This study thoroughly explores the reliability of a statistical multi-site diffusion-MRI harmonization method, ComBat, using a large diffusion-MRI dataset involving 5 sites. Additionally, we demonstrate the effects of using different software packages to estimate the diffusion tensor on the harmonization performance of ComBat. Even when the processing steps (and software) were identical for all sites, ComBat altered the inter-group variability at each site and in some regions flipped the direction of effect-sizes. Finally, when different software packages were used to estimate the diffusion tensor at different sites, ComBat introduced much drastic effects, thereby reducing the reliability of the results.
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