Magnetic resonance imaging to study white matter damage in multiple sclerosis

The integrity of white matter (WM) pathways is important for the preservation of brain functioning and the promotion of recovery after structural damage. Multiple sclerosis (MS) is characterized by the presence of both macroscopic (focal) and “occult” (diffuse) WM damage, whose features may range from reversible inflammation and edema to persistent demyelination and loss of axons. In the present chapter, we review the contributions given by magnetic resonance (MR)-based techniques to our understanding of the pathophysiology of WM damage in MS, as well as their possible role as paraclinical markers to monitor the evolution of such a damage over time. Conventional magnetic resonance imaging T2-weighted MR images are highly sensitive for the detection of MS WM lesions. Such a sensitivity makes them very useful for diagnosing MS, monitoring its short-term activity, and assessing the overall WM disease burden. Post-contrast (gadolinium – Gd) T1-weighted scans allow one to distinguish active from inactive MS lesions, since Gd enhancement occurs as a result of increased blood-brain barrier (BBB) permeability and corresponds to areas with ongoing inflammation, but does not provide information on tissue damage. Chronically hypointense lesions on T1-weighted images (known as “black holes”) represent areas where severe demyelination and axonal loss have occurred. Measurements of brain and cervical cord atrophy can also be obtained from MRI scans to assess the extent of tissue loss in MS. Thanks to the advances in image post-processing, it is now possible to assess the evolution of WM and gray matter (GM) atrophy separately.