Superior visibility of paramagnetic rim lesions on filtered phase versus SWI

BACKGROUND AND PURPOSE: Paramagnetic rim lesions (PRL) are a highly specific imaging biomarker for multiple sclerosis (MS) that are now integrated into the 2024 McDonald criteria. PRL 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 versus phase images is unknown and it remains unclear which of these contrasts should be used for PRL evaluation in the clinical setting. MATERIALS AND METHODS: 20)-both selected from a prospective, multicenter observational study (CAVS-MS) 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 PRL or non-PRL in separate trials by trained, blinded raters (two raters in the primary cohort, one rater in the secondary cohort). Differences in PRL frequency between SWI and phase images were analyzed with McNemar test in the primary, secondary, and pooled cohorts. RESULTS: 155 PRL 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 to SWI in the primary (43 vs. 16, p<0.001) and secondary (109 vs. 73, p<0.001) cohorts, respectively, and 1.7-fold greater in the pooled cohort (152 vs. 89, p<0.001). The main reasons for non-PRL classification on SWI were nodular appearance (37.9%) and discontinuity of the rim (28.8%). CONCLUSIONS: 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.