MRI machine

Magnetic resonance imaging
Medical diagnostics
Play media Para-sagittal MRI of the head, with aliasing artifacts (nose and forehead appear at the back of the head)
Synonyms	nuclear magnetic resonance imaging (NMRI), magnetic resonance tomography (MRT)
ICD-9-CM	88.91
MeSH	D008279
MedlinePlus	003335
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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 in both health and disease. MRI scanners use strong magnetic fields, radio waves, and field gradients to generate images of the organs in the body.

MRI does not involve x-rays, which distinguishes it from computed tomography (CT or CAT). While the hazards of x-rays are now well-controlled in most medical contexts, MRI still may be seen as superior to CT in this regard.

MRI often may yield different diagnostic information compared with CT. There may be risks and discomfort associated with MRI scans. Compared with CT, MRI scans typically take greater time, are louder, and usually require that the subject go into a narrow, confined 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 is based upon the science of nuclear magnetic resonance (NMR). Certain atomic nuclei are able to absorb and emit radio frequency energy when placed in an external magnetic field. In clinical and research MRI, hydrogen atoms are most-often used to generate a detectable radio-frequency signal that is received by antennas in close proximity to the anatomy being examined. Hydrogen atoms exist naturally in people and other biological organisms in abundance, 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 signal 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.