Hi Thibo,
The different output you get from mrinfo and mrconvert is known. Even more, it’s intentional: mrinfo aims to export the gradient table in a format as closely related to the input image as possible, whereas mrconvert exports the gradient table to match with the output image. If you convert between file types and associated axes conventions (such as going from .mif to .nii), or select a subset of the diffusion volumes, this makes a difference.
The specific issue you describe is indeed counterintuitive. In this instance, mrconvert prepares the gradient table for the output .nii format by rescaling the b-values with the norm of the gradient direction vectors, which in your case results in 0. I am a bit surprised to see that mrinfo does not do this too; in my mind the difference between both should only relate to the gradient directions, not to the b-values.
However, I wonder if, at te end of the day, it matters in your case. What use is a b800 image if you don’t know what gradient direction was used? I’d say you either discard it, or dive in the DICOM headers to figure out what it is. Could it be a trace map with isotropic diffusion weighting perhaps?
Cheers,
Daan