TECHNETIUM-99M-PERTECHNETATE AS A WHOLE-BLOOD MARKER FOR BRAIN PERFUSION STUDIES

In the brain, diffusible Tc-99m-pertechnetate behaves as an intravascu lar indicator because it is confined within the circulation by the blo od-brain barrier, allowing its use for noninvasive dynamic evaluation of cerebral circulation. For this application (TC)-T-99m has often bee n claimed to be a plasma marker. This study examines the validity of s uch a claim which has not yet been proven in vivo. Methods: The relati ve amount of Tc-99m in the red cells circulating in large vessels was compared to the corresponding hematocrit (LV Hct) during the rapid (t/ 2 = 1.98 min) and slow (t/2 = 84 min) phases of (TC)-T-99m disappearan ce from the circulation after bolus intravenous injection. These compa risons were performed on rats at 2 (n = 3), 5 (n = 6), 10 (n = 6) and 20 (n = 9) sec after intravenous injection for the rapid phase and 5 ( n = 5), 30 (n = 4), 60 (n = 6) and 120 (n = 6) min after intravenous i njection for the slow phase. Results: The results show that the relati ve amount of intravascular Tc-99m fixed to red cells did not differ st atistically from LV Hct until at least 1 hr after intravenous administ ration. This homogeneous distribution of Tc-99m in blood was indisputa ble during the first 20 sec but became progressively less evident and disappeared after 2 hr. Such behavior was attributed to a progressive increase of free Tc-99m, which, in whole blood, amounted to 4% at 20 s ec and 25% at 2 hr after injection. Conclusion: Because it is a 96% wh ole blood marker early after intravenous administration, (TC)-T-99m is a reliable agent for first-pass studies of whole blood circulation in the brain.