I have experiences, oh so many experiences!
MRI, CT, PET scans, more than I can count.
The first time I refused to let them put me in the tube, they ended up taking an hour and I went in backwards. I also refused to breathe when they tried to give me an anaesthetic with gas:) I'm laughing now because really, it's so easy.
You are not being ridiculous!
Firstly, yes, valium. Plenty of it. Just calms down the panic a bit.
Secondly, realise that it's not always so claustrophobic, the ones I've had are mostly open at the other end, donut style. So as they move you around and you are mostly out of it the whole time. That thought scared me until someone said "look, thats the other side, it's open".
Thirdly, genius tip from a nurse one. She said "close your eye" just as you start to move. The ones I've had scans with have the part you lie on (they tuck you up, it's cosy), they raise this up and move it into the scanner. The lifting up gives you the feeling of less space, so close your eyes for that bit. I thought that made a huge difference. Keep your eyes closed and think of something else. The time goes quickly.
At any time you can say, "Stop" and they will immediately take you out.
Take that valium and if that doesn't work, get a stronger medication from the same group for a one off, it's not unreasonable at all
(I'm a bit panicky writing about it, but it's the results that always terrified me, the scan part is easy).
Oh, I get an IV for mine, PET/CT, once the IV tube disconnected flew off my arm inside the scanner and warm liquid flew everywhere, all over my face, I thought it was blood. I was fine because in my mind I was imagining I was at the beach, even with that I was still relaxed. It was just saline:)
Definitely get valium, or lorazepam, or temazepam, something strong to help you really chill out. Someone will have to drive you home for the stronger ones, but I'd go for that to chill out as much as possible.
I refused to breathe and had an MRI upside down, now I'm chilled out with PET/CTs. Sedatives and close your eyes.
You'll be fine