A 51-year-old man referred to our department to undergo an F-FDG PET/CT to detect primary tumor 7 days after diagnosis of one left brain metastasis on MRI in the aftermath of acute right-sided hemiparesis. F-FDG PET/CT was performed without sufficient fasting period. It showed abnormal uptake in the left apical lung nodule, suspicious for primary tumor. Moreover, F-FDG PET/CT showed extensive skeletal muscle accumulation which was more significant as expected but concerning only the left side of the body. So, acute right-sided hemiparesis was responsible for no muscle uptake, probably explained by denervation and altered muscle energetics.