Magnetic Resonance Imaging (MRI) has rapidly become one of the most important and fruitful tool in both neuroscience research and clinical practice.
The functional approach, specifically functional Magnetic Resonance Imaging (fMRI), is the most generally used technique for the investigation of human cognition.  fMRI is able to map activated brain regions by taking advantage of the local relation between physiological function, energy metabolism and blood supply. In fact, the Blood Oxygenation Level Dependent (BOLD) signal, on which fMRI is based, reflects a complex relationship between changes in local blood volume (CBV) blood flow (CBF) and oxygen metabolism (CMRO2), as a consequence of neuronal activity.
BOLD contrast derives from magnetic field inhomogeneities induced by deoxyhemoglobine concentration in red blood cells in blood vessels with respect to surrounding space.  Immediately after neuronal activation, a decrease of BOLD signal would be expected, due to the increase of oxygen consumption. Actually, an increase of BOLD signal is found, due to the increase in cerebral blood flow determining an oversupply of oxygen in working areas. fMRI sequences are designed to be sensitive to this gradient of susceptibility generating BOLD signal alteration, which has been suggested to reflect  the intra-cortical information processing of a given brain area.
The principal advantages of fMRI are its non-invasive nature, the high spatio-temporal resolution, and the ability in investigating the entire network of brain areas engaged during a tasks.  To date fMRI is surely one of the most exciting and promising technique for the in vivo study of brain function and dysfunction, and it is able to optimize clinical processes providing essential information for diagnosis and therapeutic monitoring.