Yes, they compare MRIs. However, this is not always as simple as it sounds. One can have seemingly healed lesions, yet have the same or worse symptoms as before. It is not always possible to correlate lesions with symptoms, if that is what you're asking.
ess
I had a lesion disappear from one MRI to the next over six months time. The Neurologist said the myelin repaired itself but other lesions appeared on the MRI. He said in the beginning of MS myelin can repair itself. My next two MRIs were unchanged, no disappearing lesions.
Alex