Superior Visibility of Paramagnetic Rim Lesions on Filtered Phase vs Susceptibility-Weighted Imaging

Paramagnetic rim lesions (PRLs) are a highly specific imaging biomarker for MS that are now integrated into the 2024 McDonald criteria. PRLs can be detected on standard SWI, which combines data from the homodyne-filtered phase (phase) and enhanced magnitude (SWI) images, as well as on the phase images alone. However, the relative visibility of PRL on SWI vs phase images is unknown, and it remains unclear which of these contrasts should be used for PRL evaluation in the clinical setting.

The study sample (n=40) consisted of 2 cross-sectional cohorts-a primary cohort including both MS and non-MS cases (n=20) and a distinct secondary cohort consisting of MS participants only (n=20)-both selected from a prospective, multicenter observational study (Central Vein Sign: A Diagnostic Biomarker in Multiple Sclerosis Study) of participants presenting for initial diagnostic evaluation of suspected MS. Standardized brain MRI sequences (3D T1WI with and without contrast, T2-FLAIR, and vendor-provided SWI) were acquired at 3T. T2-hyperintense lesions were evaluated on SWI and phase images for classification as PRLs or non-PRLs in separate trials by trained, blinded raters (2 raters in the primary cohort, 1 rater in the secondary cohort). Differences in PRL frequency between SWI and phase images were analyzed with the McNemar test in the primary, secondary, and pooled cohorts.

One hundred fifty-five PRLs were identified in the pooled cohort, of which 66 (42.6%) were detected only on phase images. The number of lesions classified as PRL was 2.7- and 1.4-fold greater when evaluated on phase images compared with SWI in the primary (43 vs 16, P < .001) and secondary (109 vs 73, P < .001) cohorts, respectively, and 1.7-fold greater in the pooled cohort (152 vs 89, P < .001). The main reasons for non-PRL classification on SWI were nodular appearance (37.9%) and discontinuity of the rim (28.8%).

Filtered phase images derived from standard SWI outperform enhanced magnitude images for PRL identification and should be primarily used for PRL evaluation in clinical practice.