I think the worry is worse than the reality. I was also worrying about what they would find, but I told myself Im so lucky to have access to MRIs. Lots of people in the world dont get such great care.
I've had head MRIs. They put headphones on you playing music (radio 2! Not ideal but at least its a distraction). You can communicate with the operator and they dont run it constantly so keep checking on you. You can have a mirror above you so you can see the operator.
You aren't "immobilised", you have to stay still so they get a good image. You aren't trapped, you can ask to come out at any time.
To get it over quicker I stayed still and stayed in and said I was OK and asked to keep going every time they checked on me.
I also tried to go to a happy place and imagine sunshine and beaches. Bit difficult as I couldn't do deep breathing as I needed to stay still. But tried counting backwards 3,2,1 relax "my name" as it says in hypnobirthing.
A calming hypnotherapy cd could really help. I used hypnobirthing techique in dentistry as well, and whenever they are doing anything medical to me as I can faint if not careful and coming round is scary.
In the MRI I tried to imagine relaxing each body part starting from my toes and working up like they do in yoga (in yoga you tighten then release each muscle group but id only do this in your feet/legs and hands/arms). So imagine relaxing slowly in turns: your toes feet ankles calves knees thighs pelvis tummy, back, fingers hands wrists arms shoulders neck head. When you are done tell yourself you are beautifully relaxed. Try and imagine a calming voice speaking to you.
When done with that imagine light flowing through you the same way, as the light travels tell yourself it is releasing tension.
Other things you can do: Think of a calming mantra like "its all fine, i am going to get through this as quickly as I can, i am ok'. Say it to yourself in calm confident manner'.
Good luck. You'll be fine.