Magnetic resonance imaging

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"MRI" redirects here. Magnetic resonance imaging_sentence_0

For other uses, see MRI (disambiguation). Magnetic resonance imaging_sentence_1

Magnetic resonance imaging_table_infobox_0

Magnetic resonance imagingMagnetic resonance imaging_header_cell_0_0_0
SynonymsMagnetic resonance imaging_header_cell_0_1_0 nuclear magnetic resonance imaging (NMRI), magnetic resonance tomography (MRT)Magnetic resonance imaging_cell_0_1_1
ICD-9-CMMagnetic resonance imaging_header_cell_0_2_0 Magnetic resonance imaging_cell_0_2_1
MeSHMagnetic resonance imaging_header_cell_0_3_0 Magnetic resonance imaging_cell_0_3_1
MedlinePlusMagnetic resonance imaging_header_cell_0_4_0 Magnetic resonance imaging_cell_0_4_1

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. Magnetic resonance imaging_sentence_2

MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves to generate images of the organs in the body. Magnetic resonance imaging_sentence_3

MRI does not involve X-rays or the use of ionizing radiation, which distinguishes it from CT and PET scans. Magnetic resonance imaging_sentence_4

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. Magnetic resonance imaging_sentence_5

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. Magnetic resonance imaging_sentence_6

MRI is widely used in hospitals and clinics for medical diagnosis and staging and follow-up of disease without exposing the body to radiation. Magnetic resonance imaging_sentence_7

An MRI may yield different information compared with CT. Risks and discomfort may be associated with MRI scans. Magnetic resonance imaging_sentence_8

Compared with CT scans, MRI scans typically take longer and are louder, and they usually need the subject to enter a narrow, confining tube. Magnetic resonance imaging_sentence_9

In addition, people with some medical implants or other non-removable metal inside the body may be unable to undergo an MRI examination safely. Magnetic resonance imaging_sentence_10

MRI was originally called NMRI (nuclear magnetic resonance imaging), but "nuclear" was dropped to avoid negative associations. Magnetic resonance imaging_sentence_11

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. Magnetic resonance imaging_sentence_12

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. Magnetic resonance imaging_sentence_13

Hydrogen atoms are naturally abundant in humans and other biological organisms, particularly in water and fat. Magnetic resonance imaging_sentence_14

For this reason, most MRI scans essentially map the location of water and fat in the body. Magnetic resonance imaging_sentence_15

Pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the polarization in space. Magnetic resonance imaging_sentence_16

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. Magnetic resonance imaging_sentence_17

Since its development in the 1970s and 1980s, MRI has proven to be a versatile imaging technique. Magnetic resonance imaging_sentence_18

While MRI is most prominently used in diagnostic medicine and biomedical research, it also may be used to form images of non-living objects. Magnetic resonance imaging_sentence_19

MRI scans are capable of producing a variety of chemical and physical data, in addition to detailed spatial images. Magnetic resonance imaging_sentence_20

The sustained increase in demand for MRI within health systems has led to concerns about cost effectiveness and overdiagnosis. Magnetic resonance imaging_sentence_21

Mechanism Magnetic resonance imaging_section_0

Construction and physics Magnetic resonance imaging_section_1

Main article: Physics of magnetic resonance imaging Magnetic resonance imaging_sentence_22

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. Magnetic resonance imaging_sentence_23

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. Magnetic resonance imaging_sentence_24

To perform a study, the person is positioned within an MRI scanner that forms a strong magnetic field around the area to be imaged. Magnetic resonance imaging_sentence_25

First, energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonance frequency. Magnetic resonance imaging_sentence_26

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. Magnetic resonance imaging_sentence_27

The atoms emit a radio frequency (RF) signal, which is measured by a receiving coil. Magnetic resonance imaging_sentence_28

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. Magnetic resonance imaging_sentence_29

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. Magnetic resonance imaging_sentence_30

The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. Magnetic resonance imaging_sentence_31

Exogenous contrast agents may be given to the person to make the image clearer. Magnetic resonance imaging_sentence_32

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. Magnetic resonance imaging_sentence_33

The whole system is controlled by one or more computers. Magnetic resonance imaging_sentence_34

MRI requires a magnetic field that is both strong and uniform to a few parts per million across the scan volume. Magnetic resonance imaging_sentence_35

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. Magnetic resonance imaging_sentence_36

Lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for claustrophobic patients. Magnetic resonance imaging_sentence_37

Lower field strengths are also used in a portable MRI scanner approved by the FDA in 2020. Magnetic resonance imaging_sentence_38

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). Magnetic resonance imaging_sentence_39

T1 and T2 Magnetic resonance imaging_section_2

Further information: Relaxation (NMR) Magnetic resonance imaging_sentence_40

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). Magnetic resonance imaging_sentence_41

To create a T1-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the repetition time (TR). Magnetic resonance imaging_sentence_42

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. Magnetic resonance imaging_sentence_43

To create a T2-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the echo time (TE). Magnetic resonance imaging_sentence_44

This image weighting is useful for detecting edema and inflammation, revealing white matter lesions, and assessing zonal anatomy in the prostate and uterus. Magnetic resonance imaging_sentence_45

The standard display of MRI images is to represent fluid characteristics in black and white images, where different tissues turn out as follows: Magnetic resonance imaging_sentence_46

Magnetic resonance imaging_table_general_1

SignalMagnetic resonance imaging_header_cell_1_0_0 T1-weightedMagnetic resonance imaging_header_cell_1_0_1 T2-weightedMagnetic resonance imaging_header_cell_1_0_2
HighMagnetic resonance imaging_cell_1_1_0 Magnetic resonance imaging_cell_1_1_1 Magnetic resonance imaging_cell_1_1_2
Inter- mediateMagnetic resonance imaging_cell_1_2_0 Gray matter darker than white matterMagnetic resonance imaging_cell_1_2_1 White matter darker than grey matterMagnetic resonance imaging_cell_1_2_2
LowMagnetic resonance imaging_cell_1_3_0 Magnetic resonance imaging_cell_1_3_1 Magnetic resonance imaging_cell_1_3_2

Diagnostics Magnetic resonance imaging_section_3

Usage by organ or system Magnetic resonance imaging_section_4

MRI has a wide range of applications in medical diagnosis and more than 25,000 scanners are estimated to be in use worldwide. Magnetic resonance imaging_sentence_47

MRI affects diagnosis and treatment in many specialties although the effect on improved health outcomes is disputed in certain cases. Magnetic resonance imaging_sentence_48

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. Magnetic resonance imaging_sentence_49

Neuroimaging Magnetic resonance imaging_section_5

Main article: Magnetic resonance imaging of the brain Magnetic resonance imaging_sentence_50

See also: Neuroimaging Magnetic resonance imaging_sentence_51

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. Magnetic resonance imaging_sentence_52

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. Magnetic resonance imaging_sentence_53

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. Magnetic resonance imaging_sentence_54

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. Magnetic resonance imaging_sentence_55

Cardiovascular Magnetic resonance imaging_section_6

Main article: Cardiac magnetic resonance imaging Magnetic resonance imaging_sentence_56

Cardiac MRI is complementary to other imaging techniques, such as echocardiography, cardiac CT, and nuclear medicine. Magnetic resonance imaging_sentence_57

It can be used to assess the structure and the function of the heart. Magnetic resonance imaging_sentence_58

Its applications include assessment of myocardial ischemia and viability, cardiomyopathies, myocarditis, iron overload, vascular diseases, and congenital heart disease. Magnetic resonance imaging_sentence_59

Musculoskeletal Magnetic resonance imaging_section_7

Applications in the musculoskeletal system include spinal imaging, assessment of joint disease, and soft tissue tumors. Magnetic resonance imaging_sentence_60

Also, MRI techniques can be used for diagnostic imaging of systemic muscle diseases. Magnetic resonance imaging_sentence_61

Liver and gastrointestinal Magnetic resonance imaging_section_8

Hepatobiliary MR is used to detect and characterize lesions of the liver, pancreas, and bile ducts. Magnetic resonance imaging_sentence_62

Focal or diffuse disorders of the liver may be evaluated using diffusion-weighted, opposed-phase imaging and dynamic contrast enhancement sequences. Magnetic resonance imaging_sentence_63

Extracellular contrast agents are used widely in liver MRI, and newer hepatobiliary contrast agents also provide the opportunity to perform functional biliary imaging. Magnetic resonance imaging_sentence_64

Anatomical imaging of the bile ducts is achieved by using a heavily T2-weighted sequence in magnetic resonance cholangiopancreatography (MRCP). Magnetic resonance imaging_sentence_65

Functional imaging of the pancreas is performed following administration of secretin. Magnetic resonance imaging_sentence_66

MR enterography provides non-invasive assessment of inflammatory bowel disease and small bowel tumors. Magnetic resonance imaging_sentence_67

MR-colonography may play a role in the detection of large polyps in patients at increased risk of colorectal cancer. Magnetic resonance imaging_sentence_68

Angiography Magnetic resonance imaging_section_9

Main article: Magnetic resonance angiography Magnetic resonance imaging_sentence_69

Magnetic resonance angiography (MRA) generates pictures of the arteries to evaluate them for stenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture). Magnetic resonance imaging_sentence_70

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"). Magnetic resonance imaging_sentence_71

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). Magnetic resonance imaging_sentence_72

Techniques involving phase accumulation (known as phase contrast angiography) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance imaging_sentence_73

Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. Magnetic resonance imaging_sentence_74

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. Magnetic resonance imaging_sentence_75

Contrast agents Magnetic resonance imaging_section_10

Main article: MRI contrast agent Magnetic resonance imaging_sentence_76

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. Magnetic resonance imaging_sentence_77

However, for more specific types of imaging, exogenous contrast agents may be given intravenously, orally, or intra-articularly. Magnetic resonance imaging_sentence_78

The most commonly used intravenous contrast agents are based on chelates of gadolinium. Magnetic resonance imaging_sentence_79

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. Magnetic resonance imaging_sentence_80

0.03–0.1%. Magnetic resonance imaging_sentence_81

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. Magnetic resonance imaging_sentence_82

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). Magnetic resonance imaging_sentence_83

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. Magnetic resonance imaging_sentence_84

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. Magnetic resonance imaging_sentence_85

The most frequently linked is gadodiamide, but other agents have been linked too. Magnetic resonance imaging_sentence_86

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. Magnetic resonance imaging_sentence_87

In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released. Magnetic resonance imaging_sentence_88

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. Magnetic resonance imaging_sentence_89

Sequences Magnetic resonance imaging_section_11

Main article: MRI sequences Magnetic resonance imaging_sentence_90

An MRI sequence is a particular setting of radiofrequency pulses and gradients, resulting in a particular image appearance. Magnetic resonance imaging_sentence_91

The T1 and T2 weighting can also be described as MRI sequences. Magnetic resonance imaging_sentence_92

Overview table Magnetic resonance imaging_section_12

This table does not include uncommon and experimental sequences. Magnetic resonance imaging_sentence_93

Magnetic resonance imaging_table_general_2

GroupMagnetic resonance imaging_header_cell_2_0_0 SequenceMagnetic resonance imaging_header_cell_2_0_1 Abbr.Magnetic resonance imaging_header_cell_2_0_2 PhysicsMagnetic resonance imaging_header_cell_2_0_3 Main clinical distinctionsMagnetic resonance imaging_header_cell_2_0_4 ExampleMagnetic resonance imaging_header_cell_2_0_5
Spin echoMagnetic resonance imaging_cell_2_1_0 T1 weightedMagnetic resonance imaging_cell_2_1_1 T1Magnetic resonance imaging_cell_2_1_2 Measuring spin–lattice relaxation by using a short repetition time (TR) and echo time (TE).Magnetic resonance imaging_cell_2_1_3 Standard foundation and comparison for other sequencesMagnetic resonance imaging_cell_2_1_4 Magnetic resonance imaging_cell_2_1_5
T2 weightedMagnetic resonance imaging_cell_2_2_0 T2Magnetic resonance imaging_cell_2_2_1 Measuring spin–spin relaxation by using long TR and TE timesMagnetic resonance imaging_cell_2_2_2 Standard foundation and comparison for other sequencesMagnetic resonance imaging_cell_2_2_3 Magnetic resonance imaging_cell_2_2_4
Proton density weightedMagnetic resonance imaging_cell_2_3_0 PDMagnetic resonance imaging_cell_2_3_1 Long TR (to reduce T1) and short TE (to minimize T2).Magnetic resonance imaging_cell_2_3_2 Joint disease and injury.Magnetic resonance imaging_cell_2_3_3 Magnetic resonance imaging_cell_2_3_4
Gradient echo (GRE)Magnetic resonance imaging_cell_2_4_0 Steady-state free precessionMagnetic resonance imaging_cell_2_4_1 SSFPMagnetic resonance imaging_cell_2_4_2 Maintenance of a steady, residual transverse magnetisation over successive cycles.Magnetic resonance imaging_cell_2_4_3 Creation of cardiac MRI videos (pictured).Magnetic resonance imaging_cell_2_4_4 Magnetic resonance imaging_cell_2_4_5
Effective T2 or "T2-star"Magnetic resonance imaging_cell_2_5_0 T2*Magnetic resonance imaging_cell_2_5_1 Postexcitation refocused GRE with small flip angle.Magnetic resonance imaging_cell_2_5_2 Low signal from hemosiderin deposits (pictured) and hemorrhages.Magnetic resonance imaging_cell_2_5_3 Magnetic resonance imaging_cell_2_5_4
Inversion recoveryMagnetic resonance imaging_cell_2_6_0 Short tau inversion recoveryMagnetic resonance imaging_cell_2_6_1 STIRMagnetic resonance imaging_cell_2_6_2 Fat suppression by setting an inversion time where the signal of fat is zero.Magnetic resonance imaging_cell_2_6_3 High signal in edema, such as in more severe stress fracture. Shin splints pictured:Magnetic resonance imaging_cell_2_6_4 Magnetic resonance imaging_cell_2_6_5
Fluid-attenuated inversion recoveryMagnetic resonance imaging_cell_2_7_0 FLAIRMagnetic resonance imaging_cell_2_7_1 Fluid suppression by setting an inversion time that nulls fluidsMagnetic resonance imaging_cell_2_7_2 High signal in lacunar infarction, multiple sclerosis (MS) plaques, subarachnoid haemorrhage and meningitis (pictured).Magnetic resonance imaging_cell_2_7_3 Magnetic resonance imaging_cell_2_7_4
Double inversion recoveryMagnetic resonance imaging_cell_2_8_0 DIRMagnetic resonance imaging_cell_2_8_1 Simultaneous suppression of cerebrospinal fluid and white matter by two inversion times.Magnetic resonance imaging_cell_2_8_2 High signal of multiple sclerosis plaques (pictured).Magnetic resonance imaging_cell_2_8_3 Magnetic resonance imaging_cell_2_8_4
Diffusion weighted (DWI)Magnetic resonance imaging_cell_2_9_0 ConventionalMagnetic resonance imaging_cell_2_9_1 DWIMagnetic resonance imaging_cell_2_9_2 Measure of Brownian motion of water molecules.Magnetic resonance imaging_cell_2_9_3 High signal within minutes of cerebral infarction (pictured).Magnetic resonance imaging_cell_2_9_4 Magnetic resonance imaging_cell_2_9_5
Apparent diffusion coefficientMagnetic resonance imaging_cell_2_10_0 ADCMagnetic resonance imaging_cell_2_10_1 Reduced T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion.Magnetic resonance imaging_cell_2_10_2 Low signal minutes after cerebral infarction (pictured).Magnetic resonance imaging_cell_2_10_3 Magnetic resonance imaging_cell_2_10_4
Diffusion tensorMagnetic resonance imaging_cell_2_11_0 DTIMagnetic resonance imaging_cell_2_11_1 Mainly tractography (pictured) by an overall greater Brownian motion of water molecules in the directions of nerve fibers.Magnetic resonance imaging_cell_2_11_2 Magnetic resonance imaging_cell_2_11_3 Magnetic resonance imaging_cell_2_11_4
Perfusion weighted (PWI)Magnetic resonance imaging_cell_2_12_0 Dynamic susceptibility contrastMagnetic resonance imaging_cell_2_12_1 DSCMagnetic resonance imaging_cell_2_12_2 Gadolinium contrast is injected, and rapid repeated imaging (generally gradient-echo echo-planar T2 weighted) quantifies susceptibility-induced signal loss.Magnetic resonance imaging_cell_2_12_3 In cerebral infarction, the infarcted core and the penumbra have decreased perfusion (pictured).Magnetic resonance imaging_cell_2_12_4 Magnetic resonance imaging_cell_2_12_5
Dynamic contrast enhancedMagnetic resonance imaging_cell_2_13_0 DCEMagnetic resonance imaging_cell_2_13_1 Measuring shortening of the spin–lattice relaxation (T1) induced by a gadolinium contrast bolus.Magnetic resonance imaging_cell_2_13_2
Arterial spin labellingMagnetic resonance imaging_cell_2_14_0 ASLMagnetic resonance imaging_cell_2_14_1 Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest. It does not need gadolinium contrast.Magnetic resonance imaging_cell_2_14_2
Functional MRI (fMRI)Magnetic resonance imaging_cell_2_15_0 Blood-oxygen-level dependent imagingMagnetic resonance imaging_cell_2_15_1 BOLDMagnetic resonance imaging_cell_2_15_2 Changes in oxygen saturation-dependent magnetism of hemoglobin reflects tissue activity.Magnetic resonance imaging_cell_2_15_3 Localizing highly active brain areas before surgery, also used in research of cognition.Magnetic resonance imaging_cell_2_15_4 Magnetic resonance imaging_cell_2_15_5
Magnetic resonance angiography (MRA) and venographyMagnetic resonance imaging_cell_2_16_0 Time-of-flightMagnetic resonance imaging_cell_2_16_1 TOFMagnetic resonance imaging_cell_2_16_2 Blood entering the imaged area is not yet magnetically saturated, giving it a much higher signal when using short echo time and flow compensation.Magnetic resonance imaging_cell_2_16_3 Detection of aneurysm, stenosis, or dissectionMagnetic resonance imaging_cell_2_16_4 Magnetic resonance imaging_cell_2_16_5
Phase-contrast magnetic resonance imagingMagnetic resonance imaging_cell_2_17_0 PC-MRAMagnetic resonance imaging_cell_2_17_1 Two gradients with equal magnitude, but opposite direction, are used to encode a phase shift, which is proportional to the velocity of spins.Magnetic resonance imaging_cell_2_17_2 Detection of aneurysm, stenosis, or dissection (pictured).Magnetic resonance imaging_cell_2_17_3 (VIPR)Magnetic resonance imaging_cell_2_17_4
Susceptibility-weightedMagnetic resonance imaging_cell_2_18_0 SWIMagnetic resonance imaging_cell_2_18_2 Sensitive for blood and calcium, by a fully flow compensated, long echo, gradient recalled echo (GRE) pulse sequence to exploit magnetic susceptibility differences between tissuesMagnetic resonance imaging_cell_2_18_3 Detecting small amounts of hemorrhage (diffuse axonal injury pictured) or calcium.Magnetic resonance imaging_cell_2_18_4 Magnetic resonance imaging_cell_2_18_5

Other specialized configurations Magnetic resonance imaging_section_13

Magnetic resonance spectroscopy Magnetic resonance imaging_section_14

Main articles: In vivo magnetic resonance spectroscopy and Nuclear magnetic resonance spectroscopy Magnetic resonance imaging_sentence_94

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. Magnetic resonance imaging_sentence_95

The MR signal produces a spectrum of resonances that corresponds to different molecular arrangements of the isotope being "excited". Magnetic resonance imaging_sentence_96

This signature is used to diagnose certain metabolic disorders, especially those affecting the brain, and to provide information on tumor metabolism. Magnetic resonance imaging_sentence_97

Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient. Magnetic resonance imaging_sentence_98

The spatial resolution is much lower (limited by the available SNR), but the spectra in each voxel contains information about many metabolites. Magnetic resonance imaging_sentence_99

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). Magnetic resonance imaging_sentence_100

The high procurement and maintenance costs of MRI with extremely high field strengths inhibit their popularity. Magnetic resonance imaging_sentence_101

However, recent compressed sensing-based software algorithms (e.g., SAMV) have been proposed to achieve super-resolution without requiring such high field strengths. Magnetic resonance imaging_sentence_102

Real-time MRI Magnetic resonance imaging_section_15

Main article: Real-time MRI Magnetic resonance imaging_sentence_103

Real-time MRI refers to the continuous imaging of moving objects (such as the heart) in real time. Magnetic resonance imaging_sentence_104

One of the many different strategies developed since the early 2000s is based on radial FLASH MRI, and iterative reconstruction. Magnetic resonance imaging_sentence_105

This gives a temporal resolution of 20–30 ms for images with an in-plane resolution of 1.5–2.0 mm. Magnetic resonance imaging_sentence_106

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. Magnetic resonance imaging_sentence_107

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. Magnetic resonance imaging_sentence_108

Interventional MRI Magnetic resonance imaging_section_16

Main article: Interventional magnetic resonance imaging Magnetic resonance imaging_sentence_109

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. Magnetic resonance imaging_sentence_110

Such procedures use no ferromagnetic instruments. Magnetic resonance imaging_sentence_111

A specialized growing subset of interventional MRI is intraoperative MRI, in which an MRI is used in surgery. Magnetic resonance imaging_sentence_112

Some specialized MRI systems allow imaging concurrent with the surgical procedure. Magnetic resonance imaging_sentence_113

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 imaging_sentence_114

Magnetic resonance guided focused ultrasound Magnetic resonance imaging_section_17

In guided therapy, high-intensity focused ultrasound (HIFU) beams are focused on a tissue, that are controlled using MR thermal imaging. Magnetic resonance imaging_sentence_115

Due to the high energy at the focus, the temperature rises to above 65 °C (150 °F) which completely destroys the tissue. Magnetic resonance imaging_sentence_116

This technology can achieve precise ablation of diseased tissue. Magnetic resonance imaging_sentence_117

MR imaging provides a three-dimensional view of the target tissue, allowing for the precise focusing of ultrasound energy. Magnetic resonance imaging_sentence_118

The MR imaging provides quantitative, real-time, thermal images of the treated area. Magnetic resonance imaging_sentence_119

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. Magnetic resonance imaging_sentence_120

Multinuclear imaging Magnetic resonance imaging_section_18

Hydrogen has the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance, and because its high gyromagnetic ratio gives a strong signal. Magnetic resonance imaging_sentence_121

However, any nucleus with a net nuclear spin could potentially be imaged with MRI. Magnetic resonance imaging_sentence_122

Such nuclei include helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. Magnetic resonance imaging_sentence_123

Na and P are naturally abundant in the body, so they can be imaged directly. Magnetic resonance imaging_sentence_124

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. Magnetic resonance imaging_sentence_125

O and F can be administered in sufficient quantities in liquid form (e.g. O-water) that hyperpolarization is not a necessity. Magnetic resonance imaging_sentence_126

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. Magnetic resonance imaging_sentence_127

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. Magnetic resonance imaging_sentence_128

In principle, hetereonuclear magnetization transfer MRI could be used to detect the presence or absence of specific chemical bonds. Magnetic resonance imaging_sentence_129

Multinuclear imaging is primarily a research technique at present. Magnetic resonance imaging_sentence_130

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. Magnetic resonance imaging_sentence_131

Inhaled hyperpolarized He can be used to image the distribution of air spaces within the lungs. Magnetic resonance imaging_sentence_132

Injectable solutions containing C or stabilized bubbles of hyperpolarized Xe have been studied as contrast agents for angiography and perfusion imaging. Magnetic resonance imaging_sentence_133

P can potentially provide information on bone density and structure, as well as functional imaging of the brain. Magnetic resonance imaging_sentence_134

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. Magnetic resonance imaging_sentence_135

Molecular imaging by MRI Magnetic resonance imaging_section_19

Main article: Molecular imaging Magnetic resonance imaging_sentence_136

MRI has the advantages of having very high spatial resolution and is very adept at morphological imaging and functional imaging. Magnetic resonance imaging_sentence_137

MRI does have several disadvantages though. Magnetic resonance imaging_sentence_138

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. Magnetic resonance imaging_sentence_139

This problem stems from the fact that the population difference between the nuclear spin states is very small at room temperature. Magnetic resonance imaging_sentence_140

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. Magnetic resonance imaging_sentence_141

Improvements to increase MR sensitivity include increasing magnetic field strength and hyperpolarization via optical pumping or dynamic nuclear polarization. Magnetic resonance imaging_sentence_142

There are also a variety of signal amplification schemes based on chemical exchange that increase sensitivity. Magnetic resonance imaging_sentence_143

To achieve molecular imaging of disease biomarkers using MRI, targeted MRI contrast agents with high specificity and high relaxivity (sensitivity) are required. Magnetic resonance imaging_sentence_144

To date, many studies have been devoted to developing targeted-MRI contrast agents to achieve molecular imaging by MRI. Magnetic resonance imaging_sentence_145

Commonly, peptides, antibodies, or small ligands, and small protein domains, such as HER-2 affibodies, have been applied to achieve targeting. Magnetic resonance imaging_sentence_146

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. Magnetic resonance imaging_sentence_147

A new class of gene targeting MR contrast agents has been introduced to show gene action of unique mRNA and gene transcription factor proteins. Magnetic resonance imaging_sentence_148

These new contrast agents can trace cells with unique mRNA, microRNA and virus; tissue response to inflammation in living brains. Magnetic resonance imaging_sentence_149

The MR reports change in gene expression with positive correlation to TaqMan analysis, optical and electron microscopy. Magnetic resonance imaging_sentence_150

Safety Magnetic resonance imaging_section_20

Main article: Safety of magnetic resonance imaging Magnetic resonance imaging_sentence_151

MRI is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error. Magnetic resonance imaging_sentence_152

Contraindications to MRI include most cochlear implants and cardiac pacemakers, shrapnel, and metallic foreign bodies in the eyes. Magnetic resonance imaging_sentence_153

Magnetic resonance imaging in pregnancy appears to be safe at least during the second and third trimesters if done without contrast agents. Magnetic resonance imaging_sentence_154

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. Magnetic resonance imaging_sentence_155

Some patients experience claustrophobia and may require sedation Magnetic resonance imaging_sentence_156

MRI uses powerful magnets and can therefore cause magnetic materials to move at great speeds posing a projectile risk. Magnetic resonance imaging_sentence_157

Deaths have occurred. Magnetic resonance imaging_sentence_158

However, as millions of MRIs are performed globally each year, fatalities are extremely rare. Magnetic resonance imaging_sentence_159

Overuse Magnetic resonance imaging_section_21

See also: Overdiagnosis Magnetic resonance imaging_sentence_160

Medical societies issue guidelines for when physicians should use MRI on patients and recommend against overuse. Magnetic resonance imaging_sentence_161

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. Magnetic resonance imaging_sentence_162

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. Magnetic resonance imaging_sentence_163

Artifacts Magnetic resonance imaging_section_22

Main article: MRI artifact Magnetic resonance imaging_sentence_164

An MRI artifact is a visual artifact, that is, an anomaly during visual representation. Magnetic resonance imaging_sentence_165

Many different artifacts can occur during magnetic resonance imaging (MRI), some affecting the diagnostic quality, while others may be confused with pathology. Magnetic resonance imaging_sentence_166

Artifacts can be classified as patient-related, signal processing-dependent and hardware (machine)-related. Magnetic resonance imaging_sentence_167

Non-medical use Magnetic resonance imaging_section_23

Main article: Nuclear magnetic resonance § Applications Magnetic resonance imaging_sentence_168

MRI is used industrially mainly for routine analysis of chemicals. Magnetic resonance imaging_sentence_169

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. Magnetic resonance imaging_sentence_170

Being non-invasive and non-damaging, MRI can be used to study the anatomy of plants, their water transportation processes and water balance. Magnetic resonance imaging_sentence_171

It is also applied to veterinary radiology for diagnostic purposes. Magnetic resonance imaging_sentence_172

Outside this, its use in zoology is limited due to the high cost; but it can be used on many species. Magnetic resonance imaging_sentence_173

In palaeontology it is used to examine the structure of fossils. Magnetic resonance imaging_sentence_174

Forensic imaging provides graphic documentation of an autopsy, which manual autopsy does not. Magnetic resonance imaging_sentence_175

CT scanning provides quick whole-body imaging of skeletal and parenchymal alterations, whereas MRI imaging gives better representation of soft tissue pathology. Magnetic resonance imaging_sentence_176

But MRI is more expensive, and more time-consuming to utilize. Magnetic resonance imaging_sentence_177

Moreover, the quality of MR imaging deteriorates below 10 °C. Magnetic resonance imaging_sentence_178

History Magnetic resonance imaging_section_24

Main article: History of magnetic resonance imaging Magnetic resonance imaging_sentence_179

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. Magnetic resonance imaging_sentence_180

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. Magnetic resonance imaging_sentence_181

Lauterbur called his imaging method zeugmatography, a term which was replaced by (N)MR imaging. Magnetic resonance imaging_sentence_182

In the late 1970s, physicists Peter Mansfield and Paul Lauterbur developed MRI-related techniques, like the echo-planar imaging (EPI) technique. Magnetic resonance imaging_sentence_183

Advances in semiconductor technology were crucial to the development of practical MRI, which requires a large amount of computational power. Magnetic resonance imaging_sentence_184

This was made possible by the rapidly increasing number of transistors on a single integrated circuit chip. Magnetic resonance imaging_sentence_185

Mansfield and Lauterbur were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discoveries concerning magnetic resonance imaging". Magnetic resonance imaging_sentence_186

See also Magnetic resonance imaging_section_25

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Magnetic resonance imaging.