Metabolic PET Studies

Metabolic studies use 18FDG to measure regional glucose metabolism. 18FDG, like all 18F compounds, has the advantage of a relatively long half-life (110 minutes). This allows the synthesis to be performed in one location, the subject injection in another location, and the scanning in yet another location. In fact, one can have a subject doing a particular task in a location remote from the PET scanner and inject 18FDG, which will be trapped in brain regions according to the local metabolic rate. This has an obvious advantage in situations in which placing the subject in the scanner would alter the conditions of the task. For example, 18FDG is used commonly in sleep studies. The main disadvantage is that the long half-life results in effectively no temporal resolution. This offers a time-averaged snapshot of a particular brain state, and the state is averaged over 20-60 minutes. Figure 10-1 shows functional localization of an epileptic focus in the right temporal lobe during presurgical workup.

FIGURE 10-1. Functional localization of an epileptic focus in the right temporal lobe during presurgical workup using magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography (FDG-PET).

FIGURE 10-1. Functional localization of an epileptic focus in the right temporal lobe during presurgical workup using magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography (FDG-PET).

Copyright © American Psychiatric Publishing, Inc., or Arm a ri can Psychiatric Association unless otherwise indicated in figure legend, All rights reserved.

Source. Image courtesy of Carolyn C. Meltzer, M.D.

Most of the FDG uptake studies are based on the assumption that the glucose uptake and neural activity at the synaptic level might be coupled. A caveat must be borne in mind when evaluating studies using FDG-PET. Glutamate is the main excitatory neurotransmitter in the brain, and it is removed from the synapse through a process of uptake by astroglial tissues, thus terminating neural activation (Magistretti and Pellerin 1999). But recent studies have shown that uptake of glutamate by astroglia can by itself stimulate glucose (and FDG) uptake (Magistretti 2006). In fact, deactivation might actually be coupled with increased glucose uptake in a variety of conditions (Magistretti 2006). Thus, the same problems that accompany studies of fMRI—i.e., whether the signal is actively excitatory versus actively inhibitory—are present in FDG-PET studies as well.

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