When starting a patient with MS on a DMT, how frequently should MRIs be ordered to assess whether the treatment is effective?  

I typically obtain an MRI brain around 6 months after initiation of a new DMT and then monitor MRIs yearly thereafter, unless new clinical symptoms develop. 

I agree with above though I typically get an MRI C/T spine without contrast every year as patients can have silent progression in the cord without changes in the brain.