I. Introduction
The topological properties of 2-D and 3-D objects are often very simple, regardless of their geometric complexity. Human anatomy generally follows this rule, even for extremely convoluted shapes like the cerebral cortex or the vasculature. It also imposes strict spatial relationships. For example, the brain is enclosed inside the skull, and the cerebellum and cerebrum are neighboring but separated organs, linked by the brainstem. These relationships can be largely captured by topology, by considering all the related structures simultaneously.