Calculation of Positron Distribution in the Presence of a Uniform Magnetic Field for the Improvement of Positron Emission Tomography (PET) Imaging Using GEANT4 Toolkit

Introduction Range and diffusion of positron-emitting radiopharmaceuticals are important parameters for image resolution in positron emission tomography (PET). In this study, GEANT4 toolkit was applied to study positron diffusion in soft tissues with and without a magnetic field for six commonly used isotopes in PET imaging including 11 C, 13 N, 15 O, 18 F, 68 Ga, and 82 Rb. Materials and Methods GEANT4 toolkit was used to simulate the transport and interactions of positrons. Calculations were performed for the soft tissue phantom (8 mm ×8 mm × 8 mm). Positrons were emitted isotropically from the center of the phantom. By the application of a magnetic field perpendicular to the path of positrons, lateral scattering of positrons could be prevented due to Lorentz force. When the positron energy was below the cutoff threshold (0.001 MeV), the simulation was terminated. Results The findings showed that the presence of a magnetic field increased the rate of positron annihilation. At magnetic field strengths of 3, 7, and 10 Tesla, 18 F with the lowest decay energy showed improvements in the ratio of full width at half maximum (FWHM) resolution to the peak of curve by 3.64%, 3.89%, and 5.96%, respectively. In addition, at magnetic field strengths of 3, 7 and 10 Tesla, 82 Rb with the highest decay energy showed improvements in resolution by 33%, 85%, and 99%, respectively. Conclusion Application of a magnetic field perpendicular to the positron diffusion plane prevented the scattering of positrons, and consequently, improved the intrinsic spatial resolution of PET imaging, caused by positron range effects.