Dexa scans are used in the screening and diagnosis of osteoporosis and osteopenia. I was diagnosed with osteoporosis in my spine after a dexa scan. It doesn't have anything to do with the diagnosis of ms. However, since long-term steroid use and sometimes other medications can cause/lead to osteoporosis, I believe many patients with ms and other auto-immune diseases on those drugs are given dexa scans to screen them.
Hope this helps,
Melissa