We are currently doing longitudinal FreeSurfer analysis on subjects some of whom are scanned over a period of 6 years. There were 2 groups. One was the control group and the other was mTBI group. The hypothesis was the control group would have a fairly stable cortical and sub-cortical volume (<1% change over time) while mTBI group would have a decreasing volume. Both the groups are relatively young (most participants between 22-30 years old). 

Unfortunately, we only have 7 controls that were scanned for over 3 years. What we have observed is there is no pattern in change in volume neither for controls nor for mTBI; with volume increasing from baseline to year1 and then decreasing and then for some increasing again. The delineation looks ok to us. There are a lot of subjects ~120 and hence manual delineation on every subject is not practical.

The question is what is the expected measurement variance due to the longitudinal algorithm for healthy controls over time in both cortical and subcortical volumes?

Since the effect that we were expecting to see is around 4% in mTBI participants and the delineation too looks okay then what is the best procedure to correct these? OR should we use cross-sectional data for our analysis?

We have used longitudinal freesurfer v6 on a centos.

I look forward to your thoughts.

Thanks

Regards

Virendra