An imaging and diagnostic technique based on the phenomenon of nuclear magnetic resonance. The body or part to be imaged is placed in an MR scanner. This creates a strong magnetic field, which causes the alignment of certain tissue nuclei, notably hydrogen nuclei in water and lipids. Pulses of high-frequency radio waves are generated, and images of internal tissues are created by a computer, according to how the radio waves are absorbed and transmitted. These images map tissue ‘slices’ and can resolve even very similar tissues by detecting subtle differences in their chemical composition. Hence, MRI can detect, for example, a brain tumour buried deep within normal brain tissue. Apart from diagnosing pathological changes, MRI is also used to guide interventional procedures. Functional MRI (fMRI) can monitor brain activity and is now commonly used in neuroscience to study brain function. It detects the minute changes in blood flow that accompany increased neural activity. So, for example, it is possible to map regions of the visual cortex that are active when a subject is exposed to different visual stimuli, such as colours, faces, or dictionaries.

http://www.cis.rit.edu/htbooks/mri/inside.htm Comprehensive online textbook covering all aspects of MRI