Parkinson's disease(PD) is a degenerative disorder of the brain which caused numerous neurological symptoms. DTI studies based on region of interest (ROI) and Tract-based Spatial Statistics (TBSS) in PD had reported these were relationship between white matter(WM) degeneration and fractional anisotropy(FA) mean diffusivity(MD). But the results of different studies are inconsistent. To certificated FA, MD alteration of WM in PD, DTI data were analyzed by TBSS [1]. Furthermore, the role of eigenvalues λ₁, λ₂ and λ₃ were discussed.This study was approved by local Institutional Review Board. 25 PD patients and 26 age-matched healthy controls were recruited and signed informed consent form was obtained for each subject. MRI data was acquired on a 3.0T whole body scanner (Discovery MR750, GE Healthcare, Milwaukee, WI). All subjects underwent a 25-direction DTI scan using diffusion weighted single-shot spin-echo EPI sequence with b-value = 1000 s/mm², TR = 8000ms, TE = 81ms, acquisition matrix = 128 x 128, slice thickness = 3mm with no gap, NEX = 2. DTI data set were exported and transferred to an offline workstation for further analysis. FSL software was used in DTI and TBSS analysis. In the beginning, skull was stripped on b₀ image using BET tool. Eddy current correction was performed before DTI quantification using DTIFIT tool. Then fractional anisotropy (FA), mean diffusivity (MD), directional diffusivity (λ₁, λ₂ and λ₃) were generated for each subject. TBSS analysis were performed on the FA, MD and three directional diffusivity following the recommended procedures [1]. Statistical significance level p < 0.05 was displayed.Regions of significant FA difference were superimposed on the MNI152_T1_1mm template. Decreased FA (Fig1) and increased MD (Fig2) were observed in PD compared with controls. MD changed more extensively than the FA. λ₁, λ₂ and λ₃ were increased at different locations (Fig3). Furthermore, the perpendicular diffusivity λ₂ and λ₃ showed more widespread increase in WM compared with axial diffusivity λ₁. The above-mentioned exceptions were slightly dominant in right hemisphere.

The TBSS analysis showed widespread decreased FA and increased MD in WM of PD which were well in agreement with Melzer's study [2]. However, Kim etc reported MD increased without FA alteration in PD. This may be due to the different severity of the disease in these studies. In this study MD showed more significant change than FA, suggesting that MD is more sensitive than FA to WM degeneration of PD. The increase of perpendicular diffusivity is more more widespread compared with axial diffusivity. This may suggest that the demyelination process in WM of PD patients was profound, which led to the increase of MD. It was reported in previous studies that WM degeneration showed dominant in different hemisphere. For the uncertainty of dominant in left or right hemisphere in Clinical PD patients, these difference may be caused by the the different number and characteristics of patients among studies.

The TBSS analysis showed widespread decreased FA and increased MD in WM of PD. MD is more sensitive to the WM degeneration in PD patients compared with FA. In addition, the increase of MD was greatly attributed to the increase of perpendicular diffusivity which may suggest the profound demyelination of WM in PD patients.