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Technetium-99m-Pertechnetate as a Whole Blood Marker for Brain Perfusion Studies

Unit of Cardiovascular Physiology, Université Catholique de Louvain, School of Medicine, Brussels, Belgium

Correspondence: For correspondence or reprints contact: A. Keyeux, MD, Catholic University of Louvain, School of Medicine, 5479, avenue Hippocrate 54, B-1200 Brussels, Belgium.

ABSTRACT

In the brain, diffusible ^99mTc-pertechnetate behaves as an intravascular indicator because it is confined within the circulation by the blood-brain barrier, allowing its use for noninvasive dynamic evaluation of cerebral circulation. For this application ^99mTc has often been claimed to be a plasma marker. This study examines the validity of such a claim which has not yet been proven in vivo. Methods: The relative amount of ^99mTc in the red cells circulating in large vessels was compared to the corresponding hematocrit (LVHct) during the rapid (t/2 = 1.98 min) and slow (t/2 = 84 min) phases of ^99mTc disappearance from the circulation after bolus intravenous injection. These comparisonswere 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 injection for the slow phase. Results: The results show that the relative amount of intravascular ^99mTc fixed to red cells did not differ statistically from LV Hct until at least 1 hr after intravenous administration. This homogeneous distribution of ^99mTc in blood was indisputable 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 ^99mTc, which, in whole blood, amounted to 4% at 20 sec and 25% at 2 hr after injection. Conclusion: Because it is a 96% whole blood marker early after intravenous administration, ^99mTc is a reliable agent for first-pass studies of whole blood circulation in the brain.

Key Words: technetium-99m-pertechnetate • blood marker • brain perfusion