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Paramagnetic Rim Lesions

Summary

  • Paramagnetic rim lesions (PRLs) are chronic active demyelinating lesions characterized by a peripheral rim of iron-laden microglia/macrophages.
  • On susceptibility-based MRI, they appear as white matter lesions with a hypointense paramagnetic rim.
  • PRLs are an emerging imaging biomarker of chronic active inflammation in multiple sclerosis (MS).

Imaging Appearance

  • Susceptibility-weighted imaging (SWI):
    • Characteristic peripheral hypointense (paramagnetic) rim surrounding an iso- to hypointense lesion core.
    • Rim reflects iron deposition within activated microglia/macrophages at the lesion edge.
  • Quantitative susceptibility mapping (QSM):
    • Considered the most specific sequence; rim appears hyperintense (paramagnetic signal) with a non-paramagnetic core.
    • Helps distinguish iron (paramagnetic) from other sources of signal.
  • T2*/gradient-echo phase imaging:
    • Rim demonstrates paramagnetic phase shift.
  • Field strength:
    • Better conspicuity at 7T, but reliably detectable at 3T; 1.5T is less sensitive.
  • Distinguishing features:
    • Rim is complete or near-complete and persists over time, unlike the transient rim of enhancement seen in acute lesions.
    • PRLs are typically non-enhancing on post-contrast T1 (chronic, not acutely inflamed).
  • Pitfalls:
    • Central vein sign may coexist and supports demyelinating etiology.
    • Must differentiate from microbleeds, calcification, and normal deep gray matter iron.

Clinical Relevance

  • Marker of chronic active (smoldering) inflammation, associated with ongoing tissue damage and lesion expansion.
  • Correlate with greater disability, more aggressive disease course, and worse long-term outcomes in MS.
  • Considered highly specific for MS, aiding differentiation from mimics (e.g., migraine, small vessel disease, NMOSD).
  • Emerging role as a prognostic and diagnostic biomarker; increasingly incorporated into research and potentially future diagnostic criteria.
  • May serve as an imaging endpoint for evaluating therapies targeting compartmentalized/chronic inflammation.

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  • In the right centrum semiovale, a small well-demarcated T2-hyperintense and T1-hypointense lesion had a rim of susceptibility artefact.