Heidi, I have a prescription for Valium. When I had a fine needle aspiration done on my thyroid I took 10mg about 1 hour before the procedure. I remember feeling very serene and not caring what they did to me. It wore off in a couple of hours.
For all my MRIs I've taken 5mg of Valium about 1 hour before; it seems to take the edge off. My last MRI, October 2011, was a pleasant experience with the Valium, a washcloth over my eyes, headphones on and music. I still felt in control of my self and the situation.
My next MRI, May 2012, I plan to take 1 1/2 of the 5 mg pills since it's at a new facility, different people, etc so hopefully that will do it for me.
When I've asked my doctors for a prescription they've written it for either 1 or 2 pills only to be used for the specific procedure. It might not be a bad idea to ask for an extra pill of whatever they prescribe to take 48 hours in advance to see how you react. One thing with that that happened to me was I took a Valium when I wasn't "totally stressed" and had a horrible hung over feeling after. I was told that happens when you don't have the adrenalin racing for the drug to work against.
Good luck.
Karen