Susceptibility source separation from gradient echo data using magnitude decay modeling

Abstract

Background and purpose: The objective is to demonstrate feasibility of separating magnetic sources in quantitative susceptibility mapping (QSM) by incorporating magnitude decay rates $R_2^{\ast }$ in gradient echo (GRE) MRI.

Methods: Magnetic susceptibility source separation was developed using $R_2^{\ast }$ and compared with a prior method using $R_2^{\text{'}}=R_2^{\ast }-R_2$ 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 $R_2^{\ast }$ -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 ${\chi }^{+}$ and ${\chi }^{-}$ susceptibility values between the $R_2^{\ast }$ - and the $R_2^{\text{'}}$ -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 ${\chi }^{-}$ 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 $R_2^{\text{'}}$ and $R_2^{\ast }$ 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 ${\chi }^{-}$ change may serve as a biomarker for myelin recovery or damage in acute MS lesions.

Keywords: iron quantification; myelin quantification; quantitative susceptibility mapping; susceptibility source separation.