In a lot of scenarios, that’s what would typically come out, but I wouldn’t personally call it “what you’re generally looking for” in all cases. In low b-value data such as @zeydabadi’s, I’ve observed exactly what Mahmoud obtained here as well. I’m not surprised here; the response furthermore does show the expected shape in the amplitude domain. Including these coefficients in CSD, in my experience, is not detrimental. Due to the low b-value and thus low angular frequencies dominating anyway, the difference is naturally so small that you can barely observe it in the outcome. That said, theoretically, it’s in your favour to not truncate, to avoid other (theoretical) artifacts. That said again, I should probably mostly stress that in practice, you won’t notice any substantial difference though.
Long story short: Mahmoud, I’ve actually inspected the response function you provided here, there’s no reason to worry about it. Note that, by default in any CSD algorithm now, the very last coefficient there won’t even be used (see CSD on standard DWI data and https://github.com/MRtrix3/mrtrix3/issues/899). Further do note that, the lmax=10 coefficient, even though seemingly discarded later on given all defaults, does serve a role in response function estimation (see https://github.com/MRtrix3/mrtrix3/pull/786#issuecomment-265947475 and https://github.com/MRtrix3/mrtrix3/pull/862 where this was implemented and brought about some other stuff). Omitting lmax=10 at that (response estimation) step is not the same as truncating it down the track: in the response estimation, it can avoid the monotonicity and other constraints to otherwise potentially bias the estimation of (some of) the response ZSH coefficients.
For intuition, you should mainly check your response functions in the amplitude domain, using
shview. use the
right arrow keys on your keyboard to switch between b-values, e.g., to switch from the b=0 to the b=700 part of the response. Use the
Escape key to rescale the visualisation if due to switching b-values, the visualisation would appear too small (when switching to a higher b-value) or too large (when switching back again to a lower b-value).