Estimating ⁵⁶Co and ⁵⁸Co contaminants in a ⁵⁷Co flood source by PET/CT

Objectives Flood sources of ⁵⁷Co are commonly used for quality control of gamma cameras. Other isotopes, namely ⁵⁶Co and ⁵⁸Co, are typical contaminants in commercial flood sources. The half-lives of ⁵⁶Co (77.2 d) and ⁵⁸Co (70.9 d) are shorter than that of ⁵⁷Co (271.7 d), and the flood source becomes purer with time. When a flood source is new, high-energy gamma emissions from ⁵⁶Co and ⁵⁸Co may cause tube-pattern non-uniform images. It is thus desirable to develop a simple method to estimate the activity of these contaminants and ensure it meets vendor specifications.

Methods Although minimally present in a flood source, both ⁵⁶Co and ⁵⁸Co are positron emitters with branching 19% and 15%, respectively. As is known from ⁹⁰Y imaging, a clinical PET/CT scanner is capable of imaging weak positron emission in a strong photon background. A ⁵⁷Co flood source was scanned multiple times over a period of months, acquiring data for 90 minutes per study at a single bed position. Total activity was estimated by measuring average counts per slice in a 80x500mm region of interest, correcting for background counts, and accounting for the positron branching of ⁵⁶Co and ⁵⁸Co relative to a reference radionuclide (⁶⁸Ge/⁶⁸Ga).

Results Time-of-flight (TOF) PET was found to be necessary for reconstructing suitable images, as non-TOF images could not discern the sheet source from background. The measured positron activity followed exponential behavior with half-life 68 ± 10 days, which is consistent with a mixture of ⁵⁶Co and ⁵⁸Co. For this particular flood source, the contaminants at the calibration date were estimated to be 0.05% of initial flood source activity, which is within the manufacturer’s specification of < 0.12%.

Conclusions A clinical TOF PET/CT is effective for imaging the weak positron signal from ⁵⁶Co and ⁵⁸Co contaminants in a ⁵⁷Co flood source. This method provides a simple estimate of flood source purity using available equipment in a clinical setting.