Quantitative susceptibility mapping using a superposed dipole inversion method: Application to intracranial hemorrhage

Purpose: To investigate gradient-echo phase errors caused by intracranial hemorrhage (ICH) of low signal magnitude, and propose methods to reduce artifacts from phase errors in quantitative susceptibility mapping (QSM) of ICH.Methods: Two QSM methods are proposed: (1) mask-inversion that masks the phase of low signal magnitude regions, and (2) ICH magnetic dipole field isolation followed by susceptibility superposition using multiple boundaries for background field removal. The reconstruction methods were tested in eight subjects with ICH using standard single-echo susceptibility-weighted imaging at 1.5 Tesla with 40 ms echo time. Different phase unwrapping algorithms were also compared.Results: Significant phase errors were evident inside ICHs with low signal magnitude. The mask-inversion method recovered susceptibility of ICH in numerical simulation and minimized phase error propagation in patients with ICH. The additional superposed dipole inversion process substantially suppressed and constrained streaking artifacts in all subjects. Using the proposed superposition method, ICH susceptibilities measured from long and short echo times were similar. Laplacian based phase unwrapping substantially underestimated the ICH dipole field as compared to a path-based method.Conclusion: The proposed methods of mask-inversion as well as ICH isolation and superposition can substantially reduce artifacts in QSM of ICH. (C) 2015 Wiley Periodicals, Inc