Abstract

Diffusion-weighted (DW) magnetic resonance imaging (MRI) is a non-invasive imaging method, which can be used to investigate neural tracts in the white matter (WM) of the brain. Significant partial volume effects (PVEs) are present in the DW signal due to relatively large voxel sizes. These PVEs can be caused by both non-WM tissue, such as gray matter (GM) and cerebrospinal fluid (CSF), and by multiple non-parallel WM fiber populations. High angular resolution diffusion imaging (HARDI) methods have been developed to correctly characterize complex WM fiber configurations, but to date, many of the HARDI methods do not account for non-WM PVEs. In this work, we investigated the isotropic PVEs caused by non-WM tissue in WM voxels on fiber orientations extracted with constrained spherical deconvolution (CSD). Experiments were performed on simulated and real DW-MRI data. In particular, simulations were performed to demonstrate the effects of varying the diffusion weightings, signal-to-noise ratios (SNRs), fiber configurations, and tissue fractions. Our results show that the presence of non-WM tissue signal causes a decrease in the precision of the detected fiber orientations and an increase in the detection of false peaks in CSD. We estimated 35–50% of WM voxels to be affected by non-WM PVEs. For HARDI sequences, which typically have a relatively high degree of diffusion weighting, these adverse effects are most pronounced in voxels with GM PVEs. The non-WM PVEs become severe with 50% GM volume for maximum spherical harmonics orders of 8 and below, and already with 25% GM volume for higher orders. In addition, a low diffusion weighting or SNR increases the effects. The non-WM PVEs may cause problems in connectomics, where reliable fiber tracking at the WM–GM interface is especially important. We suggest acquiring data with high diffusion-weighting 2500–3000 s/mm2, reasonable SNR (~30) and using lower SH orders in GM contaminated regions to minimize the non-WM PVEs in CSD.

Highlights

  • Diffusion-weighted (DW) magnetic resonance imaging (MRI) is a non-invasive imaging method to investigate tissue microstructure via the measurement of the displacement of water molecules (Stejskal and Tanner, 1965; Jones, 2010)

  • MATERIALS AND METHODS We investigated the isotropic partial volume effects (PVEs) caused by non-white matter (WM) tissue on fiber orientation distribution functions (fODFs) estimated with constrained spherical deconvolution (CSD)

  • ESTIMATION OF FIBER ORIENTATIONS WITH CONSTRAINED SPHERICAL DECONVOLUTION In CSD, the full fODF is deconvolved from the DW signal using a kernel constructed from the single-fiber response function (RF), which can be estimated from the data (Tournier et al, 2004; Tax et al, 2014)

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Summary

Introduction

Diffusion-weighted (DW) magnetic resonance imaging (MRI) is a non-invasive imaging method to investigate tissue microstructure via the measurement of the displacement of water molecules (Stejskal and Tanner, 1965; Jones, 2010). Diffusion in white matter (WM) neural tracts is anisotropic: it is larger parallel to the tract than in the perpendicular direction In liquid, such as cerebrospinal fluid (CSF), diffusion is isotropic, i.e., equal in all directions. Significant partial volume effects (PVEs) are present in the measured signal (Alexander et al, 2001; Vos et al, 2011) These may be caused by multiple non-parallel neural tracts passing through a voxel (Vos et al, 2011; Jeurissen et al, 2013), or several tissue types present in a voxel (Pasternak et al, 2009; Metzler-Baddeley et al, 2012a)

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