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

AIM: To determine hemispheric asymmetries in major white matter tracts using diffusion tensor imaging (DTI) metrics and morphometric measurements, as comprehensive multitract analyses remain limited. MATERIAL and METHODS: Diffusion MRI data from 102 healthy adults enrolled in the Human Connectome Project (HCP) were analyzed using DSI Studio to reconstruct 17 major white- matter tracts bilaterally. For each tract, diffusion metrics (fractional anisotropy [FA], mean diffusivity [MD], radial diffusivity [RD], axial diffusivity [AD], quantitative anisotropy [QA], isotropic value [ISO], restricted diffusion imaging [RDI], and normalized RDI [nrdı]) and morphometric parameters (fiber count, mean length, trunk volume, branch volume, total volume, and termination region areas) were quantified. Paired t-tests compared left-right hemispheric differences. Multiple comparison corrections were performed with the Benjamini-Krieger-Yekutieli procedure, and effect sizes were reported as Cohen's d. RESULTS: Significant hemispheric asymmetries were observed in most tracts. FA was significantly higher in the left hemisphere (p<0.001) for the arcuate fasciculus (AF), inferior fronto-occipital fasciculus (IFOF), inferior longitudinal fasciculus (ILF), and uncinate fasciculus (UF). Mean diffusivity (MD) demonstrated left dominance in the UF and SLF2-3 (p=0.009). QA was predominantly left- lateralized across multiple tracts, including AF, IFOF, ILF, and frontal aslant tract (FAT) (p<0.01, Cohen's d: 0.42 to 1.17), except for the 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). Rightward dominance was observed in SLF3, parietal aslant tract (PAT), IFOF, ILF, and middle longitudinal fasciculus (MLF) (p<0.05). Similar asymmetries were observed in termination region areas, fiber counts, and fiber lengths (p<0.001). CONCLUSION: Widespread hemispheric asymmetries were observed across multiple pathways. Such asymmetry may reflect distinct hemispheric functional specialization and the neurological basis of lateralized functions.