Relationship between diffusion MRI parameters and disease severity according to Unified Parkinson's Disease Rating Scale (UPDRS)‐III–motor subscale score. GBSS analysis identified a significant correlation between UPDRS‐III–motor score and the diffusion MRI parameters of the cortical GM in the frontal, temporal, limbic, and paralimbic areas, as well as in the basal ganglia (P < 0.05, corrected for multiple comparisons, gender and age at the time of MRI). Red–yellow voxels demonstrate a significant positive correlation between the UPDRS‐III–motor score and diffusion MRI parameters. Blue–light blue voxels demonstrate a significant negative correlation between the UPDRS‐III–motor score and diffusion MRI parameters. To aid visualization the results (corrected P < 0.05) are thickened using the fill script implemented in FSL. AD, axial diffusivity; AK, axial kurtosis; FA, fractional anisotropy; MD, mean diffusivity; MK, mean kurtosis; OD, orientation dispersion index; RD, radial diffusivity; RK, radial kurtosis; ICVF, intracellular volume fraction; ISOVF, isotropic volume fraction. [Color figure can be viewed at http://wileyonlinelibrary.com]