FMRIPrep produces a preprocessed version of the T1-weighted image stored in the ‘anat’ folder with the pattern ‘sub*_desc-preproc_T1w.nii.gz’.
Do you think using this preprocessed image is preferred for voxel-based morphometry (VBM) instead of using the initial non-preprocessed T1-weighted image?
What preprocessing steps have been performed to this image?
Yes. You may also consider a surface based morphometry study instead with your freesurfer outputs, which enable cortical thickness and surface area estimations, in addition to volume.