I’ve had several MRIs, but mostly of my feet and legs. My experience has been mostly that it’s boring and a little uncomfortable, but basically benign.
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I removed all my clothes, glasses, jewelry, etc., and put on a cotton hospital gown and socks with some rubber tread.
I lay down in a bed, with my feet aimed towards the machine. They spent some time positioning my body the way they wanted it, double checked about any metal that might be on me or in me (any metal stitches or devices?) and gave me earphones and a panic button. They promise to stop immediately if I press the panic button.
The bed slides foot-first into the machine. When it was just my ankle, my head actually stayed outside the machine, but I assume you will go all the way in. My head was just barely inside for my knee.
The hole at the head-end stays open. There’s light. I can see. (not that there’s anything to look at) There’s a little air current.
The tech speaks to me over the headphones, and tells me they are starting the first series. There are really loud clanking noises, I try really hard to be still.
The noises subside, the tech asks if I’m okay, and then warns me they are starting the next series. Rinse and repeat.
Once, I really wanted to move some part, and asked if I could between images, and the tech said that was okay. That was a long scan, though, with lots of images.
I’ve never actually fallen asleep during one (they are too noisy for me to sleep) but a lot of people do. You are just lying there, and it’s reasonably comfortable other than the sound.
Eventually, they are finished, the the bed slides out of the hole. The tech comes in and offers help getting out of the bed. They won’t even bring my glasses into the room, due to fears that the frame might be pulled by the enormous magnet and break or something, but I notice the tech wears his glasses. I leave and change back into my things.
Easy peasy. But I don’t get horrible claustrophobia.
I am terrified of valium. My mom took it once and slept for a week afterwards, rousing enough to use the restroom, but not enough to actually do anything. I haven’t taken valium, but I’ve had sedatives in the same family, and I felt like shit for a long time after it was supposed to have worn off. YMMV, I seem to be at the extreme end of “react unpleasantly to this class of drugs”. But if you think you can lie still for 45 minutes (or whatever they tell you it will be) I’d skip the drugs.