I don't know much about spinal diseases, but I do know that MRIs have several ways of looking at the body.
http://www.medhelp.org/health_pages/Multiple-Sclerosis/How-does-an-MRI-work/show/79?cid=36
FLAIR, or fluid-attenuated recovery, is a pulse sequence that blacks out the CSF so that the rest of the brain can be seen better. So it's possible that what you're looking at is one of the sequences from the MRI.
Hi and welcome. You're posted this in te Multiple Sclerosis forum - we have lots of very smart people here and someone may know the answer to your question, but I don't
Perhaps with his next doctor visit someone whould go with him to help remember the details - that's what we advise everyone here.
good luck, Lulu