Modeling metabolic processes in the brain in vivo

Positron emission tomography provides a means to measure local concentrations of positron-emitters in brain and to reconstruct pictorial images of the distribution of isotope in the brain. This capability is useful when the isotope is confined to a defined chemical species, and its concentration when combined with other measurable variables can be used to assay a physiological or biochemical process. Common variables are concentration and/or specific activity of the precursor tracer molecule in blood or plasma, the kinetics of exchange of the precursor molecule between blood and/or plasma and tissue, and the distribution space of the tracer in the tissue. These variables must be related, usually by an equation based on a kinetic model of the process under study. An example of such a method is the 2-deoxyglucose method which measures local rates of glucose utilization in brain. It was first developed with [14C]deoxyglucose and autoradiography in animals and has demonstrated a close relationship between local functional activity and glucose utilization in brain. It has proved useful to map regions of altered functional activity in the central nervous system in a variety of physiological and pharmacological states. By a combination of this technique with positron emission tomography and the positron-emitting analog, [18F]fluorodeoxyglucose, the method has been adapted for use in humans.