Fractional anisotropy (FA) is a
scalar value between zero and one that describes the degree of
anisotropy of a
diffusion process. A value of zero means that diffusion is isotropic, i.e. it is unrestricted (or equally restricted) in all directions. A value of one means that diffusion occurs only along one axis and is fully restricted along all other directions. FA is a measure often used in
diffusion imaging where it is thought to reflect
fiber density,
axonal diameter, and
myelination in
white matter. The FA is an extension of the concept of eccentricity of conic sections in 3 dimensions, normalized to the unit range.
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*
Original Research
Diffusion tensor fractional
anisotropy of the normal-appearing seven segments of the corpus callosum in
healthy adults and relapsing-remitting multiple sclerosis patients†
- Khader M. Hasan PhD1,*,
- Rakesh K. Gupta MD1,
- Rafael M. Santos MD, FRCS1,
- Jerry S. Wolinsky MD2,
- Ponnada A. Narayana PhD1
Article first published online: 19 MAY 2005
DOI: 10.1002/jmri.20296
Copyright © 2005 Wiley-Liss, Inc.
Keywords:
- diffusion tensor imaging;
- multiple sclerosis;
- corpus callosum
Abstract
Purpose
To investigate the utility of whole-brain diffusion tensor imaging (DTI) in
elucidating the pathogenesis of multiple sclerosis (MS) using the
normal-appearing white matter (NAWM) of the corpus callosum (CC) as a marker of
occult disease activity.
Materials and Methods
A high signal-to-noise ratio (SNR) and optimized entire brain DTI data were
acquired in 26 clinically-definite relapsing and remitting multiple sclerosis
(RRMS) patients and 32 age-matched healthy adult controls. The fractional
anisotropy (FA) values of seven functionally distinct regions in the
normal-appearing CC were compared between patients and controls.
Results
This study indicates that 1) there was a gender-independent FA heterogeneity
of the functionally specialized CC segments in normal volunteers; 2) FA in the
MS group was significantly decreased in the anterior (P = 0.0039) and
posterior (P = 0.0018) midbody subdivisions of the CC, possibly due to
a reduction of small-caliber axons; and 3) the FA of the genu of the CC was
relatively intact in the MS patients compared to the healthy age-matched
controls (P = 0.644), while the splenium showed an insignificant trend
of reduced FA values (P = 0.248). The decrease in FA in any of the CC
subdivisions did not correlate with disease duration (DD) or the expanded
disability status scale (EDSS) score.
Conclusion
The preliminary results are consistent with published histopathology and
clinical studies on MS, but not with some published DTI reports. This study
provides insights into the pathogenesis of MS, and the role played by
compromised axonal integrity in this disease. J. Magn. Reson. Imaging
2005;21:735–743. © 2005 Wiley-Liss, Inc.
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Estimates of the fractional anisotropy slope parameters with standard errors are presented for the ASD-positive and -negative groups for all tracts in
Table 2. Both groups showed significant increases in fractional anisotropy from 6 to 24 months, though the rate of change for the ASD-negative group was significantly greater than that for the ASD-positive group in the bilateral limbic (fornix) and association (inferior longitudinal fasciculus and uncinate) fiber tracts. Individual and mean group trajectories for these tracts are presented in
Figure 1. The changes from 6 to 24 months in fractional anisotropy for the corpus callosum subdivisions are shown in
Figure 2; the change for the body was significantly different in the two groups. For projection tracts, the growth trajectories of the left anterior thalamic radiation and all internal capsule divisions were significantly steeper for the ASD-negative infants (
Figure 3).
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