Have you had a spinal MRI? If your brain is clean, you might have lesions in your spine. I'd look in to getting one if I were in your situation. Other reasons it might be clean: MRI wasn't powerful enough, Gd enhancement wasn't dosed high enough.
There is also the consideration of other diseases that look like MS. I'm assuming you had a battery of tests for differential diagnosis. MS is a clinical diagnosis of exclusion: there's not test for it and it's general contingent on a neuro exam and the exclusion of a number of diseases that look a lot like MS. For a good list, look at the last 2 pages of this form http://www.bostoncure.org/downloads/BCP-Questionnaire.pdf
I should point out you would not be expected to get tested for each of those diseases, they're just a list of diseases you *could* check for.