OK, that should work better than that.
However, maybe I’ve got the wrong end of the stick. Are you showing the noise map (as produced by
dwidenoise -noise), or the residuals as suggested in the docs? If the former, then what you showed is very unexpected. However, if your image is a snapshot of the residuals, and this corresponds to the first volume, it might be that there’s not a lot of denoising going on for the b=0 (which is presumably the first volume?) - this wouldn’t be wholly unexpected particularly if you have a single b=0 volume. But the other volumes might still behave as expected. Have a look through all the volumes (use the left/right arrows), hopefully they’ll look more convincing. You could also compute an image of the RMS residuals across all volumes, which might be a better way to inspect the residuals in one image:
mrmath residuals.mif rms -axis 3 rms_residuals.mif