 Multiple Sclerosis MS is a common disease affecting 11000 Canadians. Cognition impairment is reported in 40-65 of patients and is socially and functionally disabling. Although multiple sclerosis is widely regarded as a white matter disease, cortical disease burden is increasingly emphasized. Studies confirm that gray matter GM disease is grossly underestimated by conventional MRI. Although the cause for MS is unknown vascular impairment is implicated in nerve cell death. Several studies have shown perfusion abnormalities in the central GM and white matter WM structure. Severity of perfusion reduction correlates with lesion load, atrophy, MS subtype and disease duration. Further extent of cortical atrophy correlates with neurocognitive impairment. 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. 