[Freesurfer] correlations with longitudinal data

Dear FreeSurfer experts,

I have a subsample of my longitudinal data with 3 scans per subjects fully preprocessed and would like to start with some simple correlations of thickness at time 1 with behavioral measures and other factors like age.

Is there a way to do that with LME (since I was planning on using that as a method of choice) or is it possible to do that with QDEC? I've prepared my data for both methods (LME and linear two stage model) but if I use the cross.qdec.table derived from the longitudinal data, it uses the registered and smoothed (with mris_slopes) surfaces from the Base, right? So, I would like to know how I can analyzse a single time point within my longitudinal design.

Cheers, Clara