Are ct scans necessary before spinal fusion surgery?

It depends: Ct is better in the identification of bony anatomy then mris. Depending on the indications for a spinal fusion the ct scan can be an important adjunct.

Bony anatomy: Ct scans are the best method to demonstrate the anatomy of the bones in great detail. Image reconstructions will help the surgeon plan the details of surgery. Surgery will be safer if the surgeon has a detailed depiction of your anatomy.