HMPAO as a regional cerebral blood flow tracer at high flow levels.

HMPAO is being used extensively to image rCBF during focal seizures in humans. It is, however, theoretically possible that back-diffusion of tracer causes retention to fall as flow rises at high levels. Methods: We used a double label Tc-99m-HMPAO/C-14-IAP autoradiographic technique to compare HMPAO retention and regional cerebral blood flow in penicillin induced focal seizures in rats. Results: Using this protocol, flows of up to 717 ml/100 g per min were observed. The same pattern of uptake was seen on IAP and HMPAO autoradiographs, with the exception of relatively high HMPAO uptake in the choroid plexus, in the fissures and, in one animal only, the supramammilary nucleus. Correlation of HMPAO retention and blood flow showed a linear relationship up to 200 ml/100 g per min in all animals. HMPAO retention then showed a falloff in its rise with blood flow, bur was still increasing, even at the highest flows seen. At 700 ml/100 g/min, HMPAO retention was 20% of that expected from a linear relationship. Conclusion: HMPAO is a suitable tracer of rCBF at high flows and is unlikely to produce anomalous images in human focal seizures.