I posted the "Dorky Question about Spaceship Earth" thread-- I'm not SUPER claustrophobic (I'm OK in elevators, for instance), but certain things set me off, like total darkness.
I also get claustrophobic with turtlenecks! I have one that doens't bother me at all, I guess it's pretty loose, but if I wear the average turtleneck, I tug at the neck all day long and start feeling like I can't breathe!
Re: MRIs--I had an MRI last summer after my dr. told me there was a 20% chance I had a brain tumor!! (Happily, I did not.) He gave me 10 mg of Valium--5 mg to take 20 minutes before, and 5 mg to take as I walked in the room for the MRI. The MRI folks were very accomodating (apparently a lot of people have to take Valium for MRIs) and told me exactly when to take the doses.
Also, when the tech who operated the MRI heard I was claustrophopic, he put a little mirror device in front of my eyes that reflected the area in front of the MRI machine where my DH was standing. So it was like a little window. It was very reassuring to have that.
However, once the MRI started, I kept my eyes closed the entire time (the way I do when I inadvertantly end up on totally-dark WDW rides
) and spent the entire 45 minutes walking through Epcot, going on rides, browsing around the World Showcase, having a drink at the Rose and Crown, etc. At least, my mind walked through Epcot while my body was stuck in the MRI machine!