@heavyphil
Wow! I'm so sorry this happened to you, Phil! Was this some time ago?
I had, I guess, a pelvic MRI when they were first investigating my prostate about a year ago, and it was nothing like that.
I wasn't restrained or weighed down in any way, just told to lie still. The tech carefully rolled up a couple of those yellow foam earplugs and inserted them for me, which I thought was very considerate.
The noise, although attenuated by the earplugs, was indeed pretty wild! I would describe it as being in a sort of demented sound bath, one that involved both mechanical and electronic sounds. It wasn't frightening so much as just plain weird.
I'm not prone to claustrophobia, so perhaps that was a factor in being able to relax enough that there wasn't anything traumatic to the experience.
Maybe "full body" is more of ordeal, though, and your experience sounds awful!