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MRI (an abbreviation of magnetic resonance imaging) is an imaging modality that uses non-ionizing radiation to create useful diagnostic images.
In simple terms, an MRI scanner consists of a large, powerful magnet in which the patient lies. A radio wave antenna is used to send signals to the body and then a radiofrequency receiver detects the emitted signals. These returning signals are converted into images by a computer attached to the scanner. Imaging of any part of the body can be obtained in any plane.
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MRI was initially called nuclear magnetic resonance imaging (NMR imaging or NMRI) after its early use for chemical analysis. The initial "nuclear" part was dropped about 25 years ago because of fears that people would think there was something radioactive involved, which there is not.
Nuclear magnetic resonance (NMR) continues to be the term of choice in the physical sciences.
In general, "MRI" or "MR imaging" are used as the abbreviations for standard MRI. Often "MR" is also used as shorthand, e.g. "the patient has had MR liver", "the patient has had MR of the liver" or "the patient has had MR scanning of the liver".
When referring to angiography performed using MRI, it is never "MRI angiography", instead the "imaging" part of the phrase/abbreviation is dropped, becoming magnetic resonance angiography (MRA). Similarly also magnetic resonance spectroscopy (MRS) and magnetic resonance enterography (MRE).
Why use MRI
Advantages of MRI include:
- ability to image without the use of ionizing x-rays, in contradistinction to CT scanning
- images may be acquired in multiple planes (axial, sagittal, coronal, or oblique) without repositioning the patient. CT images have only relatively recently been able to be reconstructed in multiple planes with the same spatial resolution (i.e. isotropic voxels)
- MRI images demonstrate superior soft-tissue contrast as compared to CT scans and plain radiographs making it the ideal examination of the brain, spine, joints, and other soft tissue body parts
- some angiographic images can be obtained without the use of contrast material, unlike CT or conventional angiography
- advanced techniques such as diffusion, spectroscopy, and perfusion allow for precise tissue characterization rather than merely 'macroscopic' imaging
- functional MRI allows visualization of active parts of the brain during certain activities and also understanding of the underlying networks
Disadvantages of MRI include:
- MRI scans are more expensive than CT scans
- MRI scans take significantly longer to acquire than CT and patient comfort can be an issue, maybe exacerbated by:
- MR image acquisition is noisy compared to CT
- MRI scanner bores tend to be more enclosed than CT with associated claustrophobia
- MR images are subject to unique artifacts that must be recognized and mitigated against (see MRI artifacts)
- MRI scanning is not safe for patients with some metal implants and foreign bodies. Careful attention to safety measures is necessary to avoid serious injury to patients and staff, and this requires special MRI compatible equipment and stringent adherence to safety protocols (see MRI safety).
History and etymology
Nuclear magnetic resonance was discovered simultaneously by two physicists, Felix Bloch and Edward Mills Purcell, just after the end of the Second World War. Bloch trained in quantum mechanics and was involved with atomic energy and then radar countermeasures. At the end of the war, he returned to his earlier work on the magnetic moment of the neutron. Purcell was involved with the development of microwave radar during the war then pursued radio waves for the evaluation of molecular and nuclear properties. They received the Nobel Prize in Physics in 1952 for this discovery.
For many years, nuclear magnetic resonance has been used by chemists to study atoms and molecules. It was Raymond Damadian (1936-fl.2022) who first demonstrated - in experimental animals - that by measuring the relaxation times of tissues (T1 and T2) it was possible to differentiate normal from pathological tissues (1971) 2.
The use of NMR to produce 2D images (later renamed MRI) was accomplished by Paul Lauterbur (1929-2007) 4, imaging water, and Sir Peter Mansfield (1933-2017) 5 who imaged the fingers of a research student, Andrew Maudsley (fl. 2022) in 1976. Maudsley continues to make a significant contribution to the development of MRI today. Raymond Damadian obtained human images a year later in 1977.
Lauterbur and Mansfield received the Nobel Prize in Physiology or Medicine in 2003 for their development of MRI. This award was controversial in that the contributions of Damadian to the development of MRI were overlooked by the Nobel Committee.
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