Lumbar/joints/Sacrum MRI Scan?

I am having a MRI tomorrow and am now really nervous. I have been experiencing lower back pain/leg pain for a couple of months now but lately it has eased off, still sore when I lay flat. If the MRI scan does not show anything, what happens next? (my GP referred me to the Orthopedics but they sent me a letter saying i need a MRI scan first)