Multiple sclerosis (MS) is an inflammatory disease of the central nervous system characterized by lesions in the brain and spinal cord. Magnetic resonance images (MRI) are sensitive to these lesions. A particular type of lesion, called a chronic active lesion, is characterized by a hyperintense rim of iron-enriched, activated microglia and macrophages, and has been linked to greater tissue damage. An MRI technique called quantitative susceptibility mapping (QSM) provides efficient in vivo quantification of susceptibility changes related to iron deposition and identifies these chronic active lesions, called QSM rim positive (rim+) lesions. QSM rim+ MS lesions and their longitudinal behavior have the potential to serve as a biomarker of chronic inflammation and to be utilized to monitor disease progression and evaluate disease-modifying therapies in MS. In this talk, I will discuss the challenges of estimating treatment effects using the longitudinal behavior of QSM rim+ lesions. I will compare two disease-modifying treatments, Tecfidera® and Copaxone®, using linear mixed effects regression models with inverse probability of censor weighting.