Simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) provide complementary structural and functional information. Recent developments in continuous infusion functional PET (fPET) have shown promising results to track dynamic changes in brain metabolism. Although fPET provides opportunities to investigate functional metabolism in the brain, the temporal resolution still remains a major challenge compared to functional MRI (fMRI). In this work, we use anatomical MRI information modeled as a Bowsher prior to improve the sensitivity of fPET at higher temporal resolution. We validate our MRI-assisted fPET analysis framework using both in-silico and in-vivo experiments.