Neurophysiologic basis of functional neuroimaging: Animal studies

Functional neuroimaging adds metabolic or biochemical information to that o btained 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 met abolism, oxygen consumption, or blood flow. Functional magnetic resonance i maging uses the different magnetic properties of oxyhemoglobin and deoxyhem oglobin to identify areas of increased blood flow, which, in turn, reflects neuronal activation. Magnetic resonance spectroscopic imaging, with magnet ically labeled molecules, can be used to follow biochemical pathways. Funct ional neuroimaging is based on the experimental data that neuronal activati on 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 en d product, lactate, is released for neuronal uptake and subsequent oxidativ e phosphorylation. To meet these metabolic needs, blood flow increases to s uch an extent that overall capillary oxyhemoglobin concentration increases. This changes the magnetic signal in the region and permits functional magn etic resonance imaging studies. Recent data suggest that there is an initia l decrease in the concentration of oxyhemoglobin which may be more spatiall y specific to the area of neuronal activation. Further refinements in funct ional neuroimaging will lead to improved understanding of the normal functi onal anatomy of the brain and will shed further light on the pathophysiolog y of many neurologic disorders.