Microstructural Evaluation of Paramagnetic Rim and Non-Rim Lesions in Multiple Sclerosis Using SANDI and NEXI

Motivation: Paramagnetic rim lesions(PRLs) reflect iron accumulation in microglia indicating chronic inflammation and link to accelerated disability in multiple sclerosis(MS). Goal(s): To characterize cell size, density and exchange rates in PRLs and non-PRLs using advanced gray-matter microstructural metrics from SANDI and NEXI. Approach: Six MS-patients and six healthy-controls underwent ultra-high gradient diffusion MRI on the Connectome2.0 scanner, and the SANDI and NEXI models were fitted. Lesion segmentation and quantitative analysis of diffusion metrics, with correlations with disability were performed. Results: PRLs showed significant neurite loss with central loss of soma fraction and increased soma size. Prolonged exchange times but preserved exchange rates were observed. Impact: Exploration of tissue microstructural alterations in MS lesions(PRLs.vs.non-PRLs) with advanced diffusion metrics such as SANDI and NEXI may help us understand lesion characteristics better and find key markers for prognostication and determining treatment course.