Effect of tissue density on PET spatial resolution in magnetic environment

Simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) can provide a strong magnetic field to improve the PET spatial resolution by restricting the positron range. In this study, an experimental system and a Geant4 model were built to investigate the difference in improved spatial resolution by using a magnetic field for PET in different human tissues. Experiment results indicated that the magnetic field enabled a greater compressive effect on the positron distribution in a low-density medium. From the simulation results, the magnetic compression was only effective in the direction perpendicular to the magnetic field. Additionally, the saturated relative change rates of the mean positron range were reached for all tissues with increasing magnetic field, and the saturated values exhibited an overall negative correlation with the tissue density. However, despite the considerable difference in the absolute range in different tissues, the saturated values were only in a very narrow range of approximately 45% to 50%, compared to the final relative change rates. It was revealed that the magnetic field can be a more effective method for improving the PET imaging quality for low-density tissues, and there should be an upper threshold for improving the PET spatial resolution using a magnetic field. A range-corrected algorithm associated with tissue density for PET-MRI systems can probably be developed based on the results.