Scatter effects of MR components in PET-MR inserts

System design research for upcoming PET-MR scanners has mainly focussed on the effect of the high magnetic field on PET performance and on the influence of the PET scanner inside the MR bore on MR image quality. However, the presence of MR components close to the PET detectors could also have an influence on PET performance. We have investigated these effects in a simulation study of the preclinical PET-MR insert and of the proposed integrated whole-body system of the HYPERimage project. Simulations were performed with the ProcessGATE extension of the GATE simulation framework, which makes it possible to determine the fractions of total scatter caused by different components. The preclinical insert was simulated inside a clinical MR scanner. All components of the clinical system and the preclinical insert were modeled in realistic dimensions and materials. The PET detector consisted of 10 detector blocks on a 100 mm radius cylinder, each containing a 44 (tangential) by 72 (axial) array of LYSO crystals. The crystal dimensions were 1.3 ? 1.3 ? 10 mm. The energy window was set to 250 - 750 keV. The integrated whole-body system was modeled as the same clinical MR system with a split gradient coil and PET detector blocks between both parts of the split gradient coil. The PET detector blocks in the whole-body system consisted of 22 detector blocks on a 35 cm radius cylinder containing 22 (tangential) by 43 (axial) LYSO crystals. The crystal dimensions were 4 ? 4 ? 22 mm. The energy window in this configuration was 410 - 700 keV. A uniform cylinder (radius 5 mm, length 100 mm) filled with 1 MBq of 18F was simulated in both the preclinical insert and the whole-body system. The simulated time was 1s yielding one million simulated decays. In the preclinical insert only 47 % of detected singles were unscattered. The clinical system and precinical insert accounted for respectively 38 % and 15 % of scattered photons. On the coincidences level the influence of the clinical system was much smaller (17 %), while the scatter effect of the insert increased (20%). In the clinical system the gradient coils scatter the largest fraction of photons (58 %). In the insert over 65 % of scatter is caused by the table and the RF screen. In the integrated whole-body system 44 % of detected singles were scattered. At coincidence level this fraction was reduced to 34 %. The largest amount of scattered coincidences is caused by the RF screen. In conclusion, it is clear that putting MR components within or close to the FOV of a PET scanner can cause significant scatter. The scattering effect of the MR components should be taken into account in the design phase.