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2oldfordrama, I'm not sure if this is the law in all states, but hospitals cannot refuse you ER care because you don't have insurance. I'm pretty sure they have to give discounted prices, etc. Go in and complain of really bad pain or something that would warrant an MRI if possible. Normally, I wouldn't advise this kind of behavior, but desperate times...
What are the expecting see on the MRI. They are not the end all be all. I have had several and noting shows up for my back pain, but it doesn't mean it's not there.
Also, do you see a doctor in addition the the NP? I would ask to speak him/her or someone else at the clinic. It could be one grumpy, judgmental witch.