I had an MRI of my brain at Mt. Sinai Hospital a few years back, scheduled at about 2:30am on a weekend. No surprise, the place was empty. I was pretty nervous about the claustrophobia aspect. The technician told me that 1 out of 3 people flee the procedure upon arrival, simply after they see the machine She said some of them come back another time armed with some sedative pills from their gp, while many others just never come back ever.
I was in the tube for about 35-40 minutes, without any sedatives, and to my astonishment it wasn't bad. The machine was a newer model by Siemens of Germany, which had mirrors angled in such a way that you could always see your feet and out the tube, even though you are actually facing straight up at a big tube just a few inches above your nose. Clever engineering: those mirrors really trick your brain and help banish the claustrophobia.
I tried to find music in all the random loud clanging noises of the whirring magnets, and the sounds became interesting. Somewhat rhythmically predictable, somewhat not. I'm a musician.
Sometime during the experience, the whole surreal experience became hypnotic and I kind of dozed off. When the technician got on the intercom and told me it was about over, I was astonished, and almost wished the ride had lasted longer.
So good luck - it's not bad if you can relax and close your eyes and concentrate on the clanging.