Mohsen Mashayekhi - Physics Department, Hakim Sabzevari University, Sabzevar, Iran.
Ali Asghar Mowlavi - Physics Department, Hakim Sabzevari University, Sabzevar, Iran. International Centre for Theoretical Physics (ICTP), Associate and Federation Schemes, Medical Physics Field, Trieste, Italy.

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 11C, 13N, 15O, 18F, 68Ga, and 82Rb. 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 cut-off 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, 18F 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, 82Rb 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.

Positron, PET Image, Magnetic Field, Geant4 toolkit