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Review
. 2010;35(3):233-56.
doi: 10.1080/87565641003689556.

Longitudinal Study of Callosal Microstructure in the Normal Adult Aging Brain Using Quantitative DTI Fiber Tracking

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Free PMC article
Review

Longitudinal Study of Callosal Microstructure in the Normal Adult Aging Brain Using Quantitative DTI Fiber Tracking

Edith V Sullivan et al. Dev Neuropsychol. .
Free PMC article

Abstract

We present a review of neuroimaging studies of normal adult aging conducted with diffusion tensor imaging (DTI) and data from one of the first longitudinal studies using DTI to study normal aging. To date, virtually all DTI studies of normal adult aging have been cross-sectional and have identified several patterns of white matter microstructural sparing and compromise that differentiate regional effects, fiber type, and diffusivity characteristics: (1) fractional anisotropy (FA) is lower and mean diffusivity is higher in older than younger adults, (2) aging is characterized by an anterior-to-posterior gradient of greater-to-lesser compromise also seen in superior-to-inferior fiber systems, and (3) association fibers connecting cortical sites appear to be more vulnerable to aging than projection fibers. The results of this longitudinal study of the macrostructure and microstructure of the corpus callosum yielded a consistent pattern of differences between healthy, young (20s to 30s) and elderly (60s to 70s) men and women without change over 2 years. We then divided the fibers of the corpus callosum into the midsagittal strip and the lateral distal fibers in an attempt to identify the locus of the age-related differences. The results indicated that, on average, mean values of FA and longitudinal diffusivity (lambdaL) were lower in the distal than midsagittal fibers in both groups, but the age effects and the anterior-to-posterior gradients were more pronounced for the distal than midsagittal fibers and extended more posteriorly in the distal than midsagittal fibers. Despite lack of evidence for callosal aging over 2 years, ventricular enlargement occurred and was disproportionately greater in the elderly relative to the young group, being 8.2% in the elderly but only 1.2% in the young group. Thus, different brain regions can express different rates of change with aging. Our longitudinal DTI data indicate that normal aging is associated with declining FA and increasing diffusivity in both lambdaL (longitudinal diffusivity) and lambdaT (transverse diffusivity), perhaps defining the normal ontological condition rather than a pathological one, which can be marked by low FA and low diffusivity.

Figures

Figure 1
Figure 1
Axial images at the level of the lateral ventricles. Top two images. Examples of conventional magnetic resonance images from a fast spin-echo sequence (FSE). On the left is an early-echo image, which differentiates gray matter (light gray) and white matter (darker gray), both of which are relatively homogeneous in intensity. On the right is a late-echo image, on which CSF is brightest. Bottom two images: Examples of images from a diffusion tensor imaging sequence. On the left is a fractional anisotropy (FA) image, on which white matter is the brightest. On the right is a diffusivity image, on which CSF is brightest. The diffusion ellipsoid model displays in red the preferred orientation of the longitudinal diffusion, λL, and in blue the two minor axes of diffusion, the mean of which is λT.
Figure 2
Figure 2
Top sagittal image: Example of fiber tracking of the corpus callosum of a 23 year-old healthy woman. The colors represent six different fiber bundles based on divisions described by Pandya and Selzer (Pandya & Seltzer, 1986) identified with fiber tracking based on the DTI-derived FA. From anterior (far left) to posterior (far right), the bundles are deemed prefrontal, premotor, precentral, postcentral, posterior parietal, and temporal-occipital. Bottom data figures: Mean±S.E. of the six callosal sectors for the young and elderly groups at each MRI session of the four principal metrics of DTI: fractional anisotropy (upper left), apparent diffusion coefficient (upper right), λL (lower left), and λT (lower right). The elderly group had lower FA and higher diffusivity than the young group at both scanning sessions, and the group differences were greatest in the anterior sectors.
Figure 3
Figure 3
Top axial image on left: Example of fiber tracking of the midsagittal corpus callosum. The colors represent six different fiber bundles defined in Figure 2. Left column of line plots: Mean±S.E. of FA, ADC, λL, and λT of the midsagittal callosal fibers. Top axial image on right: Example of fiber tracking of the distal fibers of the corpus callosum. Right column of line plots: Mean±S.E. of FA, ADC, λL, and λT of the distal callosal fibers.
Figure 4
Figure 4
Mean±S.E. of the volumes of the corpus callosum and lateral ventricles for the young and elderly groups at each MRI session. While the callosal volumes showed no detectable change in either group over the 2-year follow-up period (group-by-time interaction (F(1,14)=.0093, p=.9245), the lateral ventricles expanded disproportionately in the elderly relative to the young group (group-by-time interaction (F(1,14)=8.921, p=.0098).
Figure 5
Figure 5
Three-dimensional rendering of three views of the ventricular system: oblique sagittal view on left (frontal to occipital is right to left); axial view in middle (frontal to occipital is top to bottom); coronal view on right. All views reveal that the ventricles of the elderly women are substantially greater than those of the younger woman.

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