Tck2fixel: streamline count or streamline density?

Dear Mrtrix experts,
I have a small doubt on the value estimated by tck2fixel.
Does it estimate

  • a) streamline/tract count per fixel
  • b) streamline density = streamline count / number of fixels within that voxel

AFD and FBA help!, I understand that is option a) according to this extract

tck2fixel, which essentially generates a fixel-wise track density image (i.e. equivalent of track count, but counted per specific fixel rather than just per voxel).".

In this other post, Tck2fixel , fixel2voxel , tract properties, I understand both options. First it says b) (or at least, I understand that), and later, a)

The part that makes me interpret it as option b) is the following

tck2fixel | fixel2voxel mean is kind of a strange operation. You’re getting the mean streamlines density across those fixels within a voxel; so more streamlines means more voxel-wise streamlines density, but if the number of fixels increases, the mean density per fixel will decrease"

The part that makes me interpret it as option a) is the following

The tck2fixel command will give you a streamline count per fixel.

Thank you very much! Really appreciate your work!


Apologies for the lack of response – it’s been a busy few weeks with the ISMRM and MRtrix workshop.

Short answer: it’s a).

The text you quote as making you interpret it as b) is only saying that if you compute the per fixel streamline count, and then average those counts across fixels, you will get a lower number than you would by summing over fixels. Main point is, this is still entirely compatible with a).

Hope this help… (?)


It does help! Thank you very much!