Simultaneous PET/MR provides an unprecedented opportunity for motion correction. We aimed at developing an integrated MR-assisted PET motion correction method which would allow accurate PET quantification in the presence of respiratory motion. In this study, we evaluated the impact of motion correction on lesions of various sizes by using a deformable motion phantom. A static scan was used as the ground truth for PET activity. The combined MR and PET motion correction recovered the activity from 55% to >97% of the static scan activity.