Hey Adam,
Yep: if you flip the x-axis, the mean streamline length is almost double that compared to if you perform no such flip, and it’s a pretty clear standout winner.
The ambiguity is whether this flip should be applied in image space (equivalent to negating values in the first row of the bvec file) or scanner space (equivalent to negating values in the first column of the gradient table in MRtrix format). If your image axes are very close to the scanner principal axes, then it’s just about impossible to figure out which of the two should be applied based on this heuristic. But your prior processing might provide a clue. E.g. If you’ve just imported DICOM data, and the nature of the data coming from your scanner vendor means that the gradient orientations are stored within the DICOM headers in scanner space, then you probably want to do the flip in scanner space. If your data have been somehow processed by some other software package, which has involved conversion to/from NIfTI / bvecs at some point, and especially if that software isn’t guaranteed to deal with the bvecs coordinate system handedness issue, more likely the error has been applied in image space and so should be corrected there. But the closer your images axes are to the scanner axes, the less important the distinction becomes.
Cheers
Rob