Study of the effect of magnetic field in positron range using GATE simulation toolkit

In simultaneous PET-MR systems, the emitted positrons trajectory is influenced by the magnetic field. The aim of this study is to define that influence to the positron annihilation distribution. Monte Carlo methods have been applied, using GATE. Several isotopes were studied, in various types of materials and with different magnetic field strengths. The results showed variations in the positron range between different components and especially between the lung and water. Measurements of the 1-D positron annihilation distance indicated a reduction of the mean positron annihilation distance for (82)Rb of similar to 25%, (68)Ga of 19% and (18)F of 3.5%, at 3 Tesla. When the magnetic field was increased to 9.5 Tesla, the reduction was significant for all isotopes, and mainly for (68)Ga and (82)Rb, with approximately 41% reduction of the mean positron annihilation distance in water. Finally, the positron annihilation distribution varies according to the alignment with the magnetic field lines. The results of this study could be used to improve positron annihilation correction algorithms for simultaneous PET-MR acquisition, by taking under consideration the non-isotropic distribution.