Functional MRI of the brain principles, applications and limitations.

MRI now allows noninvasive monitoring of brain function with a combined spatial and temporal resolution never achieved by other imaging modalities. Among several methods proposed to evaluate changes in blood volume, flow or oxygenation during mental activity, the most successful is based on the sensitivity of MRI to magnetic effects induced by the modulation of the oxygenation status of hemoglobin (oxy/deoxyhemoglobin) which results from local variations in blood flow. In the brain cortex, such variations may be induced by task activation or by cognitive processes, such as language or mental imagery. Typically, MRI signal is increased by a few percents when brain is activated due to sharp increase in oxygen supply (blood flow). Brain activation maps obtained with MRI using various task paradigms agreed well with previous PET results. However MRI permits direct correlation of function with underlying anatomy within a single imaging modality and repetitive studies on the same individuals. These studies suggest that MRI may be the method of choice in conjunction with other functional techniques, to study mental and cognitive processes underlying the function of the human brain. Clinically, potential applications include presurgical mapping, recovery monitoring of stroke or head injuries, exploration of seizure disorders or monitoring of the effects of neuropharmaceuticals.