title: Diffusion tensor imaging of nigral degeneration in Parkinson's disease: A region-of-interest and voxel-based study at 3 T and systematic review with meta-analysis.
abstract: There is increasing interest in developing a reliable, affordable and accessible disease biomarker of Parkinson's disease (PD) to facilitate disease modifying PD-trials. Imaging biomarkers using magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) can describe parameters such as fractional anisotropy (FA), mean diffusivity (MD) or apparent diffusion coefficient (ADC). These parameters, when measured in the substantia nigra (SN), have not only shown promising but also varying and controversial results. To clarify the potential diagnostic value of nigral DTI in PD and its dependency on selection of region-of-interest, we undertook a high resolution DTI study at 3 T. 59 subjects (32 PD patients, 27 age and sex matched healthy controls) were analysed using manual outlining of SN and substructures, and voxel-based analysis (VBA). We also performed a systematic literature review and meta-analysis to estimate the effect size (DES) of disease related nigral DTI changes. We found a regional increase in nigral mean diffusivity in PD (mean ± SD, PD 0.80 ± 0.10 vs. controls 0.73 ± 0.06 · 10(- 3) mm(2)/s, p = 0.002), but no difference using a voxel based approach. No significant disease effect was seen using meta-analysis of nigral MD changes (10 studies, DES = + 0.26, p = 0.17, I(2) = 30%). None of the nigral regional or voxel based analyses of this study showed altered fractional anisotropy. Meta-analysis of 11 studies on nigral FA changes revealed a significant PD induced FA decrease. There was, however, a very large variation in results (I(2) = 86%) comparing all studies. After exclusion of five studies with unusual high values of nigral FA in the control group, an acceptable heterogeneity was reached, but there was non-significant disease effect (DES = - 0.5, p = 0.22, I(2) = 28%). The small PD related nigral MD changes in conjunction with the negative findings on VBA and meta-analysis limit the usefulness of nigral MD measures as biomarker of Parkinson's disease. The negative results of nigral FA measurements at regional, sub-regional and voxel level in conjunction with the results of the meta-analysis of nigral FA changes question the stability and validity of this measure as a PD biomarker.
authors.affiliation: Radiological and Imaging Sciences, University of Nottingham, Queen's Medical Centre, Derby Rd, Nottingham, NG7 2UH, United Kingdom.
authors.forename: Stefan T
authors.initials: ST
authors.lastname: Schwarz
authors.forename: Maryam
authors.initials: M
authors.lastname: Abaei
authors.forename: Vamsi
authors.initials: V
authors.lastname: Gontu
authors.forename: Paul S
authors.initials: PS
authors.lastname: Morgan
authors.forename: Nin
authors.initials: N
authors.lastname: Bajaj
authors.forename: Dorothee P
authors.initials: DP
authors.lastname: Auer
keywords: ACE, Addenbrooke's cognitive examination test battery; ADC, Apparent diffusion coefficient; DES, Effect size of disease related nigral changes; DTI, Diffusion tensor imaging; Diffusion weighted imaging; EPI, Echo planar imaging; Fractional anisotropy; ICC, Intraclass correlation coefficient; MD, Mean diffusivity; MRI, Magnetic resonance imaging; Magnetic resonance imaging; PD, Parkinson's disease; Parkinson's disease; Parkinsonism; ROI, Region/regions of interest; SN, Substantia nigra; SNc, Substantia nigra pars compacta; Substantia nigra; TCS, Transcranial sonography; UPDRS, Unified Parkinson's disease rating scale; VBA, Voxel based analysis