T₁-Dark Rim as a Marker of New and Chronic Active Multiple Sclerosis Lesions: A Serial Study With Frequent 7T MRI

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).

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.

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.

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.