I am a nurse when I am not camping out on this forum, and I can tell you that Valium, Xanax, Ativan, etc. all usually help "take the edge off" for most people. BUT, as with any medication, one cannot truly know with 100% certainty how it will affect him/her until they take it. There are a few out there who take these meds, and have adverse reactions (not necessarily allergic) but it just doesn't do what it was intended...like you may be more anxious. That is most likely a low probability. The same thing can happen with Tylenol or Advil too, so please don't be afraid to use one. That is what they are there for. As stated by the others, you will need someone to drive you home, as they will not let you drive if you take one.
For me, MRI's are no problem. I consider myself lucky on that. "Closed" MRI's often give better quality images than "open" MRI's. So it may depend on what exactly they are looking for. Most places have protocols, and certain tests must be done in certain MRI's, etc. I didn't get headphones in mine, but I got earplugs, and it didn't take too long. The lady talked w/ me through a speaker and kept me up to date and relaxed. You may want to talk with your doc or have him put you in touch with a radiologist from the facility you will be having this done at, if you have questions. Try your best to relax and I am sure you will find out that it may not be as bad as you thought
Hope everything goes well!!!