I've had the great misfortune of having had several MRI's over the course of my life. I also never knew I had claustrophobia issues until going into an MRI tube. Wowza, that's a tight little space!
Anyhow, the last time, they offered to put a cloth over my eyes and for as ridiculous as it sounds, it really helped.
If they don't do that, keep your eyes closed and focus on your breathing.
I've had to have a lot of MRI's with my cancer and seizers. I am extremely claustrophobic and I couldn't have done it without those great little pills that totally calm you gown.