A second problem we needed to resolve was how to correct the existing defects and obtain an accurate surface model. An example of a defect localized with this procedure is given in Fig. 4, left, as well as two possible corrections show in the center (cutting the handle) and right (filling the hole). In this case it turns out that the right-hand correction leads to a geometrically accurate surface, while the center one eliminates some of the cortex from being modeled. 1 Note that it is impossible to tell from the rendering of the surface models which is the correct solution. It is only by considering the intensity volume—that is that the surface should contain white matter in the interior and gray matter outside—that one is able to choose the correct topological manipulation to generate an accurate surface (see Fig. 5 for an example of what the MRI volume looks like after filling a hole).
