Impact of magnetic susceptibility anisotropy at 3 T and 7 T on T2*-based myelin water fraction imaging

Abstract

Purpose: Myelin Water Fraction (MWF) mapping can be achieved by fitting multi-gradient recalled echo (MGRE) magnitude images with a three-component model or a pseudo-continuous T₂^∗ distribution. Recent findings of compartment-specific orientation-dependent magnetic susceptibility shifts have spurred the inclusion of frequency offset (Δf) terms in the fitting models. In this work, we performed simulations to assess the impact of Δf's on the MWF, derived from three different fitting models, at two field strengths.

Theory and methods: White matter MGRE signals were simulated using the Hollow Cylinder Fiber Model at 3 and 7 T, for a range of fiber orientations (θ), and analyzed using: 1) a multi-component T₂^∗ signal magnitude model (MCMT₂^∗); 2) a three-component T₂^∗ signal magnitude model (3CMT₂^∗); and, 3) a three-component complex T₂^∗ signal model (3CCT₂^∗).

Results: At 3 T, MCMT₂^∗ & 3CMT₂^∗ yielded accurate MWFs: (11.9±1.1)% and (11.7±1.0)% (mean± standard deviation across 1000 simulations, true MWF = 12%), respectively. 3CCT₂^∗ MWFs were less accurate and had the largest variability: (9.2±5.0)%. At 7 T, MCMT₂^∗ and 3CMT₂^∗ MWFs became less accurate as θ increased. This was remedied by 3CCT₂^∗, at the expense of accuracy for small θ.

Conclusion: This work suggests that if no information regarding Δf is sought, MCMT₂^∗ and 3CMT₂^∗ are preferable at 3 T. At 7 T, Δf cannot be overlooked.

Keywords: Gradient echo imaging; Microstructure; Multi-component T(∗)(2); Myelin water; Phase contrast; Susceptibility anisotropy.