CT scan of the robot. Note the lack of artifacts that would be induced by materials with a high atomic number and which would show up as dense white streaks on the images. The end effector is positioned at the surface of a gel phantom (a,b). An ellipse, a line, and four balls are embedded on the end effector of the robot as registration markers. They can be seen clearly on the CBCT images (a,b,c); they are also visible under MR (d).