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Magnetic resonance imaging ( MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong , magnetic field gradients, and to form images of the organs in the body. MRI does not involve X-rays or the use of ionizing radiation, which distinguishes it from CT scan (CT) and positron emission tomography (PET) scans. MRI is a medical application of nuclear magnetic resonance (NMR) which can also be used for imaging in other NMR applications, such as NMR spectroscopy.
MRI is widely used in hospitals and clinics for medical diagnosis, cancer staging and follow-up of disease. Compared to CT, MRI provides better contrast in images of soft tissues, e.g. in the brain or abdomen. However, it may be perceived as less comfortable by patients, due to the usually longer and louder measurements with the subject in a long, confining tube, although "open" MRI designs mostly relieve this. Additionally, implants and other non-removable metal in the body can pose a risk and may exclude some patients from undergoing an MRI examination safely.
MRI was originally called NMRI (nuclear magnetic resonance imaging), but "nuclear" was dropped to avoid Radiophobia. (2026). 9781139457194, Cambridge University Press. ISBN 9781139457194 Certain Atomic nucleus are able to absorb radio frequency (RF) energy when placed in an external magnetic field; the resultant evolving spin polarization can induce an RF signal in a radio frequency coil and thereby be detected. In other words, the nuclear magnetic spin of protons in the hydrogen nuclei resonates with the RF incident waves and emit coherent radiation with compact direction, energy (frequency) and phase. This coherent amplified radiation is then detected by RF antennas close to the subject being examined. It is a process similar to masers. In clinical and research MRI, hydrogen are most often used to generate a macroscopic polarized radiation that is detected by the antennas. Hydrogen atoms are naturally abundant in humans and other biological organisms, particularly in water and Lipid. 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, such as Mummy. Diffusion MRI and functional MRI extend the utility of MRI to capture neuronal tracts and blood flow respectively in the nervous system, in addition to detailed spatial images. The sustained increase in demand for MRI within has led to concerns about cost effectiveness and overdiagnosis. (2026). 9789264075559 ISBN 9789264075559
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 oscillation 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 by a Radio frequency pulse and the resultant signal is measured by one or more receiving coils. 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. Exogeny may be given to the person to make the image clearer. (2026). 9780521683845, Cambridge University Press. ISBN 9780521683845
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. 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. Whole-body MRI systems for research applications operate in e.g. 9.4T, 10.5T, 11.7T. Even higher field whole-body MRI systems e.g. 14 T and beyond are in conceptual proposal or in engineering design. Most clinical magnets are 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 patients. Lower field strengths are also used in a portable MRI scanner approved by the FDA in 2020. 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 ().
To create a T2-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, and assessing zonal anatomy in the prostate and uterus.
The information from MRI scans comes in the form of image contrasts based on differences in the rate of relaxation of nuclear spins following their perturbation by an oscillating magnetic field (in the form of radiofrequency pulses through the sample). 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, B0, such that the in the sample will, on average, align with the z-axis summing to a total magnetization Mz. 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 B0, 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, Mz, is referred to as the longitudinal relaxation time, T1. Subsequently, the rate at which this happens is simply the reciprocal of the relaxation time: . Similarly, the time in which it takes for Mxy to return to zero is T2, with the rate . Magnetization as a function of time is defined by the Bloch equations.
T1 and T2 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:
| + Signals from different materials ! scope="col" | Signal ! scope="col" | T1-weighted ! scope="col" | T2-weighted |
MRI is the investigation of choice in the preoperative cancer staging of rectal and prostate cancer and has a role in the diagnosis, staging, and follow-up of other tumors, as well as for determining areas of tissue for sampling in biobanking.
The record for the highest spatial resolution of a whole intact brain (postmortem) is 100 microns, from Massachusetts General Hospital. The data was published in Nature in October 2019.
Though MRI is used widely in research on mental disabilities, based on a 2024 systematic literature review and meta analysis commissioned by the Patient-Centered Outcomes Research Institute (PCORI), available research using MRI scans to diagnose ADHD showed great variability. The authors conclude that MRI cannot be reliably used to assist in making a clinical diagnosis of ADHD.
Swallowing movements of the throat and esophagus can cause motion artifacts over the imaged spine. Therefore, a saturation pulse applied over this region can help to avoid these artifacts. Motion artifacts arising due to the pumping of the heart can be reduced by timing the MRI pulse according to heart cycles. Blood vessel flow artifacts can be reduced by applying saturation pulses above and below the region of interest.
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. (1999). 9780471351283, J. Wiley & Sons. ISBN 9780471351283
Gadolinium-based contrast reagents are typically polydentate complexes of gadolinium. The complex is very stable (log K > 20) so that, in use, the concentration of the un-complexed Gd3+ 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. (2026). 9783110685701, Walter de Gruyter. ISBN 9783110685701
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. 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. 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 should be performed as soon as possible after the scan to remove the agent from the body promptly.
In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released. In 2008, a new contrast agent named Gadoxetic acid, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: This has the theoretical benefit of a dual excretion path.
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). The high procurement and maintenance costs of MRI with extremely high field strengths inhibit their popularity. However, recent compressed sensing-based software algorithms ( e.g., SAMV) have been proposed to achieve super-resolution without requiring such high field strengths.
A specialized growing subset of interventional MRI is intraoperative MRI, in which an MRI is used in surgery. Some specialized MRI systems allow imaging concurrent with the surgical procedure. More typically, the surgical procedure is temporarily interrupted so that MRI can assess the success of the procedure or guide subsequent surgical work.
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. In principle, heteronuclear magnetization transfer MRI could be used to detect the presence or absence of specific chemical bonds.
Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1H MRI (e.g., lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized 3He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13C or stabilized bubbles of hyperpolarized 129Xe have been studied as contrast agents for angiography and perfusion imaging. 31P can potentially provide information on bone density and structure, as well as functional imaging of the brain. Multinuclear imaging holds the potential to chart the distribution of lithium in the human brain, this element finding use as an important drug for those with conditions such as bipolar disorder.
To achieve molecular imaging of disease biomarkers using MRI, targeted MRI 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. A new class of gene targeting MR contrast agents has been introduced to show gene action of unique mRNA and gene transcription factor proteins. These new contrast agents can trace cells with unique mRNA, microRNA and virus; tissue response to inflammation in living brains. The MR reports change in gene expression with positive correlation to TaqMan analysis, optical and electron microscopy.
After a number of early suggestions for using arrays of detectors to accelerate imaging went largely unremarked in the MRI field, parallel imaging saw widespread development and application following the introduction of the simultaneous acquisition of spatial harmonics (SMASH) technique in 1996–7. The sensitivity encoding (SENSE) and generalized autocalibrating partially parallel acquisitions (GRAPPA) techniques are the parallel imaging methods in most common use today. The advent of parallel MRI resulted in extensive research and development in image reconstruction and RF coil design, as well as in a rapid expansion of the number of receiver channels available on commercial MR systems. Parallel MRI is now used routinely for MRI examinations in a wide range of body areas and clinical or research applications.
Examples of quantitative MRI methods are:
Quantitative MRI aims to increase the reproducibility of MR images and interpretations, but has historically require longer scan times.
Quantitative MRI (or qMRI) sometimes more specifically refers to multi-parametric quantitative MRI, the mapping of multiple tissue relaxometry parameters in a single imaging session. Efforts to make multi-parametric quantitative MRI faster have produced sequences which map multiple parameters simultaneously, either by building separate encoding methods for each parameter into the sequence, or by fitting MR signal evolution to a multi-parameter model.
MRI uses powerful magnets and can therefore cause Ferromagnetism to move at great speeds, posing a projectile risk, and may cause fatal accidents. However, as millions of MRIs are performed globally each year, fatalities are extremely rare.
MRI machines can produce loud noise, up to 120 A-weighted. This can cause hearing loss, tinnitus and hyperacusis, so appropriate hearing protection is essential for anyone inside the MRI scanner room during the examination.
Being non-invasive and non-damaging, MRI can be used to study the anatomy of plants, their water transportation processes and water balance. 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.
In palaeontology it is used to examine the structure of fossils.
Forensic science imaging provides graphic documentation of an autopsy, which manual autopsy does not. CT scanning provides quick whole-body imaging of skeletal and alterations, whereas MR imaging gives better representation of soft tissue pathology. All that being said, MRI is more expensive, and more time-consuming to utilize. Moreover, the quality of MR imaging deteriorates below 10 °C.
Raymond Damadian's work into nuclear magnetic resonance (NMR) has been incorporated into MRI, having built one of the first scanners.
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. (2026). 9780199792955, Oxford University Press. ISBN 9780199792955 Mansfield and Lauterbur were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discoveries concerning magnetic resonance imaging".
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