Diffusion Tensor Imaging Metrics Reveal Pronounced Hemispheric Asymmetry in White Matter Tracts

Introduction: Tractography enables in vivo analysis of white matter pathways. DTI metrics reveal hemispheric distribution, yet full assessment across multiple pathways is limited. This study addresses this by analyzing asymmetry in 17 major white matter tracts bilaterally.

Methods: Using diffusion MRI data of 102 healthy subjects from the human connectome project (HCP) database, 17 major white matter tracts were bilaterally reconstructed. DSI Studio was employed for visualization, tractographic measurements, and calculations. Measurements included fiber counts, mean fiber lengths, trunk volumes, branch volumes, total tract volumes, termination region areas, values of fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), axial diffusivity (AD), quantitative anisotropy (QA), isotropic value (ISO), restricted diffusion imaging (RDI), normalized RDI (nRDI). Paired t-tests measured hemispheric significance, and Cohen's d quantified effect sizes for each tract.

Results: Statistical analysis showed significant hemispheric asymmetries for almost all tracts. Fractional anisotropy (FA) was generally higher in left hemisphere (p < 0.001) in tracts arcuate fasciculus (AF), inferior frontooccipital fasciculus (IFOF), inferior longitudinal fasciculus (ILF) and uncinate fasciculus (UF). However, this did not extend to superior longitudinal fasciculus-3 (SLF3). Mean diffusivity (MD) demonstrated left dominance in pathways frontal aslant tract (FAT), UF, and SLF2-3 (p=0.009). Quantitative anisotropy (QA) exhibited leftward asymmetry across most tracts, including AF, IFOF, ILF, and FAT (p < 0.01, Cohen’s d: 0.42 to 1.17), except for vertical occipital fasciculus (VOF), where rightward dominance was noted (Cohen’s d: -0.55). Tract-trunk-branch volumes were predominantly left-lateralized in frontal lobe pathways such as AF, SLF1, SLF2, FAT, and UF (p < 0.01). In contrast, rightward dominance was observed in SLF3, parietal aslant tract (PAT), IFOF, ILF, and middle longitudinal fasciculus (MLF) (p < 0.05). Similar asymmetries appeared in termination region areas, fiber counts, and fiber lengths (p < 0.001).

Conclusion : Our study reveals significant hemispheric asymmetry of white matter pathways in the human brain. Such asymmetry may underlie distinct functional specializations between two hemispheres, offering insights into neuroscientific explanation of lateralization of functions.