Is proper sedation an option?
Hope you get the answers you are looking for, im claustraphobic also and hate MRIs etc but i dont actually freak out, i manage to talk myself down. If its as bad as you say, surely they could give you something to calm you?
Explain to your doctor the severity of your symptoms and see if he can prescribe something specifically for MRI or Cat Scan. You shouldnt have to panic your way through it, its unfair. Xanax works well for me in similar situations, even Valium helps.
Good luck!
Maz XX