About "axial" and "radial" diffusivities

University College London, Institute of Neurology, Department of Neuroinflammation, London, UK.
Magnetic Resonance in Medicine (Impact Factor: 3.57). 03/2009; 61(5):1255-60. DOI: 10.1002/mrm.21965
Source: PubMed

ABSTRACT This article presents the potential problems arising from the use of "axial" and "radial" diffusivities, derived from the eigenvalues of the diffusion tensor, and their interpretation in terms of the underlying biophysical properties, such as myelin and axonal density. Simulated and in vivo data are shown. The simulations demonstrate that a change in "radial" diffusivity can cause a fictitious change in "axial" diffusivity and vice versa in voxels characterized by crossing fibers. The in vivo data compare the direction of the principle eigenvector in four different subjects, two healthy and two affected by multiple sclerosis, and show that the angle, alpha, between the principal eigenvectors of corresponding voxels of registered datasets is greater than 45 degrees in areas of low anisotropy, severe pathology, and partial volume. Also, there are areas of white matter pathology where the "radial" diffusivity is 10% greater than that of the corresponding normal tissue and where the direction of the principal eigenvector is altered by more than 45 degrees compared to the healthy case. This should strongly discourage researchers from interpreting changes of the "axial" and "radial" diffusivities on the basis of the underlying tissue structure, unless accompanied by a thorough investigation of their mathematical and geometrical properties in each dataset studied.

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Available from: Mara Cercignani, Sep 26, 2015
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    • "Clinical and pathologic studies have found that axial diffusivity tends to reflect axonal integrity and radial diffusivity corresponds more closely to myelin integrity (Song et al., 2002, 2003; Kim et al., 2006). Any changes in their values are believed to represent axon loss or demyelination (Song et al., 2005; Wheeler-Kingshott and Cercignani, 2009). Therefore, the MD and FA values along with kk and k\ may be of great significance in understanding the mechanism underlying depression. "
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    ABSTRACT: Depression is amongst the most debilitating diseases worldwide. Long term exposure to stressors plays major role in development of human depression. Chronic mild stress (CMS) seems to be a valid animal model for depression. Diffusion Tensor Imaging (DTI) is capable of inferring microstructural abnormalities of the white matter and has shown to serve as non-invasive marker of specific pathology. We developed a CMS rat model of depression and validated with behavioral experiments. We measured the diffusion indices (Mean Diffusivity (MD), Fractional Anisotropy (FA), axial (λ∥) and radial (λ⊥) diffusivity) to investigate the changes in CMS rat brain during depression onset. Diffusion indices have shown to be useful to discriminate myelin damage from axon loss. DTI was performed in both control and CMS rats (n=10, in each group) and maps of FA, MD, λ∥ and λ⊥ diffusivity values were generated using in house built software. The diffusion indices were calculated by ROI analysis in different brain regions like frontal cortex, hippocampus, hypothalamus, cingulum, thalamus, caudate putamen, corpus callosum, cerebral peduncle and sensory motor cortex. The results showed signs of demyelination, reflected by increased MD, decreased FA and increased λ⊥. The results also suggest a possible role of edema or inflammation concerning the brain morphology in CMS rats. The overall finding using DTI suggests there might be a major role of loss of myelin sheath, which leads to disrupted connectivity between the limbic area and the prefrontal cortex during the onset of depression. Our findings indicate that interpretation of these indices may provide crucial information about the type and severity of mood disorders.
    Neuroscience 05/2014; 275. DOI:10.1016/j.neuroscience.2014.05.037 · 3.36 Impact Factor
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    • "In comparison, voxel-based indices like FA and MD demonstrate only minor variations and in some configurations for a constant decrease in the anisotropy of the first fiber we observed a nonmonotonic behavior in the measured FA (a decrease followed by a subsequent increase in FA). This result confirms previous findings where DTI measurements performed in complex white matter configurations may sometimes lead to results that are difficult to interpret [Wheeler-Kingshott and Cercignani, 2009]. Although, in these simulations, most changes were observed along the corresponding fiber orientation, minor changes were also observed along the second fiber orientation . "
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    ABSTRACT: Diffusion tensor imaging (DTI) methods are widely used to reconstruct white matter trajectories and to quantify tissue changes using the average diffusion properties of each brain voxel. Spherical deconvolution (SD) methods have been developed to overcome the limitations of the diffusion tensor model in resolving crossing fibers and to improve tractography reconstructions. However, the use of SD methods to obtain quantitative indices of white matter integrity has not been extensively explored. In this study, we show that the hindrance modulated orientational anisotropy (HMOA) index, defined as the absolute amplitude of each lobe of the fiber orientation distribution, can be used as a compact measure to characterize the diffusion properties along each fiber orientation in white matter regions with complex organization. We demonstrate that the HMOA is highly sensitive to changes in fiber diffusivity (e.g., myelination processes or axonal loss) and to differences in the microstructural organization of white matter like axonal diameter and fiber dispersion. Using simulations to describe diffusivity changes observed in normal brain development and disorders, we observed that the HMOA is able to identify white matter changes that are not detectable with conventional DTI indices. We also show that the HMOA index can be used as an effective threshold for in vivo data to improve tractography reconstructions and to better map white matter complexity inside the brain. In conclusion, the HMOA represents a true tract-specific and sensitive index and provides a compact characterization of white matter diffusion properties with potential for widespread application in normal and clinical populations. Hum Brain Mapp, 2012. © 2012 Wiley Periodicals, Inc.
    Human Brain Mapping 10/2013; 34(10). DOI:10.1002/hbm.22080 · 5.97 Impact Factor
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    • "The main scalar maps derived from the diffusion tensors, namely fractional anisotropy (FA) and mean diffusivity (MD), enable the indirect assessment of tissue microarchitecture and complexity, which may aid in the detection of myelin sheath and cell membrane damage (7). The combination of the three eigenvalues (λ1, λ2, and λ3), derived from the diffusion tensor eigenvectors offers two other and more specific parameters that can be assessed, namely axial diffusivity (AD) and radial diffusivity (RD), which are known to represent axon loss and demyelination, respectively (8,9). "
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    ABSTRACT: The aim of this study was to characterize the microscopic damage to the corpus callosum in relapsing-remitting multiple sclerosis (RRMS) with diffusion tensor imaging and to investigate the correlation of this damage with disability. The diffusion tensor imaging parameters of fractional anisotropy and mean diffusivity provide information about the integrity of cell membranes, offering two more specific indices, namely the axial and radial diffusivities, which are useful for discriminating axon loss from demyelination. Brain magnetic resonance imaging exams of 30 relapsing-remitting multiple sclerosis patients and 30 age- and sex-matched healthy controls were acquired in a 3T scanner. The axial diffusivities, radial diffusivities, fractional anisotropy, and mean diffusivity of five segments of the corpus callosum, correlated to the Expanded Disability Status Scale score, were obtained. All corpus callosum segments showed increased radial diffusivities and mean diffusivity, as well as decreased fractional anisotropy, in the relapsing-remitting multiple sclerosis group. The axial diffusivity was increased in the posterior midbody and splenium. The Expanded Disability Status Scale scores correlated more strongly with axial diffusivities and mean diffusivity, with an isolated correlation with radial diffusivities in the posterior midbody of the corpus callosum. There was no significant correlation with lesion loads. Neurological dysfunction in relapsing-remitting multiple sclerosis can be influenced by commissural disconnection, and the diffusion indices of diffusion tensor imaging are potential biomarkers of disability that can be assessed during follow-up.
    Clinics (São Paulo, Brazil) 08/2013; 68(8):1115-20. DOI:10.6061/clinics/2013(08)09 · 1.19 Impact Factor
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