 MS is a poorly understood chronic disease of the central nervous system characterized by focal inflammatory demyelinating lesions and diffuse axonal degeneration. Recent magnetic resonance imaging MRI studies demonstrated a widespread decrease in cerebral perfusion - expressed as cerebral blood flow CBF - throughout the normal-appearing white matter of patients with MS, which seems not just to be secondary to axonal degeneration with reduced metabolic demands, but in contrast may actively contribute to MS pathology. 