Neurophysiologic basis of functional neuroimaging: animal studies

Functional neuroimaging adds metabolic or biochemical information to that obtained with anatomic imaging, allowing localization of a neural function. Positron emission tomography and single photon emission tomography make use of radioactive tracers tagged to a molecule which can indicate glucose metabolism, oxygen consumption, or blood flow. Functional magnetic resonance imaging uses the different magnetic properties of oxyhemoglobin and deoxyhemoglobin to identify areas of increased blood flow, which, in turn, reflects neuronal activation. Magnetic resonance spectroscopic imaging, with magnetically labeled molecules, can be used to follow biochemical pathways. Functional neuroimaging is based on the experimental data that neuronal activation leads to increased metabolism. Uptake of glucose and oxygen increases to meet increased energy needs. The fractionally increased glucose appears to be taken up mostly by glia, which metabolize it through glycolysis. The end product, lactate, is released for neuronal uptake and subsequent oxidative phosphorylation. To meet these metabolic needs, blood flow increases to such an extent that overall capillary oxyhemoglobin concentration increases. This changes the magnetic signal in the region and permits functional magnetic resonance imaging studies. Recent data suggest that there is an initial decrease in the concentration of oxyhemoglobin which may be more spatially specific to the area of neuronal activation. Further refinements in functional neuroimaging will lead to improved understanding of the normal functional anatomy of the brain and will shed further light on the pathophysiology of many neurologic disorders.