Monte Carlo Simulations of the GE Signa PET/MR for Different Radioisotopes

NEMA characterization of PET systems is generally based on (18)F because it is the most relevant radioisotope for the clinical use of PET. (18)F has a half-life of 109.7 min and decays into stable (18)O via β+ emission with a probability of over 96% and a maximum positron energy of 0.633 MeV. Other commercially available PET radioisotopes, such as (82)Rb and (68)Ga have more complex decay schemes with a variety of prompt gammas, which can directly fall into the energy window and induce false coincidence detections by the PET scanner. METHODS: Aim of this work was three-fold: (A) Develop a GATE model of the GE Signa PET/MR to perform realistic and relevant Monte Carlo simulations (B) Validate this model with published sensitivity and Noise Equivalent Count Rate (NECR) data for (18)F and (68)Ga (C) Use the validated GATE-model to predict the system performance for other PET isotopes including (11)C, (15)O, (13)N, (82)Rb, and (68)Ga and to evaluate the effect of a 3T magnetic field on the positron range. RESULTS: Simulated sensitivity and NECR tests performed with the GATE-model for different radioisotopes were in line with literature values. Simulated sensitivities for (18)F and (68)Ga were 21.2 and 19.0/kBq, respectively, for the center position and 21.1 and 19.0 cps/kBq, respectively, for the 10 cm off-center position compared to the corresponding measured values of 21.8 and 20.0 cps/kBq for the center position and 21.1 and 19.6 cps/kBq for the 10 cm off-center position. In terms of NECR, the simulated peak NECR was 216.8 kcps at 17.40 kBq/ml for (18)F and 207.1 kcps at 20.10 kBq/ml for (68)Ga compared to the measured peak NECR of 216.8 kcps at 18.60 kBq/ml and 205.6 kcps at 20.40 kBq/ml for(18)F and (68)Ga, respectively. For (11)C, (13)N, and (15)O, results confirmed a peak NECR similar to (18)F with the effective activity concentration scaled by the inverse of the positron fraction. For (82)Rb, and (68)Ga, the peak NECR was lower than for (18)F while the corresponding activity concentrations were higher. For the higher energy positron emitters, the positron range was confirmed to be tissue-dependent with a reduction of the positron range by a factor of 3 to 4 in the plane perpendicular to the magnetic field and an increased positron range along the direction of the magnetic field. CONCLUSION: Monte-Carlo simulations were used to predict sensitivity and NECR performance of GE Signa PET/MR for (18)F, (15)O, (13)N, (11)C, (82)Rb, and (68)Ga radioisotopes and were in line with literature data. Simulations confirmed that sensitivity and NECR were influenced by the particular decay scheme of each isotope. As expected, the positron range decreased in the direction perpendicular to the 3T magnetic field. However, this will be only partially improving the resolution properties of a clinical PET/MR system due to the limiting spatial resolution of the PET detector.