For the PET/CT, they do want you to have as little "extra" glucose in your system as possible, so that the only glucose is the radioactive one they inject. That would jive with the "last meal should be protein" recommendation.
I've also been instructed to drink water but I don't remember why. Maybe to help flush the contrast out. They inject the glucose then you have to wait 45 minutes - so they give me a bunch of water to drink while I'm waiting, then have me empty my bladder so I can handle laying in the scanner for the next half hour! Sorry if TMI
Afterward, they also have EVERYONE IN THE BUILDING tell me to drink lots of water and sometimes they give me a water bottle on the way out the door.
For PET/CT I've not had to drink the contrast - although I did for CT only. Not too disappointed on that, since they give "berry smoothie" a bad name.
Also, Stephanie's right - no strenuous exercise the day before and the morning of the scan. This instruction has been down to the level of "don't even talk or walk around while waiting for the isotope to travel around". This is because any movement or inflammation will cause your muscles to to "take up" the contrast. So less easy to get clear images.
Here's hoping that "NOTHING" shows up on the scan.