[Freesurfer] lack of convergence when analyzing tbss-data with FreeSurfer's lme code

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Dear Freesurfer Experts

I used the FreeSurfer univariate lme_fit_FS.m on longitudinal DTI data (FA) from tbss, converted from the skeleton to a matlab matrix so as to be able to use the FreeSurfer lme code. However, the model fit converges in only 20 % of the cases, which is too low for me to be able to use these data.

I would appreciate any suggestion as to how I can rectify this situation. How do I troubleshoot this in order to increase the convergence? Can I adjust the convergence epsilon (gradient's norm) in order to increase the number of iterations, is that possible and would that help?

Best regards Knut J Bjuland PhD