Study of the effects of source type and magnetic field on the spatial distribution of positron annihilation events in PET/MRI applications

. In this paper, we investigate the spatial distribution of positron annihilation events in PET//MRI systems. A spherical source ranging from 0mm to 10mm in radius was placed in water to represent the tumor region. The magnetic field strength was adjusted from 0T to 30T, and three positron nuclides, 18 F , 11 C , 68 Ga , were used. The positron annihilation distribution was compared with the nuclide distribution (to represent pathological tumor), and the differences in these distribution with and without the magnetic field were evaluated. The compression effect of magnetic field on positron distribution was also investigated. For 11 C and 68 Ga sources with radioactive source size (r s ) of less than 4 mm and 6 mm, respectively, it was found that the critical radius ( d of the sphere, which contains 90% annihilated positrons, was significantly larger than its original radioactive source size r s . When the magnetic field was increased to 15T, it was found that the greatest compression occurred with a 2 mm 68 Ga source (which also exhibited the greatest contraction in volume V 1 although this was larger than the source volume V . Our proposed model of the volume source indicates that the positron distribution deviates greatly from the nuclide distribution for high-energy positron emitted in small source.