What is functional MRI (fMRI) of the brain?
When a region of the brain is activated, it receives increased blood flow so much, in fact, that the venous and capillary concentrations of deoxyhemoglobin actually decreases. This is significant in MRI because deoxyhemoglobin is paramagnetic, and therefore causes minute variations in magnetic field strength. These inhomogeneities cause the signal to decay faster. So when the flow goes up and the deoxyhemoglobin concentration goes down, you get more signal from more stuff. The result is a slight (~2%) increase in MR signal from active regions of the brain. Gradient echoes are particularly sensitive to magnetic field inhomogeneity and so are often used in fMRI.
