Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) is a medical imaging technique primarily used in Radiology to visualize the structure and function of the body. It provides detailed images of the body in any plane.
MRI provides much greater contrast between the different soft tissues of the body than does computed tomography (CT), making it especially useful in neurological, musculoskeletal, cardiovascular, and oncological imaging.
Unlike CT it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body.
Radiofrequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner.
This signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body.
See Wikipedia, Magnetic resonance imaging.