Review: Mapping Brain Pathophysiology and Higher Cortical Function with Magnetic Resonance Imaging

Advances in magnetic resonance imaging (MRI) have moved the technology beyond its application solely as a diagnostic test to become a tool for addressing questions of in vivo pathophysiology and higher cortical function in humans. Diffusion-weighted MRI measures the apparent rate of translational movement of water molecules through brain parenchyma. This measurement can be used to determine axonal orientation within white matter, to define regions of tissue edema, and to permit early identification of ischemic neuronal injury related to impairment of Na+-K +-ATPase activity in experimental and human stroke. Changes in various aspects of cerebral perfusion—blood volume, blood flow, and hemoglobin oxygen saturation—can be mea sured with MRI, and altered cerebrovascular circulation and regional brain activation can thereby be inves tigated. Echo planar imaging is a method of ultrafast data acquisition with MRI—individual images are ac quired on the order of 100 msec. Echo planar imaging makes diffusion and perfusion measurements more practicable for diverse applications and allows for the study of temporal characteristics of regional brain responses to stimuli. Diffusion and perfusion MRI, generally termed functional MRI, are tools for studying in vivo brain physiology with MRI and are being applied to a broad range of questions in neuroscience.