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Diffusion Magnetic Resonance Imaging in the Central Nervous System
Published in Shoogo Ueno, Bioimaging, 2020
Kouhei Kamiya, Yuichi Suzuki, Osamu Abe
The unique ability of dMRI tractography to visualize white matter fiber pathways non-invasively made it useful both for clinical medicine and neuroscience research. Despite its limitations (Section 6.5.4), dMRI tractography has at least enabled virtual white matter dissection (Catani & Thiebaut de Schotten, 2008) (Figure 6.12) that roughly matches the results of postmortem studies and has provided good explanations for syndromes caused by disconnection between particular brain regions (Thiebaut de Schotten et al., 2015). Currently, dMRI tractography is an indispensable tool for mapping before neurosurgery (Calabrese, 2016; Voets et al., 2017). The use of higher-order methods has been reported to better predict functional outcomes of surgery for brain tumors by improving the delineation of tracts passing through a peri-tumoral region (Caverzasi et al., 2016).
Big Data Era in Magnetic Resonance Imaging of the Human Brain
Published in Ervin Sejdić, Tiago H. Falk, Signal Processing and Machine Learning for Biomedical Big Data, 2018
Xiaoyu Ding, Elisabeth de Castro Caparelli, Thomas J. Ross
DTI is mostly used to evaluate white matter, where the location, orientation, and anisotropy of the tracts can be evaluated. This is because the architecture of the axons, which is in parallel bundles, and their myelin sheaths constrain the diffusion of the water molecules along their main direction. Consequently, the information about the water diffusion in the white matter can be used to perform tractography, allowing an estimation of white-matter connection patterns in three dimensions. Fiber tracking algorithms follow the coherent spatial patterns in the major eigenvectors of the diffusion tensor field to track a fiber along its whole length [4]. The ability of DTI to estimate fiber orientation and strength is increasingly accurate; however, it cannot directly image multiple fiber orientations within a single voxel (usually 3 mm isotropic). To address this limitation, diffusion spectrum imaging (DSI) [6] was developed to reveal the complex distributions of intravoxel fiber orientation, demonstrating the capability to image crossing fibers in neural tissue.
Future of Medical Imaging
Published in Troy Farncombe, Krzysztof Iniewski, Medical Imaging, 2017
Evolution from slow and fuzzy to fast and highly detailed: Today's magnetic resonance imagers (MRIs) can provide higher quality images in a fraction of the time it took state-of-the-art machines just a few years ago. These digital MRIs are also highly flexible, with the ability to image, for example, the spine while it is in a natural, weight-bearing, standing position. With diffusion MRIs, researchers can use a procedure known as tractography to create brain maps that aid in studying the relationships between disparate brain regions. Functional MRIs, for their part, can rapidly scan the brain to measure signal changes due to changing neural activity. These highly detailed images provide researchers with deeper insights into how the brain works—insights that will be used to improve treatment and guide future imaging equipment.
Effect of axonal fiber architecture on mechanical heterogeneity of the white matter—a statistical micromechanical model
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2022
Hesam Hoursan, Farzam Farahmand, Mohammad Taghi Ahmadian
The recent research concerning the mechanical behavior of the white matter have been influenced much with the anisotropic structure of the tissue, unveiled by the diffusion tensor imaging (DTI) and fiber tractography techniques. Several experimental studies have examined the mechanical anisotropy of white matter, i.e., directional dependence of the mechanical properties within the tissue, and found a significant (Velardi et al. 2006; van Dommelen et al. 2010; Rashid et al. 2012; Jin et al. 2013, Feng et al. 2013) or marginal (Budday et al. 2017) higher stiffness along the axonal tracts. Moreover, it has been revealed that the mechanical properties of the white matter are region-dependent due to the change in its microstructural arrangement. This heterogeneous mechanical behavior is again thought to be associated with the changing architecture of the axonal fibers, i.e., volume fraction, caliber and orientation (Arbogast and Margulies 1997; Rashid et al. 2012; MacManus et al. 2018; Budday et al. 2017; Hoursan et al. 2013, 2015, 2018, 2020; Shafiee et al. 2015), as well as the variations in the distribution of the cells, interconnections and capillary density (Budday et al. 2017) in different regions. For instance, the brain stem has a considerably larger mechanical stiffness than the corona radiata and the corpus callosum, particularly along inferior-superior axis (Chatelin et al. 2011), which is consistent with its relatively parallel orientation of axonal fibers (Javid et al. 2014; Labus and Puttlitz 2016; Coelho et al. 2018). Nevertheless, there are rather large variations in the mechanical properties of the intra-regional (from a specific region of white matter, e.g., brain stem), as well as inter-regional (from two different regions of the white matter) samples. The former variation is often reflected by the large standard deviations (SDs) of the stress/strain measurements (Rashid et al. 2012; Jin et al. 2013).