Because I have a brain tumor, I have them all the time - way too much, if you ask me :(
Anyway, I too can get very uncomfortable in there, as I'm not a small person to begin with and it's a very tight, small space. So what I do is close my eyes, and daydream. I don't let myself think of anything else other than a daydream. I even sometimes MAKE myself walk through a thought, just to keep me occupied (like actually sewing a quilt block in my mind, seam by seam). I just make sure they're happy thoughts/dreams, and it can range from playing with my dog, to quilting, to reviewing a movie scene by scene, an upcoming vacation plan, whatever - just so it's happy. It took some practice, but I'm pretty good at it now, and can almost fall asleep in there.
I also ask for the most open MRI machine I can have (I'm limited, as it must be a stong machine). The newer machines are so much better than the older ones that wrap you from head to toe, now they're only about 1/2 the body, and are faster so request these. But, since I have to have my head in a 4 way lock-down helmet complete with bars over the face, it means nothing except that when using the mirror, when I can see my feet - I don't feel quite so closed in. And I take a lot of deep breaths :)
Good luck with the MRI. Hope it turns out well.
Debbie in Austin