How is MS neuropathology reflected in MRIs?

Lancet Neurol. 2012 Apr;11(4):349-360. Epub 2012 Mar 19. Association between pathological and MRIfindings in multiple sclerosis. Filippi M, et al; for the Attendees of theCorrelation between Pathological MRI findings in MS workshop.

The identification of pathological processes that could be targeted by therapeutic interventions is a major goal of research into MS. Pathological assessment is the gold standard for such identification, but has intrinsic limitations owing to the limited availability of autopsy and biopsy tissue. MRI has gained a leading role in the assessment of MS because it allows neurologists to obtain an ante mortem (before death) picture of the degree of brain and spinal cord involvement. A number of correlative pathological and MRI studies have helped to define in vivo the pathological substrates of MS in focal lesions and normal-appearing white matter, not only in the brain, but also in the spinal cord. These studies have resulted in the identification of aspects of pathology and function that were previously neglected, including grey matter involvement and vascular pathology. Despite these important achievements, numerous open questions still need to be addressed to resolve controversies about how the pathology of MS results in fixed neurological disability.

Gad-ehancing lesions; this indicates active inflammation. 
This is an MRI that shows lesions up as white blobs. It is unable to distinguish old from new lesions, which is why we need to compare new and old MRIs to know which lesions have developed since the last scan.
"Although MRI has had a major impact on the field of MS and has demonstrated the iceberg phenomenon, i.e. for every clinical attack there 10 or more lesions that come and go without causing overt clinical symptoms or signs. In other words the majority of MS disease activity is initially asymptomatic, therefore we need MRI to monitor this disease."

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