I had a pet scan after being diagnosed (via an x-ray, ct scan and needle biopsy). So I knew I had lung cancer, and which type, before my scan. The scan was to see if it had spread (good news for me, it hadnt). This is all twelve years ago so my info may be outdated.
I was advised to wear warm clothes with no metal parts (it was in December, so warm wasn't a problem, but the no metal but was). I got there anas has been said, I was given an injection. I then had to wait 30 minutes. strictly no reading or music allowed, as they highlights areas off the brain that they don't want to be highlighted. So that was boring.
The scan itself was ok, much like an MRI or CT, but in my memory it felt longer. And I had to lie perfectly still with my hands stretched over my head. That was the painful thing.
I was also advised to avoid old people, young children and pregnant mothers for twelve hours afterwards.
I wish you all the luck in the world. And as someone who has gone down this path, I know how hard the waiting is. It was over three months for me from x-ray to operation. Doesn't seem so long in retrospect, at the time it felt like aeons were passing.