Abnormalities in normal appearing tissues in early primary progressive MS and their relation to disability: a tissue specific magnetism transfer study: "BACKGROUND: Patients with primary progressive multiple sclerosis (PPMS) often develop severe disability despite low levels of abnormality on conventional magnetic resonance imaging (MRI)....CONCLUSION: MTR abnormalities are seen in NAWM and NAGM in early PPMS and both are associated with disability. NAWM MTR abnormalities are more closely related to conventional MRI measures than those seen in NAGM...."