Magnetic resonance imaging
"MRI" redirects here.
For other uses, see MRI (disambiguation).
|Magnetic resonance imaging|
|Synonyms||nuclear magnetic resonance imaging (NMRI), magnetic resonance tomography (MRT)|
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.
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.
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.
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.
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.
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.
Construction and physics
Main article: Physics of magnetic resonance imaging
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.
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 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.
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.
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.
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 (SQUIDs).
T1 and T2
Further information: Relaxation (NMR)
Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T1 (spin-lattice; that is, magnetization in the same direction as the static magnetic field) and T2 (spin-spin; transverse to the static magnetic field).
To create a T1-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 post-contrast imaging.
To create a T2-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the echo time (TE).
The standard display of MRI images is to represent fluid characteristics in black and white images, where different tissues turn out as follows:
|Inter- mediate||Gray matter darker than white matter||White matter darker than grey matter|
Usage by organ or system
MRI has a wide range of applications in medical diagnosis and more than 25,000 scanners are estimated to be in use worldwide.
MRI affects diagnosis and treatment in many specialties although the effect on improved health outcomes is disputed in certain cases.
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, as well as for determining areas of tissue for sampling in biobanking.
Main article: Magnetic resonance imaging of the brain
See also: Neuroimaging
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.
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.
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.
Main article: Cardiac magnetic resonance imaging
It can be used to assess the structure and the function of the heart.
Also, MRI techniques can be used for diagnostic imaging of systemic muscle diseases.
Liver and gastrointestinal
Extracellular contrast agents are used widely in liver MRI, and newer hepatobiliary contrast agents also provide the opportunity to perform functional biliary imaging.
Anatomical imaging of the bile ducts is achieved by using a heavily T2-weighted sequence in magnetic resonance cholangiopancreatography (MRCP).
Functional imaging of the pancreas is performed following administration of secretin.
MR enterography provides non-invasive assessment of inflammatory bowel disease and small bowel tumors.
MR-colonography may play a role in the detection of large polyps in patients at increased risk of colorectal cancer.
Main article: Magnetic resonance angiography
MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off").
A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium) or using a technique known as "flow-related enhancement" (e.g., 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood that recently moved into that plane (see also FLASH MRI).
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.
Main article: 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.
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.
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.
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 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 gadoxetate, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: This has the theoretical benefit of a dual excretion path.
Main article: MRI sequences
An MRI sequence is a particular setting of radiofrequency pulses and gradients, resulting in a particular image appearance.
The T1 and T2 weighting can also be described as MRI sequences.
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|
|Proton density weighted||PD||Long TR (to reduce T1) and short TE (to minimize T2).||Joint disease and injury.|
|Gradient echo (GRE)||Steady-state free precession||SSFP||Maintenance of a steady, residual transverse magnetisation over successive cycles.||Creation of cardiac MRI videos (pictured).|
|Effective T2 or "T2-star"||T2*||Postexcitation refocused GRE with small flip angle.||Low signal from hemosiderin deposits (pictured) and hemorrhages.|
|Inversion recovery||Short tau inversion recovery||STIR||Fat suppression by setting an inversion time where the signal of fat is zero.||High signal in edema, such as in more severe stress fracture. Shin splints pictured:|
|Fluid-attenuated inversion recovery||FLAIR||Fluid suppression by setting an inversion time that nulls fluids||High signal in lacunar infarction, multiple sclerosis (MS) plaques, subarachnoid haemorrhage and meningitis (pictured).|
|Double inversion recovery||DIR||Simultaneous suppression of cerebrospinal fluid and white matter by two inversion times.||High signal of multiple sclerosis plaques (pictured).|
|Diffusion weighted (DWI)||Conventional||DWI||Measure of Brownian motion of water molecules.||High signal within minutes of cerebral infarction (pictured).|
|Apparent diffusion coefficient||ADC||Reduced T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion.||Low signal minutes after cerebral infarction (pictured).|
|Diffusion tensor||DTI||Mainly tractography (pictured) by an overall greater Brownian motion of water molecules in the directions of nerve fibers.|
|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.||In cerebral infarction, the infarcted core and the penumbra have decreased perfusion (pictured).|
|Dynamic contrast enhanced||DCE||Measuring shortening of the spin–lattice relaxation (T1) induced by a gadolinium contrast bolus.|
|Arterial spin labelling||ASL||Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest. It does not need gadolinium contrast.|
|Functional MRI (fMRI)||Blood-oxygen-level dependent imaging||BOLD||Changes in oxygen saturation-dependent magnetism of hemoglobin reflects tissue activity.||Localizing highly active brain areas before surgery, also used in research of cognition.|
|Magnetic resonance angiography (MRA) and venography||Time-of-flight||TOF||Blood entering the imaged area is not yet magnetically saturated, giving it a much higher signal when using short echo time and flow compensation.||Detection of aneurysm, stenosis, or dissection|
|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.||Detection of aneurysm, stenosis, or dissection (pictured).||(VIPR)|
|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.|
Other specialized configurations
Magnetic resonance spectroscopy
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, and to provide information on tumor metabolism.
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.
Main article: Real-time MRI
Real-time MRI refers to the continuous imaging of moving objects (such as the heart) in real time.
This gives a temporal resolution of 20–30 ms for images with an in-plane resolution of 1.5–2.0 mm.
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.
Main article: Interventional magnetic resonance imaging
The lack of harmful effects on the patient and the operator make MRI well-suited for interventional radiology, where the images produced by an MRI scanner guide minimally invasive procedures.
Such procedures use no ferromagnetic instruments.
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.
Magnetic resonance guided focused ultrasound
In guided therapy, high-intensity focused ultrasound (HIFU) beams are focused on a tissue, that are controlled using MR thermal imaging.
Due to the high energy at the focus, the temperature rises to above 65 °C (150 °F) which completely destroys the tissue.
This technology can achieve precise ablation of diseased tissue.
MR imaging provides a three-dimensional view of the target tissue, allowing for the precise focusing of ultrasound energy.
The MR imaging provides quantitative, real-time, thermal images of the treated area.
This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment.
However, any nucleus with a net nuclear spin could potentially be imaged with MRI.
Na and P are naturally abundant in the body, so they can be imaged directly.
Gaseous isotopes such as He or Xe must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions.
Using helium or xenon has the advantage of reduced background noise, and therefore increased contrast for the image itself, because these elements are not normally present in biological tissues.
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, hetereonuclear 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 H MRI (e.g., lungs and bones) or as alternative contrast agents.
Inhaled hyperpolarized He can be used to image the distribution of air spaces within the lungs.
Injectable solutions containing C or stabilized bubbles of hyperpolarized Xe have been studied as contrast agents for angiography and perfusion imaging.
P 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.
Molecular imaging by MRI
Main article: Molecular imaging
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 mol/L to 10 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.
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.
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.
Main article: Safety of magnetic resonance imaging
MRI is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error.
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.
Some patients experience claustrophobia and may require sedation
MRI uses powerful magnets and can therefore cause magnetic materials to move at great speeds posing a projectile risk.
Deaths have occurred.
However, as millions of MRIs are performed globally each year, fatalities are extremely rare.
See also: Overdiagnosis
Medical societies issue guidelines for when physicians should use MRI on patients and recommend against overuse.
MRI can detect health problems or confirm a diagnosis, but medical societies often recommend that MRI not be the first procedure for creating a plan to diagnose or manage a patient's complaint.
A common case is to use MRI to seek a cause of low back pain; the American College of Physicians, for example, recommends against this procedure as unlikely to result in a positive outcome for the patient.
Main article: MRI artifact
Many different artifacts can occur during magnetic resonance imaging (MRI), some affecting the diagnostic quality, while others may be confused with pathology.
Artifacts can be classified as patient-related, signal processing-dependent and hardware (machine)-related.
Main article: Nuclear magnetic resonance § Applications
MRI is used industrially mainly for routine analysis of chemicals.
The nuclear magnetic resonance technique is also used, for example, to measure the ratio between water and fat in foods, monitoring of flow of corrosive fluids in pipes, or to study molecular structures such as catalysts.
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.
But MRI is more expensive, and more time-consuming to utilize.
Moreover, the quality of MR imaging deteriorates below 10 °C.
Main article: History of magnetic resonance imaging
In 1971 at Stony Brook University, Paul Lauterbur applied magnetic field gradients in all three dimensions and a back-projection technique to create NMR images.
He published the first images of two tubes of water in 1973 in the journal Nature, followed by the picture of a living animal, a clam, and in 1974 by the image of the thoracic cavity of a mouse.
Lauterbur called his imaging method zeugmatography, a term which was replaced by (N)MR imaging.
Mansfield and Lauterbur were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discoveries concerning magnetic resonance imaging".
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Magnetic resonance imaging.