Dear FSL,
We have been doing DTI analysis using TBSS successfully for some time
now. Our cohort consists of normally developing adolescents.
As a small example of our analysis consider a sample of FA images from
six subjects (we have many more, but this is for simplicity), ages
12-17. I do a "within group" design in which the log of age is the
regressor. The resulting design.mat and design.con files are as
follows:
**************
design.mat
**************
/NumWaves 1
/NumPoints 6
/PPheights 1
/Matrix
-0.145
-0.145
-0.065
0.009
0.143
0.203
*************
design.con
*************
/NumWaves 1
/NumContrasts 1
/PPheights 1
/Matrix
1
-1
The values in design.mat are the demeaned natural log of the subjects' ages.
So far, so good.
Now fast forward a couple of years....This is actually a longitudinal
study, with each subject scanned on an annual basis. As is common in
longitudinal studies, not every subject is scanned every year, for a
variety of reasons.
If we consider only the first year scans as we are doing currently,
each subject can be treated as an independent sample. However, now
some subjects have had three scans, some have two, and a few didn't
make it past the first year.
Is there a way to set up a model in randomise where we can include
multiple DTI scans from the same subject? This would violate the
assumption of independence in the simple model, but we're looking for
a way to account for the within-subject correlation in a mixed-model
design.
Many thanks,
Robert Terwilliger
Laboratory of Neurocognitive Development
University of Pittsburgh
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