@Siewli PET scans can be a bit daunting--such mixed emotions where you hope the news will be good, but are terrified it will be bad.
I don't know what info about the test you have been given, but one of the most important things is to be relaxed.. which is something you have to fake on the odd occasion.
You will need to be really still for about an hour. Try to concentrate on that, rather than what the results might be. The people doing the tests are absolute pros who will not give you the slightest clue about what they think. So don't look for any of that.
Breathe slowly--I use a counting game where I start counting backward from 27 to 0. Then 26 - 0. Etc until 1-0. One long inhale and exhale per number. If I slow right down, that program can take me over an hour. It's surprisingly relaxing and distracting. But the, I'm a massive nerd. Why 27? If you finish the count, do the maths. :)