Let me try to be more clear.
When I say that voxels are negative, I’m talking about pre-processed DWI.
We did “dwidenoise”, “mrdegibbs”, “dwifslpreproc”, “dwibiascorrect” and “mrgrid” on the raw DWI data and then used this pre-processed image to achieve the DTI coefficient images.
Yes, these voxels are still NaN in the DTI coefficient image.
In the raw DWI image and the intermediate images created during these pre-processing, I also checked the region where are NaN in DTI coefficeint image. In the images after dwidenoise or mrdegibbs, the values of this region are still positive. After dwifslpreproc or dwibiascorrect, the values of this region are 0. But after mrgrid, the values of this region are negative.