DTI tractography and white matter fiber tract characteristics in euthymic bipolar I patients and healthy control subjects - PubMed (original) (raw)

DTI tractography and white matter fiber tract characteristics in euthymic bipolar I patients and healthy control subjects

Carinna M Torgerson et al. Brain Imaging Behav. 2013 Jun.

Abstract

With the introduction of diffusion tensor imaging (DTI), structural differences in white matter (WM) architecture between psychiatric populations and healthy controls can be systematically observed and measured. In particular, DTI-tractography can be used to assess WM characteristics over the entire extent of WM tracts and aggregated fiber bundles. Using 64-direction DTI scanning in 27 participants with bipolar disorder (BD) and 26 age-and-gender-matched healthy control subjects, we compared relative length, density, and fractional anisotrophy (FA) of WM tracts involved in emotion regulation or theorized to be important neural components in BD neuropathology. We interactively isolated 22 known white matter tracts using region-of-interest placement (TrackVis software program) and then computed relative tract length, density, and integrity. BD subjects demonstrated significantly shorter WM tracts in the genu, body and splenium of the corpus callosum compared to healthy controls. Additionally, bipolar subjects exhibited reduced fiber density in the genu and body of the corpus callosum, and in the inferior longitudinal fasciculus bilaterally. In the left uncinate fasciculus, however, BD subjects exhibited significantly greater fiber density than healthy controls. There were no significant differences between groups in WM tract FA for those tracts that began and ended in the brain. The significance of differences in tract length and fiber density in BD is discussed.

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Figures

Figure 1

Figure 1

A) DTI tractography entails propagation of fibers along the path of least diffusivity from each voxel, creating a 3D map of all white matter connections. B) Programs like TrackVis enable an investigator to select voxels and view all fiber orientations that pass through the ROI. Coloration of fibers then allows the investigator to create ROIs (such as the pink sphere) which can turn off all voxels that pass through the original ROI but do not belong to the tract of interest. C) Each tract in this study was then assigned a different color. This panel depicts all of the tracts analyzed.

Figure 2

Figure 2

The first graph depicts the significant differences in fiber density. The values were obtained by dividing the number of fibers in the tract of interest by the total number of WM fibers in the subject’s DTI. BD subjects showed lower densities in all significant regions except the UF, in which we found significantly higher fiber density than in healthy controls. The second graph shows all of the significant differences in tract length (after normalizing lengths to head circumference of the subjects, yielding unitless results). BD subjects had shorter tracts in all significant regions. The third graph displays the significant FA differences

Figure 3

Figure 3

All fiber tracts are colored according to the value of their FA at each point; fibers with an FA close to 0 are shown as red, while increasing values shift toward blue, as elucidated by the colorbar in the upper right. Each row shows a comparison of the control and BD subjects in whom the tract of interest was closest to the median sample value for the group. The first row illustrates the greater fiber density in the ILF-R of controls. The second row depicts the greater length of splenium fibers in control subjects. Finally, the third row demonstrates the higher median FA in the CST-R2 of controls.

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