Providing you make them aware well in time, they can arrange a mild sedative for you.
Modern MRI scanners are open at both ends and it may be possible for you to have your head, or part of it outside, or at least not so far inside that it feels it is inside. I was able to look at an interesting mural on the ceiling of the scanner-room just by moving my eyes up towards the top of my head.
I have also had a couple of scans in an open scanner. In these you are posted in sideways and being open on 3 sides they are far more comfortable if you don't like being enclosed.