Editing Magnetic resonance imaging (section)

===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/>
|}