I had a brain scan in the hospital which showed too much white brain matter relative to gray matter for someone of my age (56 at the time). The hospital neuro did a spinal tap, which came back clean, and I was basically given a clean bill of health. A few months later my symptoms got worse and I went to different neuro who is a social friend, and ended up with the MS dx. He felt that since I was still in the mild stages of MS it had not yet migrated to the spinal cord, with the implication it might show up there later.
But, to answer your specific question, it is a positive sign but it does not rule out MS.