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

We describe an evaluation of the effect of the magnetic field on the PET spatial resolution and contrast recovery for long range positron emitters on Siemens' Biographm-MR (mMR) and Biograph-40 (B40) multi-modality PET scanners as well as using electron transport simulations (EGS). A ⁶⁸Ga line source surrounded by water and a ⁶⁸Ga contrast phantom with various sphere sizes were scanned on the mMR (magnetic field strength: 3T) and B40 (magnetic field strength: 0T). The line source was aligned with the magnetic field and positioned near the center of the field of view. The acquired data were reconstructed with FBP. The EGS simulations were performed from 0T to 11T for both ⁶⁸Ga and ⁷⁶Br (a longer range positron emitter) for the contrast phantom and line source. The FWHM and FWTM of the line source as well as the sphere contrast of the contrast phantom were measured for the experimental and simulated images for the plane perpendicular to the magnetic field. The resolution and contrast obtained from the plane parallel to the magnetic field were also investigated using phantom experiments and EGS. Slight improvements in spatial resolution and contrast were observed on the plane perpendicular to the magnetic field from phantom experiments and simulations. The degree of the 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. For the plane parallel to the magnetic field, slightly worse resolution and better contrast were observed. 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 in the plane parallel to the magnetic field.