PT - JOURNAL ARTICLE AU - DiFilippo, Frank TI - Estimating <sup>56</sup>Co and <sup>58</sup>Co contaminants in a <sup>57</sup>Co flood source by PET/CT DP - 2014 May 01 TA - Journal of Nuclear Medicine PG - 2150--2150 VI - 55 IP - supplement 1 4099 - http://jnm.snmjournals.org/content/55/supplement_1/2150.short 4100 - http://jnm.snmjournals.org/content/55/supplement_1/2150.full SO - J Nucl Med2014 May 01; 55 AB - 2150 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 &lt; 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.