Diffusion tensor imaging (DTI) is a well-established magnetic resonance imaging (MRI)
technique used for studying microstructural changes in the white matter. As with many
other imaging modalities, DTI images suffer from technical between-scanner variation
that hinders comparisons of images across imaging sites, scanners and over time. Using
fractional anisotropy (FA) and mean diffusivity (MD) maps of 205 healthy participants
acquired on two different scanners, we show that the DTI measurements are highly site-specific,
highlighting the need of correcting for site effects before performing downstream
statistical analyses. We first show evidence that combining DTI data from multiple
sites, without harmonization, may be counter-productive and negatively impacts the
inference. Then, we propose and compare several harmonization approaches for DTI data,
and show that ComBat, a popular batch-effect correction tool used in genomics, performs
best at modeling and removing the unwanted inter-site variability in FA and MD maps.
Using age as a biological phenotype of interest, we show that ComBat both preserves
biological variability and removes the unwanted variation introduced by site. Finally,
we assess the different harmonization methods in the presence of different levels
of confounding between site and age, in addition to test robustness to small sample
size studies.