Author:

Keywords:

small vessel disease, myelin, leukoaraiosis, diffusion

Abstract:

White matter hyperintensities (WMH) are commonly observed on T2weighted MRI scans in older adults. Deep(D) and periventricular (PV) WMH may arise from different pathological mechanisms that cannot be distinguished by standard imaging sequences. Multiexponential T2 relaxation (MET2) yields specific markers for myelination [1], but has not yet been applied to study age related WMH. Here we present data characterising MET2, DTI and FLAIR properties of WMH and normal appearing white matter (NAWM) in healthy older adults. FLAIR, DWI (60 x b=1300s/mm2) and MET2 (32echo GraSE, TE=10,20..320ms) data were acquired from 25 healthy volunteers (age 73.8 +5.5; 6 male) on a 3T Philips Achieva system. Periventricular, deep WM and NAWM ROIs were defined by regional masking of semiautomatic segmentions using the W2MHS toolbox [2]. Calibrated FLAIR intensity values, fractional anisotropy (FA), mean, radial and axial diffusivity (MD, RD, AD), myelin water fraction (MWF), intraand extracellular water fraction (IEWF) and intraand extracellular geometric mean T2 time (IEWgmT2) in PVWMH and DWMH were compared to NAWM using Wilcoxon signed ranks tests. Spearman correlations were used to assess relationships between both normalized lesion volume and age with each metric in both WMH and NAWM. All metrics except IEWF differed between WMH compared to NAWM irrespective of location (p< /0.001) (figure 1). Myelin water fraction was the only metric that differed between PV and DWMH compared to NAWM, suggesting differences in myelin content between PV and DWMH (p< /0.001). Age and normalized lesion size correlated significantly with multiple, but not all measures in both WMH and NAWM (table 1). Notably, diffusion, but not MET2 metrics were associated with lesion load in NAWM, but these effects disappeared when correcting for age. Myelin water imaging, but not DTI or FLAIR may distinguish between different microstructural properties underlying age related WMH. Reduced MWF in PVWMH supports both postmortem and magnetization transfer ratio evidence for periventricular demyelination [3, 4]. Age and total lesion size have similar effects on MRI based measures of tissue properties suggesting a complex relationship between normal neurodegenerative processes, lesion load and WM microstructure.