Objective: Chronic active multiple sclerosis (MS) lesions represent a particularly destructive subset of lesions on pathology. However, their imaging correlates, including paramagnetic rim lesions (PRLs) detected on susceptibility-weighted imaging, lack sensitivity and are difficult to implement in clinical practice. This exploratory, longitudinal study investigates the prevalence and temporal dynamics of a novel imaging marker, T1-dark rims, and their relationship with PRLs observed on quantitative susceptibility mapping (QSM).

Methods: Four untreated people with MS underwent 7-Tesla MRI scanning six times over a period of 36 weeks. New and pre-existing lesions were analyzed for the presence and temporal evolution of T1-dark and QSM rims. Quantitative T1 values were derived using B1 maps, and the relationship between rim status and lesion size was evaluated.

Results: Of the 159 baseline lesions, 22 (14%) had T1-dark rims, 11 (7%) had QSM rims, and five lesions had both. T1-dark and QSM rims showed temporal changes, with T1-dark rims preceding new QSM rim appearance in three out of four (75%) lesions. Eleven out of 20 (55%) newly formed lesions had T1-dark rims, with a T1-dark rim present in all new lesions over 100 mm3. Small new lesions lacked discernible rims, but their overall T1 values aligned with those of larger lesion T1-dark rims implying shared pathological processes.

Conclusions: T1-dark rims were more common than QSM rims, with greater prevalence in newly formed lesions. We propose they represent edema and inflammation associated with early stages of chronic active lesion formation visible despite, not because of, iron accumulation.