Computational protocol: The sexually dimorphic impact of maltreatment on cortical thickness, surface area and gyrification

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Protocol publication

[…] All T1-weighted images were initially manually inspected for any deformations or inconsistencies that may impede its processing such as movement artefacts or structural abnormalities. If the image was thought to represent poor quality or had any ostensible deformations, the participant was excluded. From an initial recruitment of 137 participants, a total of 15 participants (MT = 8, non-MT = 7) were excluded from the analysis due to concerns over image quality, resulting in a final sample of 122 (Table ).Cortical reconstruction was performed with the FreeSurfer image analysis suite (Dale et al. ; Fischl and Dale ; Fischl et al. , ). In brief, the initial steps in this well-validated (Dale et al. ; Fischl et al. ; Ségonne et al. ) surface based morphometric pipeline were as follows. Removal of non-brain tissue using a hybrid watershed/surface deformation procedure (Ségonne et al. ), automated Talairach transformation, intensity normalization (Sled et al. ), tessellation of the gray matter white matter boundary, automated topology correction (Fischl et al. ; Ségonne et al. ), and surface deformation following intensity gradients to optimally place the gray/white and gray/cerebrospinal fluid borders at the location where the greatest shift in intensity defines the transition to the other tissue class (Dale et al. ; Dale and Sereno ; Fischl and Dale ). The structural measures were calculated in native space of each participant and transformed into a spherical representation which are registered to a common spherical atlas. The atlas is based on individual cortical folding patterns to match cortical geometry across subjects, preserving the vertex identities (Fischl et al. ). All participant’s surface models were inspected for accuracy and manual edits were made when there were inconsistencies in the differentiation between pial surface and other organic matter, including dura and bone.Cortical thickness at each vertex was measured by calculating the shortest distance from the gray/white matter boundary to the pial surface (in millimetres). Surface area measurement was quantified by assigning an area to each vertex equal to the average of its surrounding triangles. When the vertex areas are summed over all vertices, the total is equal to the sum of the areas of the triangles. The surface area was calculated at the pial level and represents the area of vertex on the gray matter surface, calculated as the average of the area of the tessellated triangles touching that vertex. Parcellation of each participant’s cortex into gyral regions was based on the Desiken-Killiany atlas (Desikan et al. ). Surface area was not analysed at the vertex-level as the values provided by the standard FreeSurfer pipeline do not provide an accurate measure of surface area, rather it indicates a measure of areal expansion to the study template. The average surface area value for each parcellated region provides an accurate measure of surface are and was extracted for all participants.The local gyrification index (lGI) is a supplementary measure incorporated within the FreeSurfer image analysis suite which takes into account the intrinsic 3D nature compared with 2D methods (Schaer et al. ). The lGI method uses the pial and white matter surface identification against an additional outer hull layer that tightly wraps the pial surface. The lGI value at each vertex is computed within 25 mm circular regions of interest and represents the ratio of pial surface to outer hull surface, an indication of sulcal cortex buried in its locality and thus the extent of cortical folding. See Schaer and colleagues (Schaer et al. ) for further details of this analytic approach. Vertex-level cortical thickness, and local gyrification index for each participant were mapped onto a normalised cortical surface to perform group analysis and to investigate the interaction between sex and maltreatment exposure. Analyses on extracted cortical surface area values were performed within SPSS v.20 (IBM, Armonk, NY). […]

Pipeline specifications

Software tools FreeSurfer, SPSS
Applications Miscellaneous, Magnetic resonance imaging
Organisms Homo sapiens
Diseases Muscular Diseases