Abstract-In this paper, we focus on the retrospective topology correction of surfaces. We propose a technique to accurately correct the spherical topology of cortical surfaces. Specifically, we construct a mapping from the original surface onto the sphere to detect topological defects as minimal nonhomeomorphic regions. The topology of each defect is then corrected by opening and sealing the surface along a set of nonseparating loops that are selected in a Bayesian framework. The proposed method is a wholly self-contained topology correction algorithm, which determines geometrically accurate, topologically correct solutions based on the magnetic resonance imaging (MRI) intensity profile and the expected local curvature. Applied to real data, our method provides topological corrections similar to those made by a trained operator.Index Terms-Homotopy, human cerebral cortex, nonseparating loop, Reeb graph, segmentation, topology.
I. THE CORTICAL RECONSTRUCTION PROBLEMT HE human cerebral cortex is a highly folded ribbon of gray matter (GM) that lies inside the cerebrospinal fluid (CSF) and outside the white matter (WM) of the brain. Locally, its intrinsic "unfolded" structure constitutes a 2-D sheet, which is several millimeters thick. In the absence of pathology and assuming that the midline hemispheric connections are artificially closed, each cortical hemisphere can be viewed as a simply connected 2-D sheet of neurons that carries the simple topology 1 of a sphere 2 [ Fig. 1 2 The true topology of the gray/white surface is not one of a sphere, as a result of the midline connections between the left and right hemisphere, such as the anterior and the posterior commisures. Fig. 1. (a) The human cerebral cortex is a highly folded ribbon of GM that lies inside the cerebrospinal build and outside the WM of the brain. The green surface represents the interface between WM and GM, and the red surface (i.e., the pial surface) models the interface between GM and CSF. When the midline connections between the left and right hemisphere are artificially closed, these two surfaces have the topology of a sphere. (b) Three-dimensional rendering of the highly folded pial surface. Opposite regions of a sulcus are often contiguous. (c) As a result of the partial volume effect, it is difficult to distinguish opposite banks of the GM from one another. (d) Segmentation algorithms that do not constrain the topology often create cortical segmentations with certain topological defects (i.e., handles). Close-up of a topologically incorrect gray/white surface representation.Recently, there has been a research focus on the extraction of accurate and topologically correct models of the brain surface. Many recent segmentation algorithms for neuroimaging data are able to identify and precisely locate diverse brain structures, although typically without ensuring the validity of the final topology (i.e., that of a sphere). Magnetic resonance images (MRIs) often contain various artifacts (e.g., image noise, image intensity inhomogeneity or nonunifo...