Quantitative imaging to guide mechanism-based modeling of cancer
Ruijiang Li, Lei Xing, Sandy Napel, Daniel L. Rubin in Radiomics and Radiogenomics, 2019
Diffusion tensor imaging (DTI) is a variant of diffusion weighted-MRI (DW-MRI) that can be used to assess the magnitude and direction of water diffusion in tissue. In DTI, pulsed gradients are applied along at least six non-collinear directions to estimate the diffusion coefficient of water in those directions and build the diffusion tensor at each voxel. Structures such as white matter or muscles which are well organized exhibit anisotropic diffusion, while tumor regions typically exhibit more isotropic diffusion. DTI measurements are commonly used to characterize structural connectivity through tractography which follows the path of the dominant diffusion direction in highly anisotropic tissues (i.e., white matter tracts). The fractional anisotropy index which ranges from 0 (fully isotropic diffusion) to 1 (fully anisotropic diffusion) is often used to assess the degree of diffusion anisotropy within a region. The diffusion tensor is useful in biophysical models as the tensor can be used to define the direction of tumor cell movement or assist in defining the movement of tumor cells along white matter tracts. A detailed review of DTI can be found in (Sundgren et al. 2004).
Diffusion Magnetic Resonance Imaging in the Central Nervous System
Shoogo Ueno in Bioimaging, 2020
Once D is obtained, one can derive several rotationally invariant scalar metrics that describe the size and shape of the diffusion tensor (Basser, 1995). These parameters are useful for summarizing the diffusion properties of a given voxel and are used as quantitative metrics in clinical studies. Here we introduce the most commonly reported metrics. Figure 6.8 shows examples of parameter maps in a normal human brain. Mean diffusivity (MD): the average of the eigenvalues. .Axial diffusivity (AD) and radial diffusivity (RD): the diffusion coefficients parallel and perpendicular to the principal direction. AD = λ1, .Fractional anisotropy (FA): a dimensionless index that describes the shape of the diffusion tensor, normalized to have a value between 0 (isotropic diffusion) and 1.
DTI of Developmental and Pediatric Disorders
Andrei I. Holodny in Functional Neuroimaging, 2019
In both preterm and term infants, apparent diffusion coefficient (ADC, which measures the mean of the diffusion tensor eigenvalues) and diffusion anisotropy measurements have been shown to correlate with gestational age (3). In one study, serial DTI scans were performed to assess maturational changes in the white matter of premature newborns that showed no abnormalities on conventional MRI (Fig. 1) (4). In this study, earlier maturing of white matter tracts showed higher fractional anisotropy (FA, which measures the fraction of diffusion tensor magnitude due to anisotropic diffusion) values than later maturing pathways, which is the same pattern found in normal adults (5). This finding suggests that anisotropy is already seen in preterm unmyelinated white matter and that, even at this very early age, differences in anisotropy can be seen across WM structures of varying degrees of myelination. Also, the investigators found that diffusion anisotropy was the most sensitive measure for detecting differences between tracts. More specifically, FA was able to detect smaller differences compared with relative anisotropy (RA, which measures the ratio of anisotropic to isotropic diffusion tensor magnitude), suggesting that FA is a superior measurement in patients with inherently low anisotropic values, such as premature infants.
Evaluation of acute anterior ischaemic optic neuropathy using diffusion tensor imaging
Published in Clinical and Experimental Optometry, 2020
Binghua Fang, Qiang Liu, Jinyan Wang, Lu Yu, Xugang Liu, Ping Ma, Bojun Zhao
The main parameter of diffusion tensor imaging is fractional anisotropy. Fractional anisotropy is a widely used parameter that can be ascribed to anisotropic diffusion, which is thought to reflect axonal diameter, fibre attenuation and myelination in white matter. In this study, it was observed that the mean fractional anisotropy from affected nerves was lower than control nerves and unaffected in the contralateral nerves, comparable with the results of Techavipoo et al.2009 Fractional anisotropy is thought to reflect axonal diameter, fibre attenuation and myelination in white matter, and represents the direction which water molecules move along in optic nerve tracts. This finding indicated that there was vasogenic oedema in the nerves of AION patients, in which tissue water content increased and cell lysis occurred. These changes may be progressive with increasing axon loss of the optic nerve.2013
Examining the relationship between perinatal depression and neurodevelopment in infants and children through structural and functional neuroimaging research
Published in International Review of Psychiatry, 2019
Christy Duan, Megan M. Hare, Morganne Staring, Kristina M. Deligiannidis
Diffusion tensor imaging (DTI) is an MR method that determines the location, orientation, and anisotropy of white matter tracts (Bandettini, 2009). Diffusion in white matter is anisotropic, greater in one direction than in others. Greater anisotropy and restricted diffusion perpendicular to the principal diffusion direction reflect healthy or mature white matter microstructure (Beaulieu, 2002). Fractional anisotropy is a value between 0 and 1 that describes the degree of anisotropy. A higher value indicates that diffusion occurs along one axis and is anisotropic, while a lower value means that the diffusion is unrestricted or isotropic. Reduced fractional anisotropy within white matter tracks is believed to reflect microstructural changes associated with reduced anatomical connectivity, with less diffusion anisotropy when axons are less myelinated (Alexander, Lee, Lazar, & Field, 2007; Soares, Marques, Alves, & Sousa, 2013). Additional DTI measures, including mean diffusivity, radial diffusivity, and axial diffusivity, characterize diffusion magnitude. The degree of anisotropic diffusion is affected by tissue barriers such as axonal fibres and the myelin sheath, which are important components to consider when imaging infants and young children when pronounced myelination and axonal fibre development occurs (Dean et al., 2017; Kunz et al., 2014).
Implications of structural and functional brain changes in amyotrophic lateral sclerosis
Published in Expert Review of Neurotherapeutics, 2018
Thanuja Dharmadasa, William Huynh, Jun Tsugawa, Yoshimitsu Shimatani, Yan Ma, Matthew C. Kiernan
The most extensively applied technique to study white matter patterns in ALS to date has utilized diffusion tensor imaging (DTI) [26]. This technique quantifies the Brownian motion of water in fiber bundles, measuring both the rate and axis of water diffusion to evaluate white matter integrity [27]. This is quantifiably summarized using measures such as fractional anisotropy (FA), which describes how strongly directional water diffusion is within the tissue, and mean diffusivity (MD), reflecting average water displacement distance independent of direction. Intact white matter restricts the diffusion of water parallel to fiber direction (resulting in higher FA and lower MD), while damage to these fibers cause less restriction of diffusion (generating a lower FA and higher MD) [28]. Importantly, diffusion metrics may also be influenced by several other underlying microstructural changes, such as axonal loss, swelling, and any fiber orientation that is less ‘organized’ (eg crossing fibers) [28]. A few approaches are performed to analyze DTI metrics: a specific brain location or a ‘region of interest’ (ROI) can be evaluated; ‘whole-brain’ voxel-wise analyses can be performed (such as in VBM-type analysis); or white matter tracts can be analyzed (tractography) based on the main diffusion direction, which characterizes these tracks and reconstructs the structural connectivity pathways of the brain [29].
Related Knowledge Centers
- Axon
- Corpus Callosum
- Diffusion
- Diffusion Mri
- Myelin
- White Matter
- Magnetic Resonance Imaging
- Liquid Crystal