Hi, Hope your results come back with a solution in mind to your back problem.
Just thought I would explain the difference between a CAT Scan and an MRI.
A CAT Scan uses x-rays to produce quite a high resolution image of what is being scanned. Cheaper to run than an MRI Scanner and is older technology. Good points: A lot quicker and not quite so enclosed, a big plus if you hate enclosed spaces. Bad Points: Radiation does nothing for your health. Ask the people around Sendai in Japan on that one. Living around there for 1 year is equivalent to having a CT Scan. Also, the quality of image of the soft tissue not as good as an MRI.
An MRI Scanner uses a different technology (Magnetic Resonance) to jiggle about the cells in your body around a huge magnet to produce an image. Good points: No known danger to your health. Better/High quality image on what is being scanned. Bad Points: You are put in a enclosed space (not good for clostrophobia) and costs more to run (only bad for Health Authority's Budget).
Here endeth the lesson