Cortical Thickness Measurement with MRI

Procedures for MRI scanning, scan processing, thickness maintenance measurement, and measurement error control and assessment have been described in detail [8 (link),9 (link)]. Salient points are as follows. T1-weighted scans of the entire brain were made with a 3T GE Signa scanner (164 continuous axial slices, voxel size 1 mm × 1 mm × 1 mm). All scans were made with the same scanner, head coil, and scan parameters. Regular quality assurance tests during the study identified no problems, and scanner upgrades were not done. Scan checks ruled out motion and other artifacts. Scan processing was done with automated FreeSurfer procedures (http://surfer.nmr.mgh.harvard.edu; accessed on 16 January 2023). To treat data from all scans as equal and independent measures, each scan was processed individually without cross-scan registration or averaging. Thickness measures were taken in native space without transformation to a template. Cortical thickness was defined at ≈150,000 vertex locations/hemisphere, and mean hemispheric cortical thickness (mm) was determined for each hemisphere using all vertex measures. To ensure uniform processing, all scans were processed at one time, after collection of all data, using one workstation, operating system, and FreeSurfer (version 4.5.1) program. Cortical borders were visually checked and judged to not require manual correction.