I know the Thonburi Hospital in Roi Et does MRI scans and are sensibly priced. Their address is 166 Patthamanon , Naimuang, Roi Et 45000. Their number is 043-527-191 (several lines from suffix 192 to 199). Not sure if you are based in UK presently but let me know if you need help calling from Uk. Roi Et airport is 50 minutes from Don Muang. Let me know if you need a reliable taxi service to pick you up at the airport. He takes my wife to and from the airport regularly and has sensible prices. I visited the hospital and the prices are really sensible. Give them a call. You may care to give the public hospital a miss - take a look at my experience in this link:
The prices of the Thonburi rooms and a few other treatments are included in the blog at the end - see the slide show. Best wishes.