Susceptibility source separation from gradient echo data using magnitude decay modeling

Background and Purpose The objective is to demonstrate feasibility of separating magnetic sources in quantitative susceptibility mapping (QSM) by incorporating magnitude decay rates R2∗$R_2^{\rm{*}}$ in gradient echo (GRE) MRI. Methods Magnetic susceptibility source separation was developed using R2∗$R_2^{\rm{*}}$ and compared with a prior method using R2′=R2∗−R2${R^{\prime}_2} = R_2^* - {R_2}$ that required an additional sequence to measure the transverse relaxation rate R2. Both susceptibility separation methods were compared in multiple sclerosis (MS) patients (n = 17). Susceptibility values of negative sources estimated with R2∗$R_2^{\rm{*}}$‐based source separation in a set of enhancing MS lesions (n = 44) were correlated against longitudinal myelin water fraction (MWF) changes. Results In in vivo data, linear regression of the estimated χ+${\chi}^{+}$ and χ−${\chi}^{-}$ susceptibility values between the R2∗$R_2^*$‐ and the R2′${R^{\prime}_2}$‐based separation methods performed across 182 segmented lesions revealed correlation coefficient r = .96 and slope close .99. Correlation analysis in enhancing lesions revealed a significant positive association between the χ−${\chi}^{-}$ increase at 1‐year post‐onset relative to 0 year and the MWF increase at 1 year relative to 0 year (β = ‐0.144, 95% confidence interval: [–0.199, –0.1], p = .0008) and good agreement between R2′${R^{\prime}_2}$ and R2∗$R_2^*$ methods (r = .79, slope = .95). Conclusions Separation of magnetic sources based solely on GRE complex data is feasible by combining magnitude decay rate modeling and phase‐based QSM and χ−${\chi}^{-}$ change may serve as a biomarker for myelin recovery or damage in acute MS lesions.