Gamma camera-based method for acceptance of new cobalt-57 flood sources to minimize image nonuniformities from radionuclidic impurities

1763 Purpose: In nuclear medicine, Cobalt 57 flood sources are commonly used to check gamma camera detector uniformity. These 57Co flood sources typically have a small concentration of 56Co and 58Co impurities which can affect detector uniformity measurements due to their high energy photon emission as noted by E. B. Sokole et al (Eur. J. Nucl. Med, 1996 Vol 23, 437-442) and R. W. Cranage et al (Br. J. Radiol, 1979 Vol 52, 81-82). The influence of these high energy photons on the overall detector count rate can be seen by scanning in an energy window that includes these high energies per E. Busemann-Sokole et al (Rad. Prot. Dosimetry, 1995 Vol 57, 281-284). Since the half-lives of 56Co and 58Co (77.24 and 70.86 days respectively) are much shorter than the 57Co half-life (271.74 days), new flood sources may be “aged” for a period of time to allow the contaminants to decay and minimize their effect on detector uniformity measurements. Contaminants in these sources can be measured using PET/CT scanners using the methods of Frank P. DiFilippo (Med. Phys. 2014, Vol 41, 112502), but many clinical users of Co-57 sources do not have access to a PET/CT. The goal of this work was to determine an appropriate aging time for new 57Co 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, with a 5/8” (coincidence/PET) crystal and LEHR collimator, using two energy windows, 122 keV, +/- 10% (57Co photopeak) and 200 - 600 keV, roughly every two weeks over a period of 4 months. The sources were centered between the two detectors approximately 10 cm from each and stop conditions were 41 million counts in any energy window. After each scan, the total counts in each window for each detector were recorded and the ratio of counts in the two energy windows was calculated and plotted over time. Uniformity was measured for the standard 57Co energy window using the Siemens Flood Uniformity tool. A sheet source with calibration date 780 days prior to the first scan date of the two new sheet sources was scanned using the same energy windows. Since 780 days is more than ten half-lives for 56Co and 58Co, 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 calculated half-life of approximately 76 days, which reasonably agrees with the known half-lives for 56Co and 58Co. At the same time, the uniformity measurements showed an improvement over the first two months, then remained fairly stable for subsequent measurements. In this case, a 20% drop in the ratio of counts corresponds with stable uniformity measurements. Conclusions: A waiting period of approximately six weeks after the source’s labeled assay date is advisable before placing new 57Co sheet sources in service for daily QC. This corresponds to a counts ratio of 1:1 in the two energy windows for this camera configuration. A longer waiting period of approximately ten weeks may be advisable before using sources to perform detector calibration and other high-count applications. Sources may be placed in service after shorter periods depending upon uniformity acceptance criteria. Future work will validate the energy window method and count ratio criteria for other gamma camera models.