Quantifying the number of crossing fibers in the aging brain using dki tractography

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Tractography is a technique that visualizes white matter (WM) fibers based on diffusion MRI data. A shortcoming of DTI is the inability to directly resolve fiber crossings. Here we reconstructed WM tracts using diffusional kurtosis imaging (DKI), which is an advanced diffusion MRI technique that quantifies the non-Gaussianity of diffusion and, allows for the detection of crossing fibers. Because aging is associated with decreased microstructural complexity in the brain, we hypothesize that this would be reflected by a decrease in the number of detected crossing fibers. The study sample is composed of 48 cognitively intact older adults (age = 68.0 ± 5.6y, range = 60-80y; MoCA score = 26.9 ± 1.8; 62.5% Female; 95.8% White). DKI was acquired for all individuals on a 3T Siemens Tim Trio. All diffusion metrics were calculated with DKE (Diffusional Kurtosis Estimator). Using in-house software, the diffusion orientation distribution function (dODF) was constructed for every voxel. The dODF quantifies the extent of diffusion as a function of direction. The local maxima of the dODF provide estimates for the fiber directions, and the number of local maxima determines the number of crossing fibers detected. The JHU WM atlas was used to identify the regions of interest. There was a significant negative correlation (r=-0.46, p<0.05) between the number of detected crossing fibers and age (Fig. 1). To verify our measurements, we compared the number of crossing fibers in areas with different tract morphologies: the corona radiata (CR) and the genu of the corpus callosum. As expected, more crossing fibers were detected in the CR, a region with fiber orientation heterogeneity, than in the more homogeneous genu (Table 1). Figure. 2 shows the fiber complexity at the level of the superior CR calculated with both DKI and DTI. Since many fibers cross the superior longitudinal fasciculus here a large part remains undetected by DTI. Advanced diffusion MRI techniques like DKI enable detection of fibers in heterogeneous regions with crossing fibers. The number of fiber crossings detected with diffusion MRI decreases with age. This may be relevant for the interpretation of tractography in older subjects.