I had an MRI on my knee several years ago, and at least in my case they stuck my legs in and left my torso outside. My understanding of the technology is that they only need to enclose the part of your body which is being scanned (assuming it isn't an open MRI, of course).
I, too, thought the racket was really bad, but I understand that more modern machines are somewhat less noisy. I wish you the best with your exam. It turned out that I no longer had my left ACL and had to have it replaced.
I won't scare you with that story. I'm sure your experience won't be like mine.
