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Fast magnetic resonance imaging is a technique using very rapidly acquired magnetic resonance images (MRIs). MRI pictures are taken less than a second apart, and are used to capture blood flow changes during a cognitive, sensory or motor task. If a particular brain area has increased blood flow during a task, it is assumed that the brain area is heavily active and thus critically involved in that task. Typically, fMRI images obtained during a task are compared against the activity patterns obtained during a control task.

Because fMRI gives a snapshot of the functioning brain, the technique is sometimes called "functional MRI". It is useful both for identifying function in the normal brain as well as detecting various abnormalities due to injury or disease. In general, the results of these studies confirm those of positron emission tomography (PET) studies in implicating particular brain regions in particular kinds of activity. fMRI provides better spatial resolution than PET, and is less intrusive -- since PET requires injection of a radioactive tracer into the bloodstream.

Further reading: Illustrated Guide to Diagnostic Tests, 2nd edition. Springhouse Corporation, Springhouse PA, 1998