Seperate fixel

Dear Expert,

I am looking for a way to seperate different fixel groups. For instance, separating the primary, secondary fixels into to separate files or visualizing them seperately. Could you give me a hint?
I know that fixel2voxel command could do genenrate scalar maps from individual fixel groups, but the directional information is sort of lost.

Dan Wu

Hi Dan,

Probably what you’re looking for is the fixel2voxel command with the “split_dir” operation. There may be further manipulation required after that point to get at exactly what you want, but it’s a start.

Couple of points:

  1. Fixels across voxels don’t actually form “fixel groups” (even if they’re presented as such in other softwares). E.g. Imagine two adjacent voxels, each of which contains an A-P bundle and a L-R bundle. In the first voxel, the former is fractionally larger than the latter; in the second voxel, the opposite is the case. If you only look at e.g. the first fixel in each voxel, or the second fixel in each voxel, both of these bundles will look discontinuous. So splitting fixels like this for independent viewing can be misleading sometimes.

  2. The ordering of fixels is actually by peak amplitude, not fibre density. I should probably change this…


Thanks Rob! I see fixels are not necessarily associated with fiber groups, and they are ordered according to the peak amplitude. I’ve been using fixel2voxel with split_dir option. I will see what I can do with these.