Just my thoughts...
A MRI is much better and safer for soft tissues. Not the best for bone, though. Your doc may be looking for bony issues that are irritating or impinging on soft tissues. You doc or nurs should be comfortable in discussing the choice since both tests are rather expensive.
Here's are links I Googled that says it better.
http://www.listen-up.org/med/ct_mri.htm
http://www.thenakedscientists.com/fo...php?topic=8971