Nuclear medicine physics
Evaluating the purity of a 57Co flood source by PET
Flood sources of 57Co are commonly used for quality control of gamma cameras. Flood uniformity may be affected by the contaminants 56Co and 58Co, which emit higher energy photons. Although vendors specify a maximum combined 56Co and 58Co activity, a convenient test for flood source purity that is feasible in a clinical environment would be desirable.
Both 56Co and 58Co emit positrons with branching 19.6% and 14.9%, respectively. As is known from 90Y imaging, a positron emission tomography (PET) scanner is capable of quantitatively imaging very weak positron emission in a high single-photon background. To evaluate this approach, two 57Co flood sources were scanned with a clinical PET/CT multiple times over a period of months. The 56Co and 58Co activity was clearly visible in the reconstructed PET images. Total impurity activity was quantified from the PET images after background subtraction of prompt gamma coincidences.
Time-of-flight PET reconstruction was highly beneficial for accurate image quantification. Repeated measurements of the positron-emitting impurities showed excellent agreement with an exponential decay model. For both flood sources studied, the fit parameters indicated a zero intercept and a decay half-life consistent with a mixture of 56Co and 58Co. The total impurity activity at the reference date was estimated to be 0.06% and 0.07% for the two sources, which was consistent with the vendor's specification of <0.12%.
The robustness of the repeated measurements and a thorough analysis of the detector corrections and physics suggest that the accuracy is acceptable and that the technique is feasible. Further work is needed to validate the accuracy of this technique with a calibrated high resolution gamma spectrometer as a gold standard, which was not available for this study, and for other PET detector models.