Letter to the editor

In a recent article in the Journal of Cerebral Blood Flow and Metabolism, Lear and Ackermann (1988) compared brain autoradiographic images from rats injected with both [¹⁸F]-fluorodeoxyglucose (FDG) and [¹⁴C]-glucose. A major contribution of this work is that it takes a significant step in the process of defining when one tracer may be preferable to another. The authors tentatively conclude that [6-¹⁴C]-glucose may be the isotope of choice for experiments in which the period of observation must be considerably less than the 45 min period used for FDG and deoxyglucose (DG). They express concern, however, that in the brain areas with the highest rates of glucose utilization in the normal rat, the values found with FDG as tracer exceed those found with [6-¹⁴C]-glucose by up to 10%. If there is a 10% discrepancy under normal conditions, and if this is due to loss of products of [6-¹⁴C]-glucose metabolism, then might not the 'underestimate' with [6-¹⁴C]-glucose as tracer become much greater under stimulated conditions? In fact, question the authors, is this not what Collins et al. (1987) found in their studies of glucose metabolic rates in superior colliculi of rats exposed to progressively more rapid rates of visual flicker stimulation? It is to this issue of increased loss with increased activity that this letter addresses itself.