Calibration of Bi-planar Radiography with a Rangefinder and a Small Calibration Object

In this paper we propose a method for geometrical calibration of bi-planar radiography that aims at minimising the impact of calibration objects on the content of radiographs, while remaining affordable. For accomplishing this goal, we propose a small extension to conventional imaging systems: a low-cost rangefinder that enables to estimate some of the geometrical parameters. Even with this extension, a small calibration object is needed for correcting scale. The proposed method was tested on 17 pairs of radiographs of a phantom object of known dimensions. For calculating scale, only a reference distance of 40mm was used. Results show a RMS error of 0.36mm with 99% of the errors inferior to 0.85mm. We conclude that the method presented here performs robustly and achieves sub-millimetric accuracy, while remaining affordable and requiring only two radiopaque marks visible in radiographs.