In this paper we propose a method to obtain a tetrahedral model of the human head by labeling elements of a tetrahedral mesh. To work with meshes as regular as possible, we use the notion of Almost Regular Tesselation (ART) providing tetrahedral meshes with good quality elements. The proposed labeling method uses segmented M.R.I. containing main tissues of the head as input. The labeling is done under topological constraints in order to preserve topological arrangement of the head tissues. This process uses a notion of simple tetrahedra.