The Origin of Chronic T2* and Phase Rims in MS Lesions: A Longitudinal 7T MRI Study (S54.001)

OBJECTIVE. Longitudinal MRI evaluation of the fate of centripetally and centrifugally enhancing multiple sclerosis (MS) lesions by examining the time evolution of a T2*/phase rim. BACKGROUND. A hypointense rim on T2*/phase MRI is visible in ~10% of chronic MS lesions and is likely related to iron deposition. In a few medium-term studies, this rim remains stable over time. Establishing the rim’s onset would help to elucidate its pathophysiological significance. We hypothesized that it develops within the first 6 months and relates spatially to the centripetal pattern of contrast enhancement. METHODS. Nine centripetally and 10 centrifugally enhancing lesions were studied prospectively at 7T in 10 MS cases. High-resolution T2* gradient-echo scans, T1 scans before/after gadolinium injection, and dynamic contrast-enhanced T1 scans were acquired at baseline, M1, M3, M6, and M12. RESULTS. In centripetal lesions, the phase rim precisely colocalized with the initial site of contrast en-hancement (mean phase rim thickness 416±138 μm). In 4/9 lesions, it persisted after enhancement re-solved (M1-M3). Rim thickness increased gradually over time (~40 μm/month; M6: 690±104 μm) and became increasingly evident on T2* (3/4 lesions had a thin T2* rim at baseline). None of the centrifugal lesions developed a phase rim at any time point. CONCLUSION. Acute phase rims are transient in most centripetal lesions, confirming their close relationship with abrupt opening of the blood-brain barrier. Centripetal enhancement is necessary but not sufficient for the subsequent development of a chronic T2*/phase rim, which reflects the local presence of paramagnetic substances. We hypothesize that a complex interplay of events, possibly including a switch toward M1-macrophage polarization, iron deposition, persistent oxidative stress, and VEGF-related neovascularization, sustains chronic inflammation at the lesion edge, where the blood-brain barrier was previously open. Rim persistence may further indicate the failure of early lesion repair, possibly including remyelination.