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.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Attention-Deficit/Hiperactivity Disorder (ADHD) is a prevalent disorder, but its neuroanatomical circuitry is still relatively understudied, especially in the adult population. The few morphometric magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) studies available to date have found heterogeneous results. This may be at least partly attributable to some well-known technical limitations of the conventional voxel-based methods usually employed to analyze such neuroimaging data. Moreover, there is a great paucity of imaging studies of adult ADHD to date that have excluded patients with history of use of stimulant medication.
    PLoS ONE 10/2014; 9(10):e110199. · 3.53 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Evidence shows that there are reductions in gray matter volume (GMV) and changes in long association white matter fibres within the left insula-temporoparietal junction (TPJ) during the early stages of psychotic disorders but less is known about short association fibres (sAFs). In this study we sought to characterise the changes in sAFs and associated volumetric changes of the left insula-TPJ during the early stages of psychosis. Magnetic resonance imaging was obtained from a sample of young people with psychosis (n = 42) and healthy controls (n = 45), and cortical parcellations of the left insula-TPJ were used as seeding masks to reconstruct 13 sAFs. Compared to healthy counterparts, the psychosis group showed significant reductions in fractional anisotropy (FA) in the sAFs connecting the superior (STG) and middle temporal gyri (MTG) and as well as reduced GMV within the inferior temporal gyrus and increased white matter volume (WMV) within Heschl's gyrus (HG). Furthermore, adolescent-onset psychosis subjects (onset 18 year or earlier) showed FA reductions in the STG-HG sAF when compared to adult-onset subjects, but this was not associated with changes in GMV nor WMV of the STG or HG. These findings suggest that during the early stages of psychosis, changes in sAFs and associated cortical GMV and WMV appear to occur independently, however age of onset of a psychotic syndrome/disorder influences the pattern of neuroanatomical abnormalities.
    PLoS ONE 11/2014; 9(11):e112842. · 3.53 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The relationship between the integrity of white matter tracts and cortical function in the human brain remains poorly understood. We investigate reversible white matter injury, in this case patients with compression of the optic chiasm by pituitary gland tumors, to study the structural and functional changes that attend spontaneous recovery of cortical function and visual abilities after surgical removal of the tumor and subsequent decompression of the nerves. We show that compression of the optic chiasm led to demyelination of the optic tracts, which reversed as quickly as 4 weeks after nerve decompression. Furthermore, variability across patients in the severity of demyelination in the optic tracts predicted visual ability and functional activity in early cortical visual areas. Preoperative measurements of myelination in the optic tracts predicted the magnitude of visual recovery after surgery. These data indicate that rapid regeneration of myelin in the human brain is a component of the normalization of cortical activity, and ultimately the recovery of sensory and cognitive function, after nerve decompression. More generally, our findings demonstrate the use of diffusion tensor imaging as an in vivo measure of myelination in the human brain.
    Science translational medicine 12/2014; 6(266):266ra173. · 14.41 Impact Factor

Full-text (2 Sources)

Available from
May 31, 2014