Background: White matter lesions (WMLs) on brain magnetic resonance imaging (MRI) in multiple sclerosis (MS) may contribute to misdiagnosis. In chronic active lesions, peripheral iron-laden macrophages appear as paramagnetic rim lesions (PRLs). Objective: To evaluate the sensitivity and specificity of PRLs in differentiating MS from mimics using clinical 3T MRI scanners. Method: This retrospective international study reviewed MRI scans of patients with MS ( n = 254), MS mimics ( n = 91) and older healthy controls ( n = 217). WMLs, detected using fluid-attenuated inversion recovery MRI, were analysed with phase-sensitive imaging. Sensitivity and specificity were assessed for PRLs. Results: At least one PRL was found in 22.9% of MS and 26.1% of clinically isolated syndrome (CIS) patients. Only one PRL was found elsewhere. The identification of ⩾1 PRL was the optimal cut-off and had high specificity (99.7%, confidence interval (CI) = 98.20%–99.99%) when distinguishing MS and CIS from mimics and healthy controls, but lower sensitivity (24.0%, CI = 18.9%–36.6%). All patients with a PRL showing a central vein sign (CVS) in the same lesion ( n = 54) had MS or CIS, giving a specificity of 100% (CI = 98.8%–100.0%) but equally low sensitivity (21.3%, CI = 16.4%–26.81%) Conclusion: PRLs may reduce diagnostic uncertainty in MS by being a highly specific imaging diagnostic biomarker, especially when used in conjunction with the CVS.

Paramagnetic rims appear as a hypointense border around the edge of Multiple Sclerosis (MS)4lesions when visualised on phase sensitive imaging such as T2*, FLAIR* or SWI. They are likely due to immune cell uptake of liberated iron following the destruction of oligodendrocytes. While rarer than the central vein sign, initial reports suggest they are a specific marker of MS. However, disagreement remains with different groups reporting varying prevalence, location, persistence over time and association with expanding lesions, disease stage and disability.Eight centres in the MAGNIMS (Magnetic Resonance Imaging in Multiple Sclerosis) study group contrib- uted phase imaging of 562 people. They included patients with MS, clinically isolated syndrome, migraine, NMOSD, CNS vasculitis, small vessel ischaemia, diabetes mellitus, and cluster headache along with healthy controls. In patients with MS and clinically isolated syndrome 129 lesions with a paramagnetic rim were detected. The prevalence was 23% and 26% of patients respectively, suggesting this biomarker may be present across all MS stages. Only one paramagnetic rim lesion was found in the rest of the cohort, in a patient with diabetes mellitus, confirming its exceptionally high specificity of 99.7%.christopher.allen@nottingham.ac.uk|ABN Bursary