I had nothing on knee mri, but a loose fragment shoes on xray. now doctor wants cat scan. is this necessary prior to arthroscopy and why?

Reasonable: Loose bodies in the joint can be invisible on mri. Ct is much better in identifying calcified bodies in the joint, determining their size, location, and where they may have come from. This information can be important to your doctor before arthroscopy so he or she "knows where to look".

See below: Xray and MRI are very different. A loose body seen on xray may not be visible on MRI if it's small and calcified and the radiologist reading the MRI did get to see the xray. It's also possible that the "loose body" on xray may not be an actual finding. Ct is recommended in this case probably to reconcile the differences as ct can see calcified loose body and the joint spaces fairly well.