Surface-based morphometry

Surface-based morphometry (SBM) involves the creation of a surface representation (i.e. a mesh) of structural boundaries defined by or on the basis of the segmentation of a brain. This does not require registering the individual brain images to a template brain, though comparisons across brains demand a reference surface that belongs to the same topological genus (i.e. 0) and is normalized in size. The brains are thus mapped to a unit sphere on which their original properties can be compared with each other, and results are mapped back to a reference brain surface.

The surfaces most appropriate for cortical analyses are the boundaries between WM and GM or between GM and CSF (the latter is also often referred to as pial surface, since the pia mater is not commonly segmented into a class of its own) but various representations of the so-called central surface (roughly corresponding to the anatomical lamina IV) are also in use. For some subcortical structures (e.g. the hippocampus or basal ganglia), appropriate surfaces can be defined in a similar way, while lateral delineation of the corpus callosum, for instance, is difficult.

Statistical analyses in SBM are based on properties of the individual mesh elements and aggregations thereof. These latter ones include, foremostly, some measure of the distance between different surfaces-- typically the cortical thickness-- or sulcal depth but also some local or global measures of area, curvature (e.g. gyrification) or overall shape (e.g. via spherical wavelets, spherical harmonics or Laplace-Beltrami spectra).