How is the brain mask outputted by qsiprep produced?

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Dear Experts,

on the qsiprep documentation (Preprocessing — qsiprep 0.22.1.dev1+gbdd0e61 documentation) I read that one of the output files is:

*desc-brain_mask.nii.gz

This file is defined as “The generous brain mask that should be reduced probably”. We would need to extract from the Qsiprep output a brain mask extracted from the average of the preprocessed B0 images.

Is *desc-brain_mask.nii.gz such a mask? Or how was it produced? why should it be reduced?

Thanks,

Fabio