Mechanisms of myelin repair, MRI techniques and therapeutic opportunities in multiple sclerosis.

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS). The remyelination process requires the activation, migration and differentiation of oligodendrocyte progenitor cells (OPC) in demyelinated areas. The metabolic dysfunction in chronic demyelinating lesions impairs the activation of OPCs, the myelin debris clearance by microglia decreases with age, along with diminished secretion of factors promoting OPC differentiation. Conventional magnetic resonance imaging (MRI) sequences have limited ability to differentiate unmyelinated and remyelinated lesions. Advanced MRI sequences based on magnetization transfer ratio (MTR), myelin water fraction (MWF) and diffusion tensor imaging (DTI) have been used to evaluate remyelination in clinical trials. More recently, the q-space myelin map (qMM) has been used on experimental and exploratory clinical studies. The improvement of myelin-specific MRI sequences with high reliability and standardization among centers will allow a more accurate evaluation of new therapies to improve remyelination. These new remyelination promoting treatments alone or in combination with current options may reduce the risk of long-term disability in MS.