Denied for 'not medically necessary' after 3 months of back pain - anyone appealed this successfully?

My insurance just denied an MRI my doctor ordered after i had persistent back pain for three months. The denial letter says it wasn't "medically necessary" even though my doctor literally wrote a referral. I'm so frustrated i don't even know where to start.

The MRI was $2,400 and they just... said no. Has anyone been through this and actually won an appeal? Did you need a lawyer or can you handle it yourself? Any advice on what to include in the appeal letter would help a lot.