Hi Marlene,
thanks for your reply. This helps a lot! I will try it out. 
Due to the fact that this command needs to run overnight, I already tried to continue with the next step for Bias Field correction yesterday (using the provided supplementary file), but there occurs an error as well if I enter: dwibiascorrect ants dwi_den_unr_preproc.mif dwi_den_unr_preproc_unbiased.mif –bias bias.mif
Traceback (most recent call last):
File “/Users/ls/mrtrix3/bin/dwibiascorrect”, line 82, in
import mrtrix3
File “/Users/ls/mrtrix3/bin/mrtrix3.py”, line 17, in
import imp, os, sys
ModuleNotFoundError: No module named ‘imp’
According to some advice I found online, I would be supposed to change the path or even the python set-up, but I do not know how to do this yet. So it is turning out to be a challenge right now - do you have any tips on how I can solve this problem?
Best,
Lea