Thereby all voxels which are lying inside the tetrahedron are counted, the majority defines the tissue class of this element, the rest, the error volume, is a measure of its quality. The strategie of mesh refinement is to place new node points inside the element with the worst quality. The exact position can be chosen by ignoring the tissue distribution within the element eg the center of the tetrahedron, the center of the innersphere or the middle of the longest edge. Using information about the tissue distribution can result in better positions for node points. This can be done by walking along a given line and detecting the first material change.
Good rays are half edges starting at the middle of edges and walking to the vertices.
Another possibility is to start at the center of an element and search on the lines connected with the vertices. Figure 6 shows the course of the error volume and the quality key of the worst tetrahedron during the presented adaptive mesh generation. With the increasing number of node points, the error volume is decreasing and the quality key is increasing. With a growing number of node points, the quality of the worst element of a FE model, created with the discribed algorithm, is increasing, the error volume is decreasing. Respectation of Boundaries Beside the presented method to get new node points, another possibility to meet the anatomical structures of the human body is to define tissue boundaries which have to be respected.