 We hypothesize that cortical perfusion is a marker of cortical disease severity and correlates with neurocognitive impairment. To show this we will measure regional cortical perfusion and regional brain and WM lesion volumes in 26 predefined brain regions using a template developed for Alzheimers disease. Regional perfusion will be correlated with neurocognitive tests validated for MS use. Patients will be divided into impaired and non impaired and perfusion compared controlling for known confounding factors. If confirmed cortical perfusion may be utilized as a surrogate marker of cognitive outcome in therapeutic studies. 