Great overlap between the arcuate fasciculus (AF) and the superior longitudinal fasciculus (SLF) II

[yutinghe99]

Dear experts,

After fixel-based analysis, I used TractSeg on my population template and tried to assign significant fixels according to the segmentation. However, I found that the AF_right greatly overlays with the SLF_II_right, and the significant fixels showed in their overlapping area. Below is the screenshot of my results, the AF_right is hot, the SLF_II_right is blue, and the green lines in them are the significant fixels (colored by directions).

I would like to know if the segmentation of the AF_right and the SLF_II_right is biologically appropriate. If it's biologically appropriate, how should I interpret the fixel results, and how should I describe the location of these fixels? within the AF_right or the SLF_II_right?

Thank you in advance for any insights.

Note: the population template is in the MNI space.

Best regards,
Yuting

[giovannividetta]

Hello @yutinghe99!

I try to answer your question having a bit knowledge of white matter (WM) neuroanatomy. I have already segmented AF and SLF in my academic carreer.

According to neuronatomy AF can be considered a part of SLF, which is defined as a complex WM system composed of several subsegments. Focusing on AF and SLF II it is normal a partial voxel overlapping due to their neuroanatomical proximity within the human brain: SLF II is localized nearby the dorsal portion of AF (references: https://doi.org/10.1038/npre.2011.5549.1; https://doi.org/10.1016/j.neuroimage.2019.116439). Morevoer, you should bear in mind that when you segment WM fascicles there are several neuroanatomical constraints (e.g.: crossing fibers) which can not be fully managed and may impact the final result. For this reason often it is challenging to clearly distinguish voxels belonging to one fascicle or another.

I hope I have helped you!

[yutinghe99]

Hello @giovannividetta ,

Thank you for your information, it's very helpful and answers my confusion.

Best wishes,
Yuting