With MS I have had more MRI than I care to count.
A few things to remember are that you will have a button and can come out as often as you need to, just know that means you won't panic just as much.
It is not a dark tunnel, it is bright, air conditioned and bigger than you think.
Scans are taken in segments, the radiographer will tell you how long the next scan will take, 2 minutes, 5, 8 etc, that is when it gets noisy but subsides quickly. They talk to you the whole time, all these segments add up quickly and makes the whole thing shorter than you think. You're not in the scanner in silence for 20 minutes.
My advice, take sedation if offered, it'll take the edge off you. Don't look at the scanner when you go into the room, I never have, just sit on table, put on eye mask (they sometimes have them but bring your own in case) put ear plugs in, get comfie, scratch your nose (I blow mine before I go in just in case), lie down, the cage is clipped on with ear protectors/petitioners. I only had music with one and it was crackly so don't assume you'll be offered it. As they chat so much it not that necessary. I sleep now but for the first one I used to lie in bed in the position with music in and do breathing exercises, then when I had my first MRI I pretended I was on my bed at home doing my breathing.
I sleep through them now and get irritated when they talk to me but the first one did worry me but it was unnecessary you will be fine. Let us know how you get on. x