I've had no end of these and they are a piece of cake.
When you arrive, you'll be fitted with a canula (The thing in your arm the dye will be injected through). You lie on a bed facing something that looks like nothing more than a giant Polo. They move you forward until you head is about level with it, then they move you back. A voice tells you to hold your breath, somebody else tells you the dye is going in, they move you forward, then they move you backward a bit more slowly. It's pretty quiet, nothing like an MRI. Whole thing takes no more than five minutes.
The only minor thing is that the dye makes you feel warm and that you're likely to pee. You won't. It's just the dye.
Nothing to worry about.
Good luck with it.