I think your braces would be a problem, you have to fill a form in beforehand about if you have any sort of metal on/in your body, but im not sure what they'd do about it. MRI's are extremely noisy- lots of different sounds though, like a very loud humming noise, lots of short bursts of banging, general being hit with a sledgehammer noises etc etc- they give you music to listen to, but you can't actually hear it above the noise. You have to keep very still for absolutely ages, which means anything other than shallow breathing is out of the question or they have to do it again... and it's surprisingly small- i think the top of it was around 5-10cm above my head at most, so yeah, it's pretty horrible
(I had one on my spine about 2 months ago, i was stuck in there for 50 minutes!)