About "axial" and "radial" diffusivities.

University College London, Institute of Neurology, Department of Neuroinflammation, London, UK.
Magnetic Resonance in Medicine (Impact Factor: 3.4). 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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Mathematical difficulties have been documented following pediatric mild traumatic brain injury (mTBI), yet a precise characterization of these impairments and their neural correlates is currently unavailable. We aimed to characterize these impairments by comparing behavioral and neuroimaging (i.e. diffusion tensor imaging or DTI) outcomes from children with subacute mTBI to typically developing controls. Twenty subacute pediatric mTBI patients and 20 well-matched controls underwent cognitive assessment and DTI examination. DTI tractography was used to detect white matter abnormalities in the corpus callosum (CC) and superior and inferior longitudinal fasciculi; these tracts are involved in mathematical performance and they are often damaged after mTBI. Behavioral results revealed that children with mTBI performed significantly more poorly on rapid apprehension of small numbers of objects (or 'subitizing'), processing of non-symbolic numerosities and procedural problem solving. These group differences were explained by differences in visuospatial working memory, which suggests that the observed mathematical difficulties may be a consequence of impairments in visuospatial abilities. DTI analysis revealed subtle group differences in the CC genu and splenium, i.e. higher fractional anisotropy and lower mean and radial diffusivity in children with mTBI, but the observed white matter abnormalities of the CC were not significantly associated with the observed mathematical difficulties in the mTBI patients.
    Journal of neurotrauma 04/2015; DOI:10.1089/neu.2014.3809 · 3.97 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The ability to image the whole brain through ever more subtle and specific methods/contrasts has come to play a key role in understanding the basis of brain abnormalities in several diseases. In magnetic resonance imaging (MRI), "diffusion" (i.e. the random, thermally-induced displacements of water molecules over time) represents an extraordinarily sensitive contrast mechanism, and the exquisite structural detail it affords has proven useful in a vast number of clinical as well as research applications. Since diffusion-MRI is a truly quantitative imaging technique, the indices it provides can serve as potential imaging biomarkers which could allow early detection of pathological alterations as well as tracking and possibly predicting subtle changes in follow-up examinations and clinical trials. Accordingly, diffusion-MRI has proven useful in obtaining information to better understand the microstructural changes and neurophysiological mechanisms underlying various neurodegenerative disorders. In this review article, we summarize and explore the main applications, findings, perspectives as well as challenges and future research of diffusion-MRI in various neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease and degenerative ataxias. Copyright © 2015. Published by Elsevier Inc.
    Magnetic Resonance Imaging 04/2015; DOI:10.1016/j.mri.2015.04.006 · 2.02 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Late-blind humans can learn to understand speech at ultra-fast syllable rates (ca. 20 syllables/s), a capability associated with hemodynamic activation of the central-visual system. Thus, the observed functional cross-modal recruitment of occipital cortex might facilitate ultra-fast speech processing in these individuals. To further elucidate the structural prerequisites of this skill, diffusion tensor imaging (DTI) was conducted in late-blind subjects differing in their capability of understanding ultra-fast speech. Fractional anisotropy (FA) was determined as a quantitative measure of the directionality of water diffusion, indicating fiber tract characteristics that might be influenced by blindness as well as the acquired perceptual skills. Analysis of the diffusion images revealed reduced FA in late-blind individuals relative to sighted controls at the level of the optic radiations at either side and the right-hemisphere dorsal thalamus (pulvinar). Moreover, late-blind subjects showed significant positive correlations between FA and the capacity of ultra-fast speech comprehension within right-hemisphere optic radiation and thalamus. Thus, experience-related structural alterations occurred in late-blind individuals within visual pathways that, presumably, are linked to higher order frontal language areas.
    PLoS ONE 01/2015; 10(4):e0122863. DOI:10.1371/journal.pone.0122863 · 3.53 Impact Factor