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Quantification of Flow in a Dynamic Phantom Using ⁸¹Rb-^81mKr, and a NaI Detector

Harvard Medical School and Peter Bent Brigham Hospital, Boston, Massachusetts

Correspondence: For reprints contact: B. Leonard Holman, Dept. of Radiology, Harvard Medical School, 25 Shattuck St., Boston, MA 02115.

ABSTRACT

Blood flow can be measured by monitoring the count rate of Krypton-81m after its parent, Rubidium-81 (a potassium analogue), has been deposited in the tissue. The steady-state Kr-81m count rate reflects both production by decay of Rb-81 and washout due to blood flow. Its use is theoretically superior to that of Xenon-133 for quantification of blood flow (cc/min per 100 gm) since: (1) multiple flow measurements can be obtained from a single arterial injection, (2) flow-dependent changes in the count rate of Kr-81m provide a steady-state measure of specific flow, and (3) errors due to uptake in fat are eliminated. The count rate of Kr-81m was measured as a function of flow in a dynamic phantom with a NaI crystal, using both pure cyclotron-produced Rb-81 and commercially available samples with as much as 25% contamination from Rb-82m. The phantom was calibrated by measuring the flow-rate constants with Xe-133. No significant difference was found between the flow-rate constants measured with three pure samples of Rb-81 and those measured with three contaminated samples. The ratio of the zero-flow Kr-81 m count rate to the rate observed in the presence of flow showed excellent correlation with calibrated flow over a range of rate constants from 0 to 0.02 sec^–1. This study suggests that regional specific flow can be measured in vivo with currently available NaI detecting systems after the intra-arterial injection of Rb-81.