Creating population templates of fixels

1

I have a set of diffusion weighted MRI data that was collected with 64 directions at 3 shells, namely, b1000 b2000 and b3000. There are 3 age groups and within each age group there are 3 cognitive groups.

I have processed to the point where I have fixel maps of each individual brain in template space. I have processed using the MRtrix cfestats command but I would like to (if possible) get “mean fixel maps” of the “average” of each of my 9 groups (3 age groups with 3 cognitive groups within each age group). Is that possible? How would I go about doing so? I’ve looked all over the documentation about it and cant find an answer.
thank you

2

Fixels are usually stored as directories containing image files that define the location (index) and direction of fixels as well as fixels data files (Fixel image (directory) format — MRtrix 3.0 documentation). As long as your subject fixels share index and directions, you can use voxel-wise operations using mrmath and mrcalc to perform operations on the fixel value files. So, you’d need to map subject fixels to a common reference fixel image (see Fibre density and cross-section - Multi-tissue CSD — MRtrix 3.0 documentation).

You can compute the mean across specific fixel value files with:
mrmath path/to/fixel_values_subj1.mif path/to/fixel_values_subj2.mif path/to/fixel_values_subj3.mif mean path/to/groupmean.mif.
Depending on your filenames on linux/macOS you could use standard globbing patterns (see Wildcards) to identify files of interest.