Room: Exhibit Hall | Forum 9
Purpose: In nuclear medicine, Cobalt 57 flood sources are commonly used to check gamma camera detector uniformity. These sources typically have small amounts of â?µâ?¶Co and â?µâ?¸Co impurities which emit high energy photons that can affect detector uniformity measurements. Since their half-lives are shorter than the â?µâ?·Co half-life, new sources may be â€œagedâ€? allowing the contaminants to decay, minimizing their effect on detector uniformity measurements. The goal of this work was to determine an appropriate aging time for new â?µâ?·Co sheet sources by a convenient method that is feasible in a clinical environment.
Methods: Two newly-manufactured sheet sources were scanned on a Siemens E-Cam, using energy windows at 122 keV, (â?µâ?·Co photopeak) and 200 â€“ 600 keV, over a period of 4 months. After each scan, the counts in each detector energy window were recorded and the ratio of counts in the energy windows was calculated over time. Uniformity was measured for the standard â?µâ?·Co energy window with Siemens Flood Uniformity tool. A sheet source with calibration date 780 days (>10 half-lives) prior to the first scan date of the two new sheet sources was also scanned. It was assumed that the impurities in this source had completely decayed and the ratio of counts in the two energy windows represented the probable endpoint ratio for both new sources.
Results: The ratio of counts in the two energy windows over time agreed with an exponential decay model with a half-life that reasonably agrees with the half-lives of â?µâ?¶Co and â?µâ?¸Co. The uniformity showed an improvement over the first two months, then remained stable for subsequent measurements.
Conclusion: A waiting period of approximately six weeks after the sourceâ€™s assay date is advisable before using new â?µâ?·Co sheet sources for daily QC.