I had one which only involved my lower body. I was in touch via headphones and a little button switch which I could press to talk to someone. It was fine. I must confess that I am a bit claustrophobic and would have been scared if my top half had been slid into the machine; but I reckon I would have coped, as I did when I had cataract operations. When you have to go, you have to go. If it's not an eye op, then you shut your eyes and count your slow breaths, focussing on how many seconds per breath and thus how many minutes are left. I am a huge physical coward, especially with eyes, but by focussing on counting, and inspired by my need to show my granddaughters that stuff like that is nothing to be too scared of, I get through it.
Good luck, and try not to worry. It's never as bad as you think it might be.