Geometric calibration is essential to counter the gantry imperfection. It is a process of projecting 3D markers to 2D and find the corresponding mapping matrices. Therefore, to achieve accurate calibration, the same marker in both 2D and 3D must be identified. However, in early reported phantom designs, most of the marker are of the same sizes. It makes the identification of the markers difficult. We proposed a size-coding approach that the markers of varying sizes are embedded into the phantom to make the marker string unique and more immune from the artifacts, such as the overlap of the markers in projection images. The experimental results showed the proposed method is simpler and effective to achieve accurate calibration that enables the cone beam CT reconstruction.

geometric calibration, size-coding markers, projection matrix