Disclosed is a fuel tank system capable of suppressing stagnation of fuel between check valves arranged in a fuel filling path. A substrate is provided that is constructed and arranged to receive printing thereon. The reference instrument preferably is located intraoperatively in a relatively constrained anatomical environment, such as in the coronary sinus of the heart, to prevent significant movement. Two pull points may further be arranged so that the application of oppositely directed forces to the first and second pull-points will lead to unfolding of the inner folding. When multiple successive frames are determined as having invalid backgrounds, the camera represents an invalid camera.