Every doctor has a different answer to this question.

Though not a medical professional , have family members who are and have told me MRI's are a snap shot in time and can vary even within an hour after one has already been taken. My Neuro also agrees with this information. My Neuro as well as I believe that routine MRI's are not required. That an MRI would only be needed if :
1. If new symptoms have developed and it's necessary to rule out other issues before settling in on it being MS related.
2. to evaluate if a change an MS disease modifying drug was being considered.

Once a yr if no flare ups in that year. When a flare happens, of course an MRI is needed. My dr will order MRI after flare up then again 6 months later. Just to compare ex rays.

I just had one after 3 years. I think my neuro doesn't have me get them as often because I've not shown any progression for more than 3 years. I had one of neck, which didn't show any lesions but showed a ruptured disc. Never had any of my thoracic or lumbar spine in relation to MS. It was my understanding that they rely on a spinal tab to determine any lesions in the spine - something I refuse to undergo ever again!

It's usually up to the neurologist. My neuro just had me get one because I have been on Gilenya for 3 years and he wanted to see how it's working. So far, so good. :)

Each doctor is different. I see my neurologist every 6 months and I get an MRI once a year. I have been seeing the same doctor since 2002. Sometimes he orders a brain and spine and then the next time he will order an MRI of the brain.