It's pretty common practice to get anti-anxiety meds for MRI's as people can get very paranoid being in such a confined space. You must also be absolutely still for the images to come out clearly. It is also very noisy and creepy in the tube and for H&N scans you are placed right in the middle of it. This is not the same as sedation - it's nothing more than a muscle relaxant. Until the MRI is actually done the doctors are guessing at this point.

Bear in mind also that the adverse reactions in the PDR are required to be listed (by the FDA) if even 1 in a million have that response (and it may not even be attributable to the drug). Many of us have been on Fentanyl and had no problems, only a relative few have had adverse reactions. The way adverse reactions are written up in the literature is a joke and almost useless. The best advice you can get about a drug is from a pharmacist. Most of those adverse reactions in your list are more indicative of an overdose.

Morphine is frequently prescribed with Fentanyl for breakthrough pain and they typically work well together.

I would be more suspicious of a reaction from a contrast agent that is frequently ordered for H&N MRI's and allergic reactions to that are not uncommon.