The use of positron emitters to evaluate linearity of radionuclide dose calibrators

Objective: Four positron emitters, F, N, 11 C and 0, were studied immediately after production for possible radionuclidic contaminants. Methods: A series of activity measurements was performed with a dose calibrator over decay periods of 20 half-lives for each radionuclide. Individual semilogarithmic time-activity curves were generated over a wide range of activities (from a few hundred MBq to the 370 kBq (10 J.LCi) level}. The presence of radionuclidic contaminants in activity measurements over reasonably long periods will manifest as deviations from linearity in these plot presentations. Results: Nitrogen-13 samples showed significant deviation from linear response even at the MBq level due to the presence of a F contaminant. However, F, 11 C and 0 samples showed a virtually linear response over the entire activity range. Conclusions: Although most of these positron emitters maintain the linear decay characteristics in the practical use range, investigators should be aware of the increasing contribution of long-lived contaminants in the low end of the activity range. Although the 2-min half-life of 0 might be sufficient to allow performance of linearity measurements within minutes, we feel that 11 C might be an ideal positron emitter to accomplish this task due to its more practical half-life period.