Functional MRI (FMRI) is a technique that has recently been introduced to obtain functional information from the central nervous system. FMRI detects subtle increases in blood flow associated with activation of parts of the brain. FMRI may be useful for preoperative neurosurgical planning, epilepsy evaluation, and "mapping" of the brain.
Activation of an area of the brain causes an increase in blood flow to that area that is greater than that needed to keep up with the oxygen demands of the tissues. This results in a net increase in intravascular oxyhemogobin and a decrease in deoxyhemoglobin. Deoxyhemoglobin is paramagnetic, resulting in shorting of the T2* of the brain and decrease in signal. Less deoxyhemoglobin as a result of increase in blood flow results in an overall increase in signal. Sophisticated image processing techniques are used to obtain images of these flow changes. The increase in signal detected ranges from 1-5% on a 1.5T MR system.