Functional Magnetic Resonance Imaging and Magnetic Resonance Spectroscopy in Schizophrenia

Abstract

Functional magnetic resonance imaging (fMRI) is a recently developed noninvasive technique for examining brain function and relies on detecting changes in MRI signals from brain regions that are differentially activated by sensory, cognitive, or pharmacological stimuli. For an in depth discussion of fMRI principles and methodology, see Sanders and Orrison (1995). fMRI relies on the fact that capillaries and red blood cells within tissues induce microscopic magnetic field gradients that shorten the effective transverse relaxation decay rate (T2 *) to a degree that depends on the precise magnetic susceptibility of blood. Magnetic susceptibility determines the intensity of magnetic field experienced within a region and depends on the local oxygen tension. Blood containing oxyhemoglobin has a magnetic susceptibility close to that of tissue water, whereas blood containing deoxyhemoglobin (which is paramagnetic) has a very different susceptibility. Neuronal activation in response to some stimulus produces a local blood flow increase whereby local oxygen delivery actually exceeds oxygen utilization, and the net amount of deoxyhemoglobin decreases. The result is that the oxygen tension of the tissue rises, and venous blood becomes more oxygenated. The intravascular magnetic susceptibility then more closely matches the surrounding tissue than it does when the vessels contain deoxyhemoglobin.