Published: July 28th, 2013
Diffusion tensor imaging (DTI) basically serves as an MRI-based tool to identify in vivo the microstructure of the brain and pathological processes due to neurological disorders within the cerebral white matter. DTI-based analyses allow for application to brain diseases both at the group level and in single subject data.
Diffusion tensor imaging (DTI) techniques provide information on the microstructural processes of the cerebral white matter (WM) in vivo. The present applications are designed to investigate differences of WM involvement patterns in different brain diseases, especially neurodegenerative disorders, by use of different DTI analyses in comparison with matched controls.
DTI data analysis is performed in a variate fashion, i.e. voxelwise comparison of regional diffusion direction-based metrics such as fractional anisotropy (FA), together with fiber tracking (FT) accompanied by tractwise fractional anisotropy statistics (TFAS) at the group level in order to identify differences in FA along WM structures, aiming at the definition of regional patterns of WM alterations at the group level. Transformation into a stereotaxic standard space is a prerequisite for group studies and requires thorough data processing to preserve directional inter-dependencies. The present applications show optimized technical approaches for this preservation of quantitative and directional information during spatial normalization in data analyses at the group level. On this basis, FT techniques can be applied to group averaged data in order to quantify metrics information as defined by FT. Additionally, application of DTI methods, i.e. differences in FA-maps after stereotaxic alignment, in a longitudinal analysis at an individual subject basis reveal information about the progression of neurological disorders. Further quality improvement of DTI based results can be obtained during preprocessing by application of a controlled elimination of gradient directions with high noise levels.
In summary, DTI is used to define a distinct WM pathoanatomy of different brain diseases by the combination of whole brain-based and tract-based DTI analysis.
Diffusion tensor imaging in the human brain
The white matter (WM) tracts in the central nervous system consist of densely packed axons in addition to various types of neuroglia and other small populations of cells. The axonal membrane as well as the well-aligned protein fibers within an axon restricts water diffusion perpendicular to the fiber orientation, leading to anisotropic water diffusion in brain WM 1. Myelin sheaths around the axons may also contribute to the anisotropy for both intra- and extracellular water 2.
The quantitative description of this anisotropy could be detect....
Analysis Methods: Pre- and Postprocessing
The task of the following protocol is to analyze diffusion properties voxelwise within white matter tracts which could be - due to the voxelwise detection - either isotropic or anisotropic, resulting in prolate or oblate diffusion tensors for the respective voxels. The parameterization of the voxel tensors is used for either the calculation of FA-maps or the identification of fibertracts (Figure 1).
In order t.......
1. QC and correction for corrupted gradient directions in application to data of patients with hyperkinetic disorders
As an example for the effect of the application of QC and subsequent volume exclusion (as a consequence from the correction for corrupted GD), Figure 8 shows differences in whole brain based spatial statistics with and without volume exclusion for group comparison of 29 premanifest Huntington's disease subjects vs. 30 age and gender matched controls. The scanning .......
Inter-subject averaging of DTI data can be achieved with respect to diffusion amplitude (by use of FA information) and diffusion direction (based upon FT), respectively. Averaging of FA maps allows for the statistical comparison of subject groups by WBSS and TFAS. This methodological framework gives an introduction to DTI techniques with inter-subject averaging and group comparison. Stereotaxic normalization and comparison of FA maps at the group level allows for several possibilities to quantify differences between subj.......
Parts of this work, i.e. the study on QC and correction for corrupted gradient directions in application to data of patients with hyperkinetic disorders, were supported by the European HD network (EHDN project 070). The MRI scans in this certain study were acquired as part of the London site TRACK-HD cohort.....
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