[Functional MRI of the brain]

Abstract

An introduction to functional MRI (fMRI) of the brain was described. Basically there are two methods in fMRI; one is using extrinsic substance and the other intrinsic substance. The blood oxygen level dependent contrast method, which uses intrinsic substance, is used commonly at present. This method is based on the idea that the signal intensity changes due to the oxygenation of hemoglobin (Hb) in the blood vessels. Oxy-Hb has a diamagnetic property which does not affect the signal intensity of water proton. On the other hand, deoxy-Hb is paramagnetic and shortens the T2 relaxation time of the water proton. By the activation of brain, blood flow increases around the activated area with a little increase of oxygen consumption, resulting in an increase of oxy-Hb in the capillary of this area. Consequently signal increase occurs in the activated area of the brain on MRI due to the decrease of deoxy-Hb. The fMRI was measured by pulse sequences sensitive to the T2 changes such as echo planar imaging (EPI) on 1.5 T systems or gradient echo imaging (GRE) on high-filed magnetic systems (3.0-4.0 T). It becomes possible to get fMRI on conventional MRI scanners using GRE pulse sequence. Many activation tasks are adopted for fMRI; not only simple tasks such as motor, photic and sensory stimulations but also complex tasks such as hearing of words, word generation, imagination, coordination motion, etc. A rapid increase of signal intensity was observed in the primary cortical area corresponding to each task, and the activated area is visualized by the subtraction imaging or statistically treated imaging. The fMRI has big advantages to get brain functional imaging because of non-invasive measurement, using intrinsic substance, highly spatial and temporal resolution and easy measurement on conventional clinical devices. Therefore, the fMRI will be used more and more widely in future, especially by introducing the EPI technique to the clinical MRI scanners.