Development of sealed 22Na phantoms for PET system QA/QC: uniformity and stability evaluation

Sodium-22 is a radioisotope which is typically used as a calibration point source for PET. Its half-life of 2.6 years is suitably long enough while its positron range is as small as that of
¹⁸
F. However, the use of
²²
Na has been limited to a point-like shape. In this work, with the help of the Japan Radioisotope Association, we proposed a wider variety of
²²
Na sealed phantoms. As examples, we developed three types of
²²
Na phantoms: a 5-cm cylinder, a 60-cm flexible tube and a Derenzo-like multi-rod phantom. These were designed for PET system quality assurance and quality control (QA/QC) and performance measurements. At first, in this work, we evaluated uniformity and long-term stability of radioactivity distributions of these phantoms. These phantoms were scanned by the Biograph mCT Flow PET/CT scanner four times during 446 days. The uniformity of radioactivity distributions was evaluated by visual qualitative assessment and region-of-interest (ROI)-based quantitative assessment. In the case of the Derenzo-like phantom, we additionally scanned the phantom using the helmet-type PET prototype and Inveon small-animal PET system. In addition, we measured the radioactivity of these phantoms using the dose calibrator (Atomlab 500) six times during 300 days and we compared the measured and ideal decay curves. As a result, we found the variability of radioactivity distributions was lower than the 5% variation for the cylinder and tube phantoms. The Inveon PET visualized the smallest hot rod (1.6 mm diameter) of the Derenzo-like phantom and the radioactivity seemed to be uniformly distributed. The measured decay curves of all phantoms almost followed their respective ideal decay curve. We confirmed these phantoms have good uniformity and enough stability for PET system evaluation. They enable to evaluate and calibrate PET systems without radioactive solutions (ex.
¹⁸
F).