Evaluation of the effect of magnetic field on the positron range/PET spatial resolution in clinical PET scanners

Objectives With the recent advance in technology, simultaneous PET/MR scanners have become a reality. In this work, we evaluate the effect of the magnetic field on the positron range/PET spatial resolution using experimental phantoms and electron transport simulations (EGS).

Methods A 68Ga (a relatively long range positron emitter) line source surrounded by water and a 68Ga contrast phantom were scanned on the Siemens’ Biograph-mMR (mMR) and on the Biograph-40 (B40) to measure spatial resolution and sphere contrast with and without magnetic field. The phantoms were aligned along the magnetic field and positioned near the center of the field of view (FOV). The EGS simulations were performed with a 3T and 0T magnetic field. The resolution and contrast for the plane perpendicular to the magnetic field were measured for the experimental and simulated images.

Results The FWHM for the 68Ga line source was 5.44 mm for the B40 and 5.28 mm for the mMR while 5.98 mm for 0T and 5.18 mm for 3T with EGS. The FWTM was 9.88 mm for the B40 and 9.45 mm for the mMR while 10.96 mm for 0T and 9.57 mm for 3T with EGS. Experimental contrasts with and without magnetic field were nearly identical, while simulations predicted a slight improvement under a 3T magnetic field. The difference between the experimental and simulated results might be due to the differences between the two scanners (e.g. different transaxial and axial FOV).

Conclusions A 3% reduction of the FWHM in resolution was observed from the 68Ga line source phantom, while the EGS simulation suggests a 13% reduction of the FWHM under a 3T magnetic field. Small effect on contrast was observed from simulations and experiments at 3T for 68Ga. Experiments and simulations will be performed for lower density materials and other long range positron emitters such as 76Br. Resolution parallel to the magnetic field will also be evaluated. A detailed discussion on the effect of the magnetic field on the point spread function (1D and 3D) as well as its effect on resolution and contrast recovery will be presented