealing [[Hyperintensity|white matter lesions]], and assessing zonal anatomy in the [[prostate]] and [[uterus]].

The information from MRI scans comes in the form of [[Contrast resolution|image contrasts]] based on differences in the rate of relaxation of [[spin quantum number|nuclear spins]] following their perturbation by an oscillating magnetic field (in the form of radiofrequency pulses through the sample).<ref>{{cite journal |last1=De Leon-Rodriguez |first1=L.M. |title=Basic MR Relaxation Mechanisms and Contrast Agent Design |journal=Journal of Magnetic Resonance Imaging |date=2015 |volume=42 |issue=3|pages=545–565 |doi=10.1002/jmri.24787 |pmid=25975847 |pmc=4537356 }}</ref> The relaxation rates are a measure of the time it takes for a signal to decay back to an equilibrium state from either the longitudinal or transverse plane.

[[Magnetization]] builds up along the z-axis in the presence of a magnetic field, B<sub>0</sub>, such that the [[magnetic dipole]]s in the sample will, on average, align with the z-axis summing to a total magnetization M<sub>z</sub>. This magnetization along z is defined as the equilibrium magnetization; magnetization is defined as the sum of all magnetic dipoles in a sample. Following the equilibrium magnetization, a 90° radiofrequency (RF) pulse flips the direction of the magnetization vector in the xy-plane, and is then switched off. The initial magnetic field B<sub>0</sub>, however, is still applied. Thus, the spin magnetization vector will slowly return from the xy-plane back to the equilibrium state. The time it takes for the magnetization vector to return to its equilibrium value, M<sub>z</sub>, is referred to as the longitudinal relaxation time, T<sub>1</sub>.<ref>{{Cite web|url=http://imserc.northwestern.edu/downloads/nmr-t1.pdf|title=T1 relaxation experiment}}</ref> Subsequently, the rate at which this happens is simply the reciprocal of the relaxation time: <math>\frac {1}{T_1} = R_1</math>. Similarly, the time in which it takes for M<sub>xy</sub> to return to zero is T<sub>2</sub>, with the rate <math>\frac {1}{T_2} = R_2</math>.<ref>{{cite book |last1=McHale |first1=J. |title=Molecular Spectroscopy |date=2017 |publisher=CRC Press/Taylor and Francis Group |pages=73–80}}</ref> Magnetization as a function of time is defined by the [[Bloch equations]].

T<sub>1</sub> and T<sub>2</sub> values are dependent on the chemical environment of the sample; hence their utility in MRI. Soft tissue and muscle tissue relax at different rates, yielding the image contrast in a typical scan.

The standard display of MR images is to represent fluid characteristics in [[black-and-white]] images, where different tissues turn out as follows:

{|class="wikitable"
|+ Signals from different materials
! scope="col" | Signal 
! scope="col" | T1-weighted 
! scope="col" | T2-weighted
|-
! scope="row" style="background: #c8ccd1;" | High
|style="vertical-align:top;"|
* [[Fat]]<ref name=wisconsin/><ref name=johnson2>{{cite web |url=http://www.med.harvard.edu/aanlib/basicsMR.html | vauthors = Johnson KA |title=Basic proton MR imaging. Tissue Signal Characteristics }}{{MEDRS|date=September 2018}}</ref>
* Subacute hemorrhage<ref name=johnson2/>
* [[Melanin]]<ref name=johnson2/>
* Protein-rich fluid<ref name=johnson2/>
* Slowly flowing blood<ref name=johnson2/>
* [[Paramagnetism|Paramagnetic]] or [[diamagnetism|diamagnetic]] substances, such as [[gadolinium]], [[manganese]], [[copper]]<ref name=johnson2/>
* [[Cortical pseudolaminar necrosis]]<ref name=johnson2/>
* Anatomy
|style="vertical-align:top;"|
* Fat
* More water content,<ref name=wisconsin/> as in [[edema]], [[tumor]], [[infarction]], [[inflammation]] and [[infection]]<ref name=johnson2/>
* [[Extracellular]]ly located [[methemoglobin]] in subacute hemorrhage<ref name=johnson2/>
* Pathology
|-
! scope="row" style="background: #a2a9b1;" | Intermediate
|style="vertical-align:top;"|
* [[Gray matter]] darker than [[white matter]]<ref name=patil>{{cite web|url=http://www.slideshare.net/DrTusharPatil/mri-sequences|title=MRI sequences| vauthors = Patil T |access-date=2016-03-14|date=2013-01-18}}</ref>
|style="vertical-align:top;"|
* [[White matter]] darker than [[grey matter]]<ref name=patil/>
|-
! scope="row" style="background: #101418; color: #fff;" | Low
|style="vertical-align:top;"|
* Bone<ref name=wisconsin>{{cite web|url=https://www.radiology.wisc.edu/education/med_students/neuroradiology/NeuroRad/Intro/MRIintro.htm|title=Magnetic Resonance Imaging|publisher=[[University of Wisconsin]]|access-date=2016-03-14|archive-url=https://web.archive.org/web/20170510065614/https://www.radiology.wisc.edu/education/med_students/neuroradiology/NeuroRad/Intro/MRIintro.htm|archive-date=2017-05-10|url-status=dead}}</ref>
* Air<ref name=wisconsin/>
* Low [[proton]] density as in [[calcification]]<ref name=johnson2/>
* Urine
* [[Cerebrospinal fluid|CSF]]
* More water content,<ref name=wisconsin/> as in [[edema]], [[tumor]], [[infarction]], [[inflammation]], [[infection]], hyperacute or chronic [[hemorrhage]]<ref name=johnson2/>
|style="vertical-align:top;"|
* [[Bone]]<ref name=wisconsin/>
* [[Air]]<ref name=wisconsin/>
* Low proton density, as in [[calcification]] and [[fibrosis]]<ref name=johnson2/>
* [[Paramagnetism|Paramagnetic]] material, such as [[deoxyhemoglobin]], intracellular [[methemoglobin]], [[iron]], [[ferritin]], [[hemosiderin]], [[melanin]]<ref name=johnson2/>
* Protein-rich fluid<ref name=johnson2/>
|}