Do you have your policy, and what your ins covers? Can you call and ask why they are refusing this service (I'm sure your MRI is listed, and the reason you need it.)
Call the ins and ask what the problem is, if the doc needs to fill out a pre-auth or something.
If they are just trying to rip you off and save a buck, threaten to have them investigated.say you want to talk to the supervisor, and tell him/her if this MRI is not paid for, you'll report this to your state's Department of Insurance.
They'll make the ins co explain WHY a covered procedure is being withheld. The ins co will not want to deal with that.
I have had to get online and fill out a couple complaints with my state's DI. Works like a charm. Things I could not resolve in months and dozens of phone call were fixed in a week - and in my favor.
Just type it in your browser - (your state) dept of insurance.
Don't get mad, get everything.
Good luck.