Fig. 4.

Visual definitions for the five principal features. a Principal feature 1 and 4: branch points and distance from an end point to its nearest point on the boundary. b Principal feature 2: vessel thickness, which is the same as the diameter (d _i) of the i ^th vessel tube. c Principal feature 3: tortuosity, which is the ratio of arc length and straight line distance between the initial and terminal nodes of the vessel segment, i.e., tortuosity of the i ^th vessel = c _i/d _i. d Principal feature 5: branch angle, which is given by the angle between the two line segments formed by connecting the initial node and the fourth pixel of each branch