Title: Validation of PET/CT measurements of 57Co sheet source radionuclidic impurities

1769 Objectives: In nuclear medicine, Cobalt 57 sheet 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 emissions, which include positron emission annihilation photons. Sheet source vendors may specify the 56Co and 58Co activity, but it can also be measured on a PET/CT scanner. This work extends previously-published methods by validating them on an additional PET/CT scanner platform using settings available to users in a clinical environment. Methods: A 57Co sheet source was scanned on a Philips Ingenuity TF PET/CT scanner using a previously-published method of Frank P. DiFilippo (Med. Phys. 2014, Vol. 41, 112502) to acquire annihilation photons from positron emission decays of 56Co and 58Co. Scans were acquired approximately every 3-4 weeks over a period of 6 months. The scanner was set for 68Ge as the administered radionuclide, and initial activity was calculated from the source manufacturer’s impurity data and entered on the scanner console. The standard whole-body PET reconstruction was used with “high quality” option enabled. No other special reconstruction or processing was used. In the reconstructed images, a region of interest was placed over the source in each slice, and mean activity concentration in Bq/mL was measured for each ROI. The concentration was multiplied by the ROI volume to obtain the activity in each slice. The slice activities were summed to determine the total activity. The resulting activity then was multiplied by the ratio of the positron emission fractions for the average of 56Co and 58Co, and 68Ge (0.172/0.866) to determine the impurity activity. The effectiveness of the scanner in quantifying the weak positron emissions from the 56Co and 58Co contaminants in the high single-photon background from the 57Co was evaluated by analyzing the scan event data stored in the rates.csv files which contain all of the detector events from a single scan. Results: Measurements of the 56Co and 58Co impurity activity agreed well with the vendor’s specified activity and with an exponential decay model with a half-life consistent with a mixture of 56Co and 58Co. Using data from the rates files, it is possible to determine the true coincidence rate for each scan. When evaluated over time, these data fit an exponential decay model with a half-life consistent with a mixture of 56Co and 58Co. In addition, the singles rate is recorded in the rates file, allowing the ratio of true coincidences versus single counts to be calculated. Conclusions: 57Co sheet source contamination can be easily measured on a PET/CT scanner in a clinical setting by scanning the source and measuring the total activity in all slices. The PET/CT scanner used in this study was effective in making these activity measurements at a ratio of true coincidences versus singles of 0.02 PPM. This work validates a previously-published method on an additional make and model of clinical scanner. Further work is needed to validate this technique for other PET/CT scanner makes and models.