Radiofrequency coil design for simultaneous PET/MR systems

Simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) can provide both functional and structural information about disease. However, for quantitative PET measurements all sources of scatter and absorption that photons undergo on their trajectory between source and detector must be assessed. In simultaneous PET/MR, this includes the scatter and absorption undergone in MR radio frequency (RF) coils that lie directly in the field of view (FOV). This work quantifies the sensitivity and resolution impact of various coil arrangements in the PET FOV using Monte Carlo simulations for a preclinical system. The sensitivity and resolution impact of five typical RF coils has been simulated by tracking γ-ray scatter locations and by reconstructing ¹⁸F and ⁶⁸Ge sources with activity in the range 0.1MBq to 1MBq. The majority of scattering is found to be from coil support structures, with sensitivity loss up to 25% and spatial resolution loss up to 0.6mm. Subsequently six common supporting polymers were compared for their scattering effect. Typically for 5mm thick material the probability of a true can be reduced by ~10%. The effect of misaligning a surface coil on scatter location is also demonstrated, and having dense electronic materials between 2mm and 6mm from the source is shown to result in a spatial resolution loss ~5% and a sensitivity loss up to 17%.