the second-largest cluster of the DTI sub-analysis cover the fornix. The fornix is a thin structure located close to the ventricles, which makes it particularly susceptible to partial volume effects [82]. This effect occurs when in a voxel the signal is not only represented by one substance (e.g., WM) but is also confounded by other substances such as cerebrospinal fluid (CSF). The mixing of signals leads to an increase in diffusivity measures and a decrease in anisotropy measures and is particularly pronounced when small, thin structures are affected and when groups with brain atrophy are compared with healthy controls [82]. Both are evident in the integrated studies. For adequate correction of the partial volume effect, voxel-wise CSF contamination correction, such as free water elimination, is recommended [82, 83]. None of the integrated studies explicitly mentioned that such a correction was performed. Despite the clear meta-analytic evidence, the ALE cluster covering the fornix should therefore be interpreted with caution, as there is a possibility that the results of the contributing studies may be biased by the partial volume effect.
