Magnetic field effects on positron range and image quality in PET/MR

This study investigates the influence of strong magnetic fields on the transport of positrons emitted by 18F and 68Ga and how these effects propagate to image quality metrics in tissue-equivalent phantoms relevant to PET/MR imaging. Monte Carlo simulations were performed using PENELOPE 2018, a well-established code for detailed low-energy radiation transport. Although electromagnetic fields are not directly implemented in its main code, the appropriate subroutines were combined to enable tracking of positrons up to annihilation in uniform magnetic fields of 0, 3, and 9 T. Simulations were carried out in four representative media (lung, soft tissue, water, and cortical bone) as well as in NEMA-like image quality phantoms used in a clinical PET/MR experimental study, to assess the impact of the magnetic fields on image quality and radiopharmaceutical quantification. The results demonstrate that positron range significantly affects recovery coefficients and spill-over ratios in NEMA-like image quality phantoms, particularly for high-energy positron emitters in low-density tissues, such as 68Ga in lung. Even under ideal Monte Carlo conditions, full signal recovery is never achieved, and recovery coefficients remain below unity due to the finite positron range. Spillover ratio results depend on the size of the volumes of interest in air cavities, as partial volume effects compromise quantitative accuracy. The magnetic field reduces these effects in the direction perpendicular to the field lines, leading to improved spatial resolution and quantitative accuracy in PET/MR. At 9 T, the dependence of positron range on tissue density is substantially reduced. These findings establish fundamental limits on image quality imposed by positron range in the presence of uniform magnetic fields, independent of detector technology or reconstruction algorithms. They provide a physical basis for the development of positron-range correction methods in PET/MR and offer quantitative insight into the potential benefits of ultra-high-field (9 T) PET/MR systems.