Hi Christian,

you are interested in the longitudinal change in your study group. The problem of course is that you are missing a longitudinal control group. Control groups are so important.

With your data you can analyze if there is change in your study group (using the 2 stage model, since time interval seems to be approximately equal across subjects and all have 2 time points). You will find some changes in some areas, but it will be unclear if that is noise (from outliers), regular aging, hydration, motion etc. You'd have to compare to a longitudinal control group to test if the changes in the study group are different from the changes that happen in controls.

Since you are looking for small changes (4 month) cross sectional analysis will probably not help either (because of the inter subject variability). Comparing time point 1 with controls and comparing time point 2 with controls is not great, you could find a difference in one of the time points (or in both) with respect to the controls but maybe that is because of subject selection (gender, age or whatever) (+- noise to move it in and out of significance).

To answer your question, since you already computed the rate (or percent change) maps (i.e. the first stage of the model), you simply run a qdec analysis on that (instead of e.g. thickness) using a cross sectional qdec table with one line per subject (2nd stage). This is described here:

http://surfer.nmr.mgh.harvard.edu/fswiki/LongitudinalTwoStageModel http://surfer.nmr.mgh.harvard.edu/fswiki/FsTutorial/LongitudinalTutorial

Best, Martin

On 06/05/2014 02:49 AM, Rummel, Christian wrote:

Dear FreeSurfers,

we have MR scans from a group of healthy subjects (about 40, equal

number of males and females, age range 25 to 65 years), who were scanned

at two time points (about four months separation). In between they

experienced exposure to a condition that might or might not influence

cortical thickness or other morphometric measures. From other studies we

have a large number of healthy controls in the same age range. The

difference is that they were scanned only once and were not exposed to

any special condition.

We want to find out where cortical thickness changes significantly

stronger in our subjects than expected from the controls for normal

aging. We know how to do that for the averages over cortical

parcellations and sub-cortical segmentations as reported in the

aseg.stats and aparc.stats.

Is it possible to use qdec to do the same thing vertex-wise? The

symmetrized percent changes have been calculated already from

FreeSurfer's longitudinal stream (long_mris_slopes). How can we feed the

information about the locally expected change rate (and maybe even its

variability) into qdec?

Thanks,

Christian Rummel

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Christian Rummel (PhD)

Support Center for Advanced Neuroimaging (SCAN)

University Institute for Diagnostic and Interventional Neuroradiology

Inselspital, 3010 Bern, Switzerland

Tel. 0041 31 6328038

Fax 0041 31 6324872

crummel@web.de

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