I had an MRE and it was as Scipio described. Just a normal MRI (lying on my back) but had to drink 1 or 2 litres of fluid before. I then had a canular inserted too. When laying down whatever was going through the canular made me feel like I was wetting myself... I wasn't lol, but that's the feeling it gives.
If you're not claustrophobic, you'll be fine with an MRI. It's just the laying so still for such a long period of time that some struggle with. I did struggle with the breathing a little bit as they seemed to want me to hold my breath for ages and then relax, but before I knew it, I had to do it again. It didn't feel like I had enough time to catch my breath again! Lol. But that could be as I was a little panicky as I'm claustrophobic.
If you do suffer with claustrophobia, just let them know beforehand and you can have a sedative. I didn't, but I know people that do xx