Exploring Pathology Related to Slowly Expanding Lesions Using Advanced Imaging

Smoldering inflammation is recognized as a critical contributor to MS progression-related central nervous system (CNS) damage. Activated microglia and macrophages particularly at chronic lesion edge are believed to promote lesion growth. Reversing their harmful activity may prove to be an efficient way to halt progression independent of relapses in MS. These smoldering, or chronic active lesions can be detected in vivo using advanced imaging techniques. 1) Specific algorithms can be used to identify lesion growth, with a hypothesis that the slowly evolving lesions (SEL) are the ones harboring a rim of activated microglial cells, which contribute to damage in the surrounding tissue, and lesion growth. 2) Lesions partially or entirely surrounded by rims of increased tissue intensity on QSM-MRI (quantitative susceptibility mapping) sequences are considered as iron rim lesions, with iron-containing proinflammatory microglia/macrophages at the lesion edge. 3) In addition, 18 kDa translocator protein-positron emission tomography (TSPO-PET) imaging can be used to identify chronic active lesions based on TSPO-expression by activated innate immune cells, and their gathering at the edges of chronic active lesions. The TSPO-PET analysis of chronic active lesions can be semi-automated, and the specific radioligand binding at the chronic active lesion edge can be quantitated, which enhances the sensitivity of this method. Despite existing preliminary data demonstrating increased QSM signal TSPO-positive lesions, it is yet to be demonstrated how these three imaging methods perform in identifying chronic active lesions when compared to each other at larger scale.