A newly developed " Flexible PET (fxPET)" is a prototype of MR-compatible mobile PET system. We tried to investigate the clinical feasibility of the fxPET with a 1.5T MRI compared with PET/CT for brain imaging. Twenty-one patients ( including 12 patients with known intracranial masses) were enrolled in this study. We compared misregistration between fxPET/MRI and PET/CT, and compared the image quality of fxPET and PET in both qualitative (visual rating) and quantitative (standardized uptake value (SUV)-based analysis ) manner. Consequently, fxPET/MRI showed acceptable misregistration and enough image quality, revealing clinical feasibility comparable to that of PET/CT.