Novel Point-Like $^{68}{\rm Ge}/^{68}{\rm Ga}$ Radioactive Source With Spherical Positron Absorber

Purpose: In the conventional techniques for determining the calibration factors of positron emission tomography (PET) scanners, cylindrical water and resin phantoms with radioisotope ¹⁸F or ⁶⁸Ge/⁶⁸Ga are used. In these methods, however, the results depend on attenuation and scatter correction. The purpose of this study is to develop a point-like ⁶⁸Ge/⁶⁸Ga radioactive source that can be used to determine the calibration factors of PET scanners without the uncertainty of attenuation and scatter correction. Methods: A spherical absorber design was employed to realize a symmetric angular distribution of emitted annihilation photons. A Geant4-based Monte Carlo simulation code was used to compare physics characteristics of point-like sources with various absorber materials. On the basis of this simulation, a point-like ⁶⁸Ge/⁶⁸Ga source with a spherical aluminum absorber was manufactured. Its radioactivity was calibrated at an accredited national calibration facility. A calibration factor of a clinical PET scanner was then obtained with a point-like source and compared with that obtained by a standard cross-calibration method. Results: The emission probability of 0.511 MeV annihilation photon pairs per positron decay was typically 0.6-0.8. The fraction of background photon pairs was 6-8% in the energy region of 0.4-0.6 MeV. Considering these two figures, lower density materials such as aluminum and pol(ymethyl methacrylate) (PMMA) were preferable. For the aluminum absorber, a diameter of 8 mm was suitable to prevent positrons from escaping. The calibration factor obtained with the point-like source agreed with that obtained by the standard method within 2-3%. Conclusion: A point-like ⁶⁸Ge/⁶⁸Ga radioactive source was successfully designed, manufactured, and used for determining a calibration factor of a PET scanner. It can be considered a practical tool for calibrating and ev- luating the quantitative aspects of PET scanners.