white matter fiber orientation wrt the B0 field

Hi everyone.

I’d like to calculate the white matter fiber orientation with respect to the B0 field from 30-diffusion axis Diffusion Tensor Image.
And I saw the other’s post and tried the code below

dwi2tensor dwi.mif dt.mif
tensor2metric dt.mif -modulate none -vec vec.mif
mrconvert vec.mif -coord 3 2 z-component.mif
mrcalc z-component.mif -acos angle.mif

However, I have some question about this point ‘tensor2metric dt.mif -modulate none -vec vec.mif’
If I scanned the human subjects’ head with slice direction set perpendicular (transverse) to the OM-Line of the subjects’ head ; I mean that the slice direction was not equal to the B0 field, does the z-component.mif still represent the direction of principal eigenvector to the B0 field? or this z-component just represent the direction of the eigenvector to the slice direction?

If anybody know this, please let me know.