Evaluation of the effect of magnetic field on PET spatial resolution and contrast recovery using clinical PET scanners and EGSnrc simulations

We describe an evaluation of the effect of the magnetic field on the PET spatial resolution and contrast recovery for short and long range positron emitters using experimental phantoms scanned on clinical PET/CT and PET/MR scanners as well as using electron transport simulations. A ²²Na (a short range positron emitter) point source surrounded by Lucite, a ⁶⁸Ga (a relatively long range positron emitter) line source surrounded by water, and a ⁶⁸Ga contrast phantom with various sphere sizes were scanned on Siemens' Biograph-mMR (magnetic field strength: 3 Tesla) and Biograph-40 (no magnetic field). The electron transport simulations were performed from 0T to 11T for ²²Na, ⁶⁸Ga, and ¹⁵O for the point source, line source, and the contrast phantom. It was observed that the magnetic field has very small effect (< 1%) on the resolution of short range nuclides such as ²²Na based on both simulation and experimental results as expected. For long range nuclides such as ⁶⁸Ga slight improvements in spatial resolution and contrast recovery were observed on the plane perpendicular to the direction of the magnetic field from phantom experiments and simulations with 3T magnetic field for the human scanner. The degree of improvement is proportional to the positron range of the nuclides as well as the strength of the magnetic field, and it saturates at ~ 7T for all nuclides used in this study according to simulation results. For the plane parallel to the direction of the magnetic field, worse resolution and better contrast recovery were observed due to more positron annihilations deposited along the direction of the magnetic field (i.e. re-distribution of positrons). With regard to the results obtained from the simulations for a scanner with better intrinsic resolution (2 mm PSF), the improvement in FWHM saturates at a higher field strength (> 11 T) as compared to that for a human scanner (4.7 mm PSF). However, worse FWHM was observed in all directions at 3T as compared to that at 0T due to re-distribution of positrons and the interaction between the low frequency high energy positrons and the scanner's narrower intrinsic resolution kernel, while the FWTM and contrast recovery still improve at 3T. FWHM was observed to improve with higher magnetic field strength (> 3 T). It was also found that the intrinsic resolution of the PET scanner needs to be worse/wider than 2.35 mm to observe an improvement in FWHM for a ⁶⁸Ga point source in water under a 3T magnetic field as compared to that under 0T. In addition, a directionally dependent resolution modeling which accounts for the effect of the magnetic field in iterative PET reconstruction was observed to improve the resolution recovery and produce more uniform resolution in all directions within the magnetic field.