ComBat scanner harmonization for fixel-based analysis

Diffusion MRI data are known to be sensitive to scanner and site effects, for which harmonization methods have been developed. However, harmonization methods have not yet been applied to many advanced diffusion imaging metrics. In this work, we apply the ComBat harmonization approach to fixel-based analysis. ComBat successfully minimizes scanner-related differences in fibre density and cross-section, and performs similarly to the inclusion of scanner as a nuisance regressor during whole-brain fixel-based analysis. Importantly, ComBat can now readily be used within the fixel-based framework, which will enable large multi-centre studies to implement this approach in the future.