Hi Blue Heron,
Sorry to come in late, but everyone has looked after you well as usual - this is a wonderful place with amazing members.
I have very similar pain to you referred from my neck down my right arm, getting worse down my left where I have numbness down to my fingers. The cause, found by an overrun head MRI is a compressed spinal cord. I will find out the details when I see my Neuro next week.
Your MRI will, most likely, show the cause of your issues, and then your Neuro or NS will be able to target the best course of action.
Good luck tomorrow, keep relaxed in that annoyingly cramped and loud machine.

Dave.