First things first, tell them well in advance about your claustrophobia, you won't be the first such person they've seen and they will have advice for you. Secondly, which part of the body are you having scanned, if it's the lower half then you shouldn't feel any problems. If it's the top half then you will be enclosed and there's no avoiding it, the space is limited and the machine gives of a variety of weird, quite loud noises which can be a little disconcerting. I had a brain scan and it took some 10-15 minutes inside the machine and it is important that you stay still throughout so they might suggest a light sedation. As long as you tell them up front they can be prepared and you won't then feel bad if you can't go through with it and waste an appointment slot. Good luck and I hope all goes well.