Fig. 2.

Entry point search algorithm. Critical structures are arteries (red), veins (cyan), and sulci (not shown). The lowest risk trajectory is displayed in purple (arrow indicating entry and sphere indicating target). a Initial set of potential entry points ${\widehat{E}}_{i,a_i}$ obtained from the skull template (semi-transparent white). b Opaque white patch corresponds to ${\widehat{E}}_{i,a_i}$ after excluding trajectories due to length or angle. c Opaque white patch corresponds to ${\widehat{E}}_{i,a_i}$ after excluding trajectories due to critical structure intersection. d Coloured patch corresponds to risk scores $R_{i,a_i}$ with low (0—green) to high (1—red) risk