Figure 2.

Flowchart of the surface-based analysis procedure. Manually segmented labels (A) are first minimally smoothed (B) and then converted to surface meshes for which a spherical parameterization (SPHARM) is computed using area-preserving, distortion-minimizing mapping (C). Based on a uniform icosahedron-triangulation of the SPHARM, we obtain a point distribution model (PDM) (D), allowing shape-inherent point (= vertices) correspondences across subjects (E) (top). Individual SPHARM-PDM surfaces are rigidly aligned to the template shape (E) (bottom). Vertex-wise displacement vectors are then calculated between each individual and the template (F) (top). Displacement vectors are diffused to voxels (F) (top) enclosed by the surface using a heat equation, yielding voxelwise vector field. To assess local volume changes, we calculate jacobian determinants (J) from this vector field. By projecting this metric back onto the surface using tri-linear interpolation, we finally obtain a metric of growth (J > 0) or shrinkage (J < 0) at each vertex (F) (bottom).