Abstract
Quantitative MRI modalities, such as diffusion tensor imaging (DTI) or magnetization transfer imaging (MTI) are sensitive to the neuronal effects of aging of the cerebral white matter (WM), but lack the specificity for myelin content. Myelin water imaging (MWI) is highly specific for myelin and may be more sensitive for the detection of changes in myelin content inside the cerebral WM microstructure. In this multiparametric imaging study, we evaluated the performance of myelin water fraction (MWF) estimates as a marker for myelin alterations during normal-aging. Multiparametric MRI data derived from DTI, MTI and a novel, recently-proposed MWF-map processing and reconstruction algorithm were acquired from 54 healthy subjects (aged 18–79 years) and region-based multivariate regression analysis was performed. MWFs significantly decreased with age in most WM regions (except corticospinal tract) and changes of MWFs were associated with changes of radial diffusivity, indicating either substantial alterations or preservation of myelin content in these regions. Decreases of fractional anisotropy and magnetization transfer ratio were associated with lower MWFs in commissural fiber tracts only. Mean diffusivity had no regional effects on MWF. We conclude that MWF estimates are sensitive for the assessment of age-related myelin alterations in the cerebral WM of normal-aging brains.
Highlights
Myelin holds a crucial role in the composition of brain’s microstructure and is an important compartment of the cerebral white matter (WM)
The mean myelin water fraction (MWF), fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and magnetization transfer ratio (MTR) values obtained from both hemispheres were comparable and no significant sidedifferences were found between the respective regions of interest (ROI) in the frontal, parietal, and occipital WM and CST of both sides (p > 0.05 for all ROIs)
Since no significant side-differences were detected, mean measurements of bilateral structures were combined into single regions and we referred to these measures as frontal WM, parietal WM, occipital WM and CST
Summary
Myelin holds a crucial role in the composition of brain’s microstructure and is an important compartment of the cerebral white matter (WM). MTI studies have reported potential evidence for age-related demyelination (Mehta et al, 1995; Pike et al, 2000; Rovaris et al, 2003; Gunning-Dixon et al, 2009), albeit MTI measures lack specificity for myelin (Serres et al, 2009; Vavasour et al, 2011) and need confirmation using techniques more sensitive to myelin content (Billiet et al, 2015). New and improved MWI approaches (enabling whole brain coverage in clinically feasible acquisition time) have been developed in the recent past, which need to be validated and tested in the clinical routine outside of a research environment In this investigation, we have employed a recently proposed iterative multi-voxel spatially regularized MWI reconstruction approach (Kumar et al, 2018), which enabled enhanced noise robustness of reconstruction along with a more accurate accounting for the stimulated echo contribution. This resulted in considerably improved MWF-quantifications, especially in the sub-corticaland major WM tract regions
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