I may be reading this too late - you may have already left for your MRI. If not, I would definitely ask for the valium. I didn't - what a mistake!
I had no idea I was claustrophobic until the MRI - I was fine with my head being completely inside the machine and my nose almost touching the top, but when they told me to put my arms next to my sides, then moved the machine so that I could barely move my fingers, I started to freak out. That, combined with how incredibly loud the test was, made for a very unpleasant experience. Valium would have made it bearable, I'm sure.
Hope all goes well and you get a good result.