Alexander Rauscher

Prospective Graduate Students / Postdocs

This faculty member is currently not actively recruiting graduate students or Postdoctoral Fellows, but might consider co-supervision together with another faculty member.

Associate Professor

Research Interests

magnetic resonance imaging
quantitative susceptibility mapping
myelin water imaging
maschine learning

Relevant Thesis-Based Degree Programs

Research Options

I am available and interested in collaborations (e.g. clusters, grants).
I am interested in and conduct interdisciplinary research.
I am interested in working with undergraduate students on research projects.

Research Methodology

Magnetic Resonance Imaging
numerical simulations
machine learning

Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Gradient and spin echo magnetic resonance imaging for the characterization of myelin health in multiple sclerosis (2020)

Multiple sclerosis (MS) is a complex, autoimmune disease that results in demyelination and neurodegeneration. Magnetic resonance imaging (MRI) is an essential cornerstone of MS diagnosis and clinical decision making, however, clinical MRIs lack specificity to the pathological mechanisms influencing myelin health.In this thesis, two quantitative MRI techniques were probed for their potential to study myelin health in MS. First, multi-echo spin-echo myelin water imaging (MWI) was tested for its specificity to myelin lipids, proteins and iron. We demonstrated to-date unknown sensitivity of MWI and identified MS lesion changes indicative of late-stage remyelination. Thereafter, the accuracy of MWI and its potential application at ultra-high magnetic fields were investigated. Using signal simulations, the dependence of the non-negative least-squares analysis on processing and tissue parameters was described. Myelin underestimations due to B⁺₁-inhomogeneities and noise were shown to be minimized by adjusting the T₂ range according to the echo time. To translate MWI to 7T, T₂ tissue properties in seven healthy subjects were studied in comparison to 3T. We demonstrated the feasibility of 7T-MWI and discussed current limitations in assessing short T₂ times.Secondly, susceptibility-sensitive MRI was explored, which provides greater sensitivity, albeit possibly lower specificity to myelin, than MWI. Using the phase component, we showed that the MS lesion contrast is typically not driven by iron accumulation. In simulations and with post-mortem data, it was demonstrated that iron and myelin loss in combination determine the lesions’ appearance. Thereafter, the potential of the technique to become a marker of tissue damage and repair was evaluated by studying the evolution and pathological underpinnings of acute MS lesions in eleven patients over five years. Current models and their shortcomings were discussed.Finally, two technical developments were introduced. First, a multi-dynamic, high-spatial resolution susceptibility-sensitive imaging approach was presented for visualizing the central vein sign. Using phantom and in vivo data, qualitative and quantitative agreement of the proposed approach with other imaging strategies was demonstrated. Secondly, FLAIR² was introduced, a novel contrast that improves contrast-to-noise, while shortening scan time. The potential of FLAIR² to aid automated lesion segmentation was demonstrated on real-world multi-centre clinical data.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Predicting primary diffusion directions in white matter from T1-weighted magnetic resonance images (2024)

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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Impact of formalin fixation on multi-compartment T2 relaxation and its tissue orientation dependence in excised pig spinal cord white matter (2023)

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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Orientation dependence of transverse relaxation rates in magnetic resonance imaging of the newborn and adult brain (2022)

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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Current Students & Alumni

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