Myelin water imaging depends on white matter fiber orientation in the human brain

Christoph Birkl,Enedino Hernández‐Torres,Michael Fan,Alexander Rauscher,Jonathan Doucette

doi:10.1002/mrm.28543

Abstract

PurposeThe multi‐exponential T2 decay of the MRI signal from cerebral white matter can be separated into short T2 components related to myelin water and long T2 components related to intracellular and extracellular water. In this study, we investigated to what degree the apparent myelin water fraction (MWF) depends on the angle between white matter fibers and the main magnetic field.MethodsMaps of the apparent MWF were acquired using multi‐echo Carr‐Purcell‐Meiboom‐Gill and gradient‐echo spin‐echo sequences. The Carr‐Purcell‐Meiboom‐Gill sequence was acquired with a TR of 1073 ms, 1500 ms, and 2000 ms. The fiber orientation was mapped with DTI. By angle‐wise pooling the voxels across the brain’s white matter, orientation‐dependent apparent MWF curves were generated.ResultsWe found that the apparent MWF varied between 25% and 35% across different fiber orientations. Furthermore, the selection of the TR influences the apparent MWF.ConclusionWhite matter fiber orientation induces a strong systematic bias on the estimation of the apparent MWF. This finding has implications for future research and the interpretation of MWI results in previously published studies.

Highlights

Myelin is a lipid bilayer membrane wrapped around the axon, protecting it from mechanical and chemical damage [1, 2] and facilitating fast saltatory signal conduction [3, 4]
The gradient and spin echo (GRASE) and CPMG sequence provide comparable results, using a CPMG sequence resulted in higher myelin water fraction (MWF)
We demonstrated that the measurement of MWF is considerably influenced by the angle between white matter fiber tracts and the main magnetic field

Summary

Introduction

Myelin is a lipid bilayer membrane wrapped around the axon, protecting it from mechanical and chemical damage [1, 2] and facilitating fast saltatory signal conduction [3, 4]. The orientation of white matter fibers with respect to the main magnetic field B0 affects the magnitude and phase of the complex MRI signal and a wide range of quantitative MRI parameters. Due to anisotropy of the cerebral vascular architecture, dynamic susceptibility contrast (DSC) perfusion measurements conducted with both gradient echo [30] and spin echo sequences [31] show an orientation dependent behaviour. The orientation dependency of the gradient echo signal of white matter is extensively studied and is ascribed mainly to the anisotropy of the magnetic susceptibility caused by the myelinated nerve fibers [24, 25, 32, 33]. We show that the MWF strongly depends on white matter fiber orientation, independent of MRI sequence

Methods

Results

Conclusion