T1 weighted images were corrected for distortions caused by gradient nonlinearity and then submitted to the standard longitudinal processing pipeline in the FreeSurfer software suite (v4.5.0, http://surfer.nmr.mgh.harvard.edu). Procedures are described in detail in publications by the FreeSurfer group (44-57). Briefly, to extract reliable volume and thickness estimates across repeated MRI measurements, T1 weighted images were processed longitudinally in FreeSurfer (54), beginning with the construction of an unbiased within-subject template space and image using robust, inverse consistent registration (56). Several processing steps in the standard FreeSurfer pipeline, such as skull stripping, Talairach transforms, atlas registration as well as spherical surface maps and parcellations are then initialized with common information from the within-subject template, significantly increasing reliability and statistical power (54).
