This analysis focused on major lobar tissue regions of the cortex, allocortex, and subcortical structures. Accordingly, the 116 ROIs available from the SRI24 atlas, derived from the Automated Anatomical Labelling (AAL) template (Tzourio-Mazoyer et al., 2002) were collapsed into 15 bilateral ROIs (Figure 2). The decision to collapse the 116 ROIs into 15 was motivated by considering the proportion of ROIs (N=15) to subjects (N=28) and confirmed by the use of these 15 ROIs in our studies of cerebrovascular perfusion (Pfefferbaum et al., 2011; Pfefferbaum et al., 2010). A further consideration was the desire to have anatomically-driven tissue subregions, and not simply gross lobar volumes, that were of similar size for statistical comparability. For each subject and time, gray matter volume was computed for each cortical region, and tissue volume for each subcortical region. Also measured were CSF-filled volumes of the lateral ventricles, third ventricle, temporal horns, cortical sulci, and sylvian fissures, based on ROIs drawn also on the SRI24 SPGR template (Figure 3) and reformatted into subject image space. The ventricles were identified by registration, and for the cortical sulci and sylvian fissure, the CSF segmentations from FAST were multiplied against the ROIs in Figure 3.
