I can't be REAL helpful, as I was diagnosed with MS long before MRI's were even invented. But here's what I can tell you:
MRI's are not an exact "science". The same MRI "read" (looked at) by two or three different doctors can sometimes be seen differently...one will see lesions, another won't.
MRI's can change from session to session, even a few days apart, certainly months or years apart. What might show up on one MRI won't be there the next, and vice versa.
MRI equipment differs. The newer machines are much more accurate. "Closed" MRI's, where you enter a tube, are generally more accurate than "open" MRI's where there is space along your sides...but even that is changing as the equipment is improving.
MRI's "with contrast" -- with a dye injected in you before the MRI, and which will show up as white bright areas in the brain -- are sometimes more accurate than MRI's "without contrast".
People nowadays who are diagnosed with MS without clear evidence on an MRI usuallly exhibit the classic signs of MS, which can be detected by the doctor during a physical exam, include altered eye movements, abnormal responses of the pupils, sublte changes in speech pattersn, altered reflex responses, impaired coordination, sensory disturbances, evidence of spasticity and weakness in the limbs, and o-bands in the spinal fluid (obtained through a spinal tap, or lumbar puncture).
I don't know if any of this fits your situation, though.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....