CT scan of the spine usually use for bone problem, usually in accident where Dr wants to look for fracture.

MRI of the spine usually looks for nerves prob and sometimes muscles, fat layers.

So, its base on what problems you have.

MRI does not harm ur cells, its safe. Only sometimes you may feel anxious in the machine tunnel (claustrophobia) for 15-20 min. While CT may harm ur cells, it is like X-rays ( just need more radiation to get the image).

If not mistaken, MRI in gov hosp range from 400-800 depends on which body part you scan. Private not sure, its around 800-1500 I think.