Novel Point-Like Radioactive Source With Spherical Positron Absorber

摘要

Purpose: In the conventional techniques for determining the calibration factors of positron emission tomography (PET) scanners, cylindrical water and resin phantoms with radioisotope $^{18} {rm F}$ or $^{68}{rm Ge}/^{68}{rm Ga}$ 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 $^{68}{rm Ge}/^{68}{rm Ga}$ 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 $^{68}{rm Ge}/^{68}{rm Ga}$ 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 w-nth that obtained by the standard method within 2–3%. Conclusion: A point-like $^{68}{rm Ge}/^{68}{rm Ga}$ 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 evaluating the quantitative aspects of PET scanners.