Luna,
There are lots of people on recumbents that start riding them due to back pain/problems.
So, depending on your MRI, personality, and desires, remember that you still have another bike option.
I'd be happy to give you choices/options, etc. if you want to try going bent. Fortunately (or unfortunately) there are as many styles of bents as there are DF (safety or upright) bikes.
Also - I had an MRI on my shoulder a few weeks ago. I totally panicked, had the worst few minutes of my life, heart racing, sweating, just awful. Then something happened and I almost laughed out loud. The rest of my 40+ minutes in the machine were boring. My point is, don't psych yourself out, like I did! If I hadn't been so concerned about panicking, I don't think I would have and the whole thing would have been boring rather than scary at first.