Alexander Rauscher

Orientation dependence studies are an ongoing research area in Magnetic Resonance Imaging (MRI), often relying on diffusion MRI to gain nerve fiber direction, a time-consuming scan not always included in research and clinical protocols. Using paired 3D T₁-weighted and diffusion images from the Human Connectome Project (HCP), a U-Net was trained to find voxel-wise principal diffusion directions from 3D T₁-weighted images. Different loss functions and data augmentations were used in model training. The models trained with different data augmentations and loss functions were evaluated using cosine distance during training. The trained model with the best performance had a group average cosine distance of 0.349 ± 0.007 and a group average difference in absolute polar angle of 8.762° ± 0.293° on the independent test set.

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In this MRI study, we investigated multi-compartment T₂ relaxation times of fresh, fixed, and washed pig spinal cord white matter (WM) and their dependence on tissue orientation relative to the main magnetic field. Multi-echo spin echo scans from eight pig spinal cord samples were acquired at 9.4 T with their axes parallel to the main magnetic field B₀. Four of these spinal cord segments were imaged at six different orientations with respect to B₀. After fixation, short fraction T₂ times and myelin water fraction (MWF) increased, while long fraction T₂ times decreased. After washing, short and long fraction T₂ times increased relative to fixed tissue, and MWF decreased relative to fresh tissue. We found a considerable orientation dependence in the short fraction relaxation rate R₂ (=1/T₂) in fresh tissue, which is reduced in washed tissue, and absent in fixed tissue. A similar, but reduced, orientation dependence was observed in long fraction R₂. This work presents a direct comparison of orientation dependence in a WM tissue sample in the fresh, fixed, and washed state. Understanding the changes caused by fixation and tissue orientation can aid in the correction of fixation and orientation-induced alterations of MRI metrics for more accurate and reliable measurements.

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In MRI the transverse relaxation rate, R₂=1/T₂, shows dependence on the orientation of ordered tissue relative to the main magnetic field. In previous studies, orientation effects of R₂ relaxation in the mature brain's white matter have been found to be described by a susceptibility-based model of diffusion through local magnetic field inhomogeneities created by the diamagnetic myelin sheaths. Orientation effects in human newborn white matter have not yet been investigated. The newborn brain is known to contain very little myelin and is therefore expected to exhibit a decrease in orientation dependence driven by susceptibility-based effects. We measured R₂ orientation dependence in the white matter of human newborns.R₂ data were acquired with a 3D Gradient and Spin Echo (GRASE) sequence and fibre orientation was mapped with diffusion tensor imaging (DTI). We found orientation dependence in newborn white matter that is not consistent with the susceptibility-based model and is best described by a model of residual dipolar coupling. In the near absence of myelin in the newborn brain, these findings suggest the presence of residual dipolar coupling between motionally restricted water molecules. This has important implications for quantitative imaging methods such as myelin water imaging, and suggests orientation dependence of R₂ as a potential marker in early brain development.

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