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

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 T-2* distribution. Recent findings of compartment-specific orientation-dependent magnetic susceptibility shifts have spurred the inclusion of frequency offset (Delta f) terms in the fitting models. In this work, we performed simulations to assess the impact of Delta f'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 (theta), and analyzed using: 1) a multi-component T-2* signal magnitude model (MCMT2*); 2) a three-component T-2* signal magnitude model (3CMT(2)*); and, 3) a three-component complex T-2* signal model (3CCT(2)*). Results: At 3 T, MCMT2* & 3CMT(2)* yielded accurate MWFs: (11.9 +/- 1.1)% and (11.7 +/- 1.0)% (mean +/- standard deviation across 1000 simulations, true MWF = 12%), respectively. 3CCT(2)* MWFs were less accurate and had the largest variability: (9.2 +/- 5.0)%. At 7 T, MCMT2* and 3CMT(2)* MWFs became less accurate as. increased. This was remedied by 3CCT(2)*, at the expense of accuracy for small theta. Conclusion: This work suggests that if no information regarding Delta f is sought, MCMT2* and 3CMT(2)* are preferable at 3 T. At 7 T, Delta f cannot be overlooked.