Magnetic resonance imaging of brain function and neurochemistry

In the past decade, magnetic resonance imaging (MRI) research has been focused on the acquisition of physiological and biochemical information noninvasively. Probably the most notable accomplishment in this general effort has been the introduction of the MR approaches to map brain function. This capability, often referred to as functional magnetic resonance imaging, or fMRI, is based on the sensitivity of MR signals to secondary metabolic and hemodynamic responses that accompany increased neuronal activity. Despite this indirect link to neurotransmission, recent studies demonstrate that under appropriate conditions, these fMRI maps have accuracy at the scale of submillimeter neuronal organizations such as the orientation columns of the visual cortex, and are directly proportional in magnitude to electrical signals generated by the neurons. High magnetic fields have been critical in achieving such specificity in functional maps because they provide advantages through increased signal-to-noise ratio, diminishing blood-related contributions to mapping signals, and enhanced sensitivity to microvasculature. Equally important is MR spectroscopy studies, which, at high magnetic fields, provide for the first time the opportunity to measure local metabolic correlates of human brain function and neurotransmission rates. Together, these MR methods provide a complementary set of approaches for probing important aspects of the nervous system.