Tell the staff of your concerns. They can explain exactly what will happen so you know what to expect. The staff where I had my scans were lovely and explained everything in detail. They totally understood that I was anxious about the cancer aspect and were very kind.
(that made me nervous as I thought they must have seen something in the scan, but it was fine).
It is not like an MRI where your whole body is in an enclosed space, it is more like a doughnut that you pass in and out of with only a part of your body enclosed at any one time. I think it helps to close your eyes. If you are really worried it might be worth getting some medication to help.
I really hope it goes well for you. Sending lots of good wishes your way. Let us know how you go. Xxxxoooo Deanne