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

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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 Symbol distribution. Recent findings of compartment-specific orientation-dependent magnetic susceptibility shifts have spurred the inclusion of frequency offset Symbol terms in the fitting models. In this work, we performed simulations to assess the impact of Symbol'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 (Symbol), and analyzed using: 1) a multi-component Symbol signal magnitude model (MCMSymbol); 2) a three-component Symbol signal magnitude model (3CMSymbol); and, 3) a three-component complex Symbol signal model (3CCSymbol).

Results

At 3 T, MCMSymbol & 3CMSymbol yielded accurate MWFs: (11.9Symbol1.1)% and (11.7Symbol1.0)% (meanSymbol standard deviation across 1000 simulations, true MWF = 12%), respectively. 3CCSymbol MWFs were less accurate and had the largest variability: (9.2Symbol5.0)%. At 7 T, MCMSymbol and 3CMSymbol MWFs became less accurate as Symbol increased. This was remedied by 3CCSymbol, at the expense of accuracy for small Symbol.

Conclusion

This work suggests that if no information regarding Symbol is sought, MCMSymbol and 3CMSymbol are preferable at 3 T. At 7 T, Symbol cannot be overlooked.

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