Magnetic Resonance Imaging
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Magnetic Resonance Imaging

Magnetic resonance imaging

Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves to generate images of the organs in the body. MRI does not involve X-rays or the use of ionizing radiation, which distinguishes it from CT and PET scans. MRI is a medical application of nuclear magnetic resonance (NMR). NMR can also be used for imaging in other NMR applications, such as NMR spectroscopy.

While the hazards of ionizing radiation are now well controlled in most medical contexts, an MRI may still be seen as a better choice than a CT scan. MRI is widely used in hospitals and clinics for medical diagnosis and staging and follow-up of disease without exposing the body to radiation. An MRI may yield different information compared with CT. Risks and discomfort may be associated with MRI scans. Compared with CT scans, MRI scans typically take longer and are louder, and they usually need the subject to enter a narrow, confining tube. In addition, people with some medical implants or other non-removable metal inside the body may be unable to undergo an MRI examination safely.

MRI was originally called NMRI (nuclear magnetic resonance imaging), but "nuclear" was dropped to avoid negative associations.[1] Certain atomic nuclei are able to absorb radio frequency energy when placed in an external magnetic field; the resultant evolving spin polarization can induce a RF signal in a radio frequency coil and thereby be detected.[2] In clinical and research MRI, hydrogen atoms are most often used to generate a macroscopic polarization that is detected by antennas close to the subject being examined.[2] Hydrogen atoms are naturally abundant in humans and other biological organisms, particularly in water and fat. For this reason, most MRI scans essentially map the location of water and fat in the body. Pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the polarization in space. By varying the parameters of the pulse sequence, different contrasts may be generated between tissues based on the relaxation properties of the hydrogen atoms therein.

Since its development in the 1970s and 1980s, MRI has proven to be a versatile imaging technique. While MRI is most prominently used in diagnostic medicine and biomedical research, it also may be used to form images of non-living objects. MRI scans are capable of producing a variety of chemical and physical data, in addition to detailed spatial images. The sustained increase in demand for MRI within health systems has led to concerns about cost effectiveness and overdiagnosis.[3][4]


Construction and physics

Schematic of construction of a cylindrical superconducting MR scanner

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 MRI scanner that forms a strong magnetic field around the area to be imaged. First, energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonance frequency. Scanning with X and Y gradient coils cause a selected region of the patient to experience the exact magnetic field required for the energy to be absorbed. The excited atoms emit a radio frequency (RF) signal, which is measured by a 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 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 equilibrium state. Exogenous contrast agents may be given to the person to make the image clearer.[5]

The major components of an MRI scanner are the main magnet, which polarizes the sample, the shim coils for correcting shifts in the 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.

a mobile MRI unit visiting Glebefields Health Centre, Tipton, England

MRI requires a magnetic field that is both strong and uniform to a few parts per million across the scan volume. The field strength of the magnet is measured in teslas - and while the majority of systems operate at 1.5 T, commercial systems are available between 0.2 and 7 T. Most clinical magnets are superconducting magnets, which require liquid helium to keep them very cold. Lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for claustrophobic patients.[6] Lower field strengths are also used in a portable MRI scanner approved by the FDA in 2020.[7] 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 (SQUIDs).[8][9][10]

T1 and T2

Effects of TR and TE on MR signal
Examples of T1 weighted, T2 weighted and PD weighted MRI scans



Signal T1-weighted T2-weighted
Inter- mediate Gray matter darker than white matter[13] White matter darker than grey matter[13]


Usage by organ or system

Patient being positioned for MR study of the head and abdomen

MRI has a wide range of applications in medical diagnosis and more than 25,000 scanners are estimated to be in use worldwide.[14] MRI affects diagnosis and treatment in many specialties although the effect on improved health outcomes is disputed in certain cases.[15][16]

MRI is the investigation of choice in the preoperative staging of rectal and prostate cancer and has a role in the diagnosis, staging, and follow-up of other tumors,[17] as well as for determining areas of tissue for sampling in biobanking.[18][19]


MRI diffusion tensor imaging of white matter tracts

MRI is the investigative tool of choice for neurological cancers over CT, as it offers better visualization of the posterior cranial fossa, containing the brainstem and the cerebellum. The contrast provided between grey and white matter makes MRI the best choice for many conditions of the central nervous system, including demyelinating diseases, dementia, cerebrovascular disease, infectious diseases, Alzheimer's disease and epilepsy.[20][21][22] Since many images are taken milliseconds apart, it shows how the brain responds to different stimuli, enabling researchers to study both the functional and structural brain abnormalities in psychological disorders.[23] MRI also is used in guided stereotactic surgery and radiosurgery for treatment of intracranial tumors, arteriovenous malformations, and other surgically treatable conditions using a device known as the N-localizer.[24][25][26]


MR angiogram in congenital heart disease

Cardiac MRI is complementary to other imaging techniques, such as echocardiography, cardiac CT, and nuclear medicine. It can be used to assess the structure and the function of the heart.[27] Its applications include assessment of myocardial ischemia and viability, cardiomyopathies, myocarditis, iron overload, vascular diseases, and congenital heart disease.[28]


Applications in the musculoskeletal system include spinal imaging, assessment of joint disease, and soft tissue tumors.[29] Also, MRI techniques can be used for diagnostic imaging of systemic muscle diseases.[30]

Liver and gastrointestinal



Magnetic resonance angiography


Techniques involving phase accumulation (known as phase contrast angiography) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method, the tissue is now excited inferiorly, while the signal is gathered in the plane immediately superior to the excitation plane--thus imaging the venous blood that recently moved from the excited plane.[36]

Contrast agents

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, exogenous contrast agents may be given intravenously, orally, or intra-articularly.[5] The most commonly used intravenous contrast agents are based on chelates of gadolinium.[37] In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare, occurring in approx. 0.03-0.1%.[38] 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 contrast-enhanced CT.[39]

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.[40] 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.[41] 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 dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.[42][43]

In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released.[44][45] In 2008, a new contrast agent named gadoxetate, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: This has the theoretical benefit of a dual excretion path.[46]


An MRI sequence is a particular setting of radiofrequency pulses and gradients, resulting in a particular image appearance.[47] The T1 and T2 weighting can also be described as MRI sequences.

Overview table

This table does not include uncommon and experimental sequences.

Group Sequence Abbr. Physics Main clinical distinctions Example
Spin echo T1 weighted T1 Measuring spin-lattice relaxation by using a short repetition time (TR) and echo time (TE).

Standard foundation and comparison for other sequences

T2 weighted T2 Measuring spin-spin relaxation by using long TR and TE times

Standard foundation and comparison for other sequences

Normal axial T2-weighted MR image of the brain.jpg
Proton density weighted PD Long TR (to reduce T1) and short TE (to minimize T2).[50] Joint disease and injury.[51] Proton density MRI of a grade 2 medial meniscal tear.jpg
Gradient echo (GRE) Steady-state free precession SSFP Maintenance of a steady, residual transverse magnetisation over successive cycles.[53] Creation of cardiac MRI videos (pictured).[53] Four chamber cardiovascular magnetic resonance imaging.gif
Effective T2
or "T2-star"
T2* Postexcitation refocused GRE with small flip angle.[54] Low signal from hemosiderin deposits (pictured) and hemorrhages.[54] Effective T2-weighted MRI of hemosiderin deposits after subarachnoid hemorrhage.png
Inversion recovery Short tau inversion recovery STIR Fat suppression by setting an inversion time where the signal of fat is zero.[55] High signal in edema, such as in more severe stress fracture.[56]Shin splints pictured: Shinsplint-mri (crop).jpg
Double inversion recovery DIR Simultaneous suppression of cerebrospinal fluid and white matter by two inversion times.[58] High signal of multiple sclerosis plaques (pictured).[58] Axial DIR MRI of a brain with multiple sclerosis lesions.jpg
Diffusion weighted (DWI) Conventional DWI Measure of Brownian motion of water molecules.[59] High signal within minutes of cerebral infarction (pictured).[60] Cerebral infarction after 4 hours on DWI MRI.jpg
Apparent diffusion coefficient ADC Reduced T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion.[61] Low signal minutes after cerebral infarction (pictured).[62] Cerebral infarction after 4 hours on ADC MRI.jpg
Diffusion tensor DTI Mainly tractography (pictured) by an overall greater Brownian motion of water molecules in the directions of nerve fibers.[63] White Matter Connections Obtained with MRI Tractography.png
Perfusion weighted (PWI) Dynamic susceptibility contrast DSC Gadolinium contrast is injected, and rapid repeated imaging (generally gradient-echo echo-planar T2 weighted) quantifies susceptibility-induced signal loss.[65] In cerebral infarction, the infarcted core and the penumbra have decreased perfusion (pictured).[66] Tmax by MRI perfusion in cerebral artery occlusion.jpg
Dynamic contrast enhanced DCE Measuring shortening of the spin-lattice relaxation (T1) induced by a gadolinium contrast bolus.[67]
Arterial spin labelling ASL Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest.[68] It does not need gadolinium contrast.[69]
Functional MRI (fMRI) Blood-oxygen-level dependent imaging BOLD Changes in oxygen saturation-dependent magnetism of hemoglobin reflects tissue activity.[70] Localizing highly active brain areas before surgery, also used in research of cognition.[71] 1206 FMRI.jpg
Phase-contrast magnetic resonance imaging PC-MRA Two gradients with equal magnitude, but opposite direction, are used to encode a phase shift, which is proportional to the velocity of spins.[73] Detection of aneurysm, stenosis, or dissection (pictured).[72] Vastly undersampled Isotropic Projection Reconstruction (VIPR) Phase Contrast (PC) sequence MRI of arterial dissections.jpg
Susceptibility-weighted SWI Sensitive for blood and calcium, by a fully flow compensated, long echo, gradient recalled echo (GRE) pulse sequence to exploit magnetic susceptibility differences between tissues Detecting small amounts of hemorrhage (diffuse axonal injury pictured) or calcium.[74] Susceptibility weighted imaging (SWI) in diffuse axonal injury.jpg

Other specialized configurations

Magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is used to measure the levels of different metabolites in body tissues, which can be achieved through a variety of single voxel or imaging-based techniques.[75] The MR signal produces a spectrum of resonances that corresponds to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain,[76] and to provide information on tumor metabolism.[77]

Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient. The spatial resolution is much lower (limited by the available SNR), but the spectra in each voxel contains information about many metabolites. Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR achievable only at higher field strengths (3 T and above).[78] The high procurement and maintenance costs of MRI with extremely high field strengths[79] inhibit their popularity. However, recent compressed sensing-based software algorithms (e.g., SAMV[80]) have been proposed to achieve super-resolution without requiring such high field strengths.

Real-time MRI


Real-time MRI refers to the continuous imaging of moving objects (such as the heart) in real time. One of the many different strategies developed since the early 2000s is based on radial FLASH MRI, and iterative reconstruction. This gives a temporal resolution of 20-30 ms for images with an in-plane resolution of 1.5-2.0 mm.[81] Balanced steady-state free precession (bSSFP) imaging has a better image contrast between the blood pool and myocardium than the FLASH MRI, yet it will produce severe banding artifact when the B0 inhomogeneity is strong. Real-time MRI is likely to add important information on diseases of the heart and the joints, and in many cases may make MRI examinations easier and more comfortable for patients, especially for the patients who cannot hold their breathings or who have arrhythmia.[82]

Interventional MRI



Magnetic resonance guided focused ultrasound


Multinuclear imaging


Moreover, the nucleus of any atom that has a net nuclear spin and that is bonded to a hydrogen atom could potentially be imaged via heteronuclear magnetization transfer MRI that would image the high-gyromagnetic-ratio hydrogen nucleus instead of the low-gyromagnetic-ratio nucleus that is bonded to the hydrogen atom.[88] In principle, hetereonuclear magnetization transfer MRI could be used to detect the presence or absence of specific chemical bonds.[89][90]


Molecular imaging by MRI

MRI has the advantages of having very high spatial resolution and is very adept at morphological imaging and functional imaging. MRI does have several disadvantages though. First, MRI has a sensitivity of around 10-3mol/L to 10-5 mol/L, which, compared to other types of imaging, can be very limiting. This problem stems from the fact that the population difference between the nuclear spin states is very small at room temperature. For example, at 1.5 teslas, a typical field strength for clinical MRI, the difference between high and low energy states is approximately 9 molecules per 2 million. Improvements to increase MR sensitivity include increasing magnetic field strength and hyperpolarization via optical pumping or dynamic nuclear polarization. There are also a variety of signal amplification schemes based on chemical exchange that increase sensitivity.[92]

To achieve molecular imaging of disease biomarkers using MRI, targeted MRI contrast agents with high specificity and high relaxivity (sensitivity) are required. To date, many studies have been devoted to developing targeted-MRI contrast agents to achieve molecular imaging by MRI. Commonly, peptides, antibodies, or small ligands, and small protein domains, such as HER-2 affibodies, have been applied to achieve targeting. To enhance the sensitivity of the contrast agents, these targeting moieties are usually linked to high payload MRI contrast agents or MRI contrast agents with high relaxivities.[93] A new class of gene targeting MR contrast agents (CA) has been introduced to show gene action of unique mRNA and gene transcription factor proteins.[94][95] This new CA can trace cells with unique mRNA, microRNA and virus; tissue response to inflammation in living brains.[96] The MR reports change in gene expression with positive correlation to TaqMan analysis, optical and electron microscopy.[97]


MRI is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error.[98]Contraindications to MRI include most cochlear implants and cardiac pacemakers, shrapnel, and metallic foreign bodies in the eyes. Magnetic resonance imaging in pregnancy appears to be safe at least during the second and third trimesters if done without contrast agents.[99] Since MRI does not use any ionizing radiation, its use is generally favored in preference to CT when either modality could yield the same information.[100] Some patients experience claustrophobia and may require sedation [101]

MRI uses powerful magnets and can therefore cause magnetic materials to move at great speeds posing a projectile risk. Deaths have occurred.[102] However, as millions of MRIs are performed globally each year,[103] fatalities are extremely rare.[104]




Motion artifact (T1 coronal study of cervical vertebrae)[105]


Non-medical use


Being non-invasive and non-damaging, MRI can be used to study the anatomy of plants, their water transportation processes and water balance.[107] It is also applied to veterinary radiology for diagnostic purposes. Outside this, its use in zoology is limited due to the high cost; but it can be used on many species.[108] In palaeontology it is used to examine the structure of fossils by getting their three-dimensional geometry.[109]



Advances in semiconductor technology were crucial to the development of practical MRI, which requires a large amount of computational power. This was made possible by the rapidly increasing number of transistors on a single integrated circuit chip.[112] Mansfield and Lauterbur were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discoveries concerning magnetic resonance imaging".[113]

See also


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