Paramagnetic Rim Lesions in Multiple Sclerosis: Comparison of Visualization at 1.5-T and 3-T MRI

BACKGROUND. Multiple sclerosis (MS) is characterized by both acute and chronic intrathecal inflammation. A subset of MS lesions show paramagnetic rims on susceptibility-weighted MRI sequences, reflecting iron accumulation in microglia. These para-magnetic rim lesions have been proposed as a marker of compartmentalized smoldering disease. Paramagnetic rim lesions have been shown at 7 T and, more recently, at 3 T. As susceptibility effects are weaker at lower field strength, it remains unclear if paramagnetic rim lesions are visible at 1.5 T. OBJECTIVE. The purpose of our study was to compare visualization of paramagnetic rim lesions using susceptibility-weighted imaging at 1.5-T and 3-T MRI in patients with MS. METHODS. This retrospective study included nine patients (five women, four men; mean age, 46.8 years) with MS who underwent both 1.5-T and 3-T MRI using a comparable susceptibility-weighted angiography (SWAN) sequence from the same manufacturer. Lesions measuring greater than 3 mm were annotated. Two reviewers independently assessed images at each field strength in separate sessions and classified the annotated lesions as isointense, diffusely paramagnetic, or paramagnetic rim lesions. Discrepancies were discussed at consensus sessions including a third reviewer. Agreement was assessed using kappa coefficients. RESULTS. Based on the 3-T consensus readings, 115 of 140 annotated lesions (82%) were isointense lesions, 16 (11%) were diffusely paramagnetic lesions, and nine (6%) were paramagnetic rim lesions; based on the 1.5-T consensus readings, 115 (82%) were isointense lesions, 14 (10%) were diffusely paramagnetic lesions, and 11 (8%) were para-magnetic rim lesions. The mean lesion diameter was 11.9 mm for paramagnetic rim lesions versus 6.4 mm for diffusely paramagnetic lesions (p = .006) and 7.8 mm for iso-intense lesions (p = .003). Interrater agreement for lesion classification as a paramagnetic rim lesion was substantial at 1.5 T (κ = 0.65) and 3 T (κ = 0.70). Agreement for paramagnetic rim lesions was also substantial between the consensus readings at the two field strengths (κ = 0.79). CONCLUSION. We show comparable identification of paramagnetic rim lesions at 1.5-T and 3-T MRI with substantial interrater agreement at both field strengths and substantial consensus agreement between the field strengths. CLINICAL IMPACT. Paramagnetic rim lesions may be an emerging marker of chronic neuroinflammation in MS. Their visibility at 1.5 T supports the translational potential of paramagnetic rim lesion identification to more widespread clinical settings, where 1.5-T scanners are prevalent.