Example of constructing 3D network graph by adding one tract at a time to the existing graph. There are four possible cases of connecting the two end points (indexed 5 and 6) of the tract to the existing graph with nodes {1,2,3,4}. Depending on the proximity of {5, 6}to nodes {1, 2,3,4}, we either do nothing, or add connect one node (either 5 or 6) or add two nodes (both 5 and 6) to the graph.