Smiffkin,
I am a radiographer, and do these scans all day and night (when on call). Shouldn't be any problems. 6ft long 50cm wide tunnel, open at both ends. Machine is noisy but ear defenders should deaden this (some places even play the radio). There's a "panic" button so if you do feel nervous you can summon someone- no one that I know/knew has ever died while scanning, and we do not tend to work alone.
The worst patients for compliance are ex-rugby players.
Only ever heard on one patient getting stuck in a scanner and that was in Japan, over a bank holiday weekend, but they managed to crawl out.
You'll be fine.