Objective: To compare the effect of implanted medical materials on 18 F-fludeoxyglucose ( 18 F-FDG) positron emission tomography (PET)/ MRI using a Dixon-based segmentation method for MRI-based attenuation correction (MRAC), PET/CT and CT-based attenuation-corrected PET (PET CTAC ).Methods: 12 patients (8 males and 4 females; age 58611 years) with implanted medical materials prospectively underwent whole-body 18 F-FDG PET/CT and PET/MRI. CT, MRI and MRAC maps as well as PET CTAC and PET MRAC images were reviewed for the presence of artefacts. Their morphology and effect on the estimation of the 18 F-FDG uptake (no effect, underestimation, overestimation compared with non-corrected images) were compared. In PET MRAC images, a volume of interest was drawn in the area of the artefact and in a reference site (contralateral body part); the mean and maximum standardised uptake values (SUV mean ; SUV max ) were measured.Results: Of 27 implanted materials (20 dental fillings, 3 injection ports, 3 hip prostheses and 1 sternal cerclage), 27 (100%) caused artefacts in CT, 19 (70%) in T 1 weighted MRI and 17 (63%) in MRAC maps. 20 (74%) caused a visual overestimation of the 18 F-FDG uptake in PET CTAC , 2 (7%) caused an underestimation and 5 (19%) had no effect. In PET MRAC , 19 (70%) caused