In infants, brain (18)F-FDG PET often requires sedation. To keep the sedation mild, the PET acquisition time should be kept short. Therefore, calculated attenuation correction is often preferred to measured attenuation correction. In addition, the infant should be positioned as comfortably as possible. Here, we report a case in which the infant's fist was near her head, resulting in severe artifacts when calculated attenuation correction was applied. Brain (18)F-FDG PET was performed for localization of a focus in an 11-mo-old girl with West's syndrome. After injection of 120 MBq of (18)F-FDG, a 20-min emission scan was obtained. Then, a 7-min hot transmission scan was acquired with 3 rotating (68)Ge/(68)Ga rod sources. Attenuation was corrected both by calculated attenuation correction and by measured attenuation correction using the transmission scan. Images reconstructed with calculated attenuation correction showed apparently increased (18)F-FDG uptake in the skin of the neck. The fist was not visible on the images because it was outside the reconstructed field of view. The lesion of increased (18)F-FDG uptake was not immediately recognizable as an artifact. It might have been misinterpreted as an active process, such as inflammation, in the skin. However, the lesion showed up neither on images reconstructed with measured attenuation correction nor on images without attenuation correction. Detailed analysis of each step in the calculated attenuation correction revealed that the fist caused the boundary detection algorithm to detect not the boundary of the head but a strongly extended boundary enclosing the fist and large "air areas" between the fist and the head. The result was a significant overestimation and overcorrection of attenuation, particularly in the region of skin near the fist.
