Dear experts
I have a couple of questions regarding multitissue FBA pipeline which I would greatly appreciate if you kindly comment on.

Should I use the cutoff value of point 0.08 in tckgen (step 19) in case I increase the fmls_peak_value to 0.08 in fod2fixel (step 12)? I guess the answer is “yes” but just want to be sure.

In a life span study (2075 years of age), should I use intracranial volume (ICV) as a covariate of no interest when analyzing FD/logFC/FDC?

Do these design (shortened version) and contrast look fine if I consider only younger and older groups and want to test whether there is any fixel showing an interaction group x performance?
Design1:
GR1 GR2 GR1per GR2per
1 0 a1 0
1 0 a2 0
1 0 a3 0
0 1 0 b1
0 1 0 b2
where a1, a2,a3, b1, b2 are demeaned values “across both groups”. >> actually not sure if this is correct and when should be otherwise (i.e., within groups demeaning).
contrast 1:
[0 0 1 1] >> add neg to the fixelcfestats to compute [0 0 1 1] as well.
And in case of adding ICV:
Design 2:
GR1 GR2 GR1per GR2per ICV
1 0 a1 0 c1
1 0 a2 0 c2
1 0 a3 0 c3
0 1 0 b1 c4
0 1 0 b2 c5
contrast 2:
[0 0 1 1 0] >> add neg to the fixelcfestats to compute [0 0 1 1 0] as well.
Where a1, a2 , b1, b2, c1, c2,…,c5 are again demeaned values “across both groups”.
In the above situations, it would be nicer not to include a column of 1s. Right?
Cheers,
Hamed