Significant head motion can cause either hypermetabolic or hypometabolic cerebral activity on 18F-FDG PET/CT.

We report 2 separate cases with significant head motion causing apparent hypometabolic and hypermetabolic cerebral activity on FDG PET/CT. Case 1 is a 57-year-old man with facial numbness status post chemotherapy for mantle cell lymphoma. Restaging attenuation-corrected PET showed increased left frontal region activity, whereas nonattenuation-corrected PET demonstrated evidence of patient motion with normal physiologic left frontal cortical activity. Case 2 is a 78-year-old man with history of malignant melanoma. Attenuation-corrected PET/CT demonstrated diffusely decreased activity in the left cerebral hemisphere, whereas nonattenuation-corrected PET showed significant head motion with intact cortical activity.