Hi MRtrix’ers,

Following the paper of Genc et al. (2018), we performed a longitudinal FBA comparing an intervention with a control group. In this analysis, the population template is generated in a two-step approach: first affine registration to the intra-subject template followed by a nonlinear registration from the intrasubject to the group template. To assess longitudinal changes in FD, log-FC and FDC within and between both groups, we subtracted each time-point 1-image from the time-point 2 and time-point 3 images and divided this difference images by the time interval (in weeks), covarying for age and TIV.

We found a significant effect of logFC in 3 fixels in the mPFC (pFWE<0.05).

When looking at the absolute effect sizes (mrstats abs_effect.mif -mask fwe_signmask.mif), you can appreciate that they are very small:

volume mean median std min max count

[ 0 ] 2.06935e-08 1.97287e-08 3.17442e-09 1.81134e-08 2.42384e-08 3

However, the standard effect is much bigger:

volume mean median std min max count

[ 0 ] 1.84473 1.89786 0.153643 1.67158 1.96476 3

If we calculate the percentage effect as described on Expressing the effect size relative to controls — MRtrix 3.0 documentation we get the following result:

volume mean median std min max count

[ 0 ] 0 0 0 0 0 3

We’re not sure how to explain these null findings, while we did find a significant difference between both groups using fixelcfestats.

Thanks in advance!

Best,

Michelle and @JeroenBlommaert