If you relax and close your eyes, you will be fine. Keep breathing deeply, and don't think about where you are, take your mind off to somewhere you enjoy and focus on that.
As advised, the machine does clank a bit, but that's just a minor irritation.
They do give you a rubber bulb attached to a tube to hold - if you find yourself distressed, squeeze it, and they will stop the scan and take you out of the scanner immediately.
I think the thought of a 'fail safe' will help to control your anxiety.
Good luck.