I was scheduled for my first post treatment PET/CT scan and the pre scan instructions were: no food 4 hours before scan, no caffine and no strenous excercise 24 hours before scan. I arrived for the scan chewing gum, the one relief I get for dry mouth. They would not do the scan due to the chewing gum since the activity of the jaw muscles would distort the PET scan results, so it was rescheduled. Be sure that you take nothing for granted and ask the dumbest questions you can ask. This can only save you time, but more importantly, a distorted scan.