Geometric calibration for offset flat-panel CBCT systems using projection matrix

Abstract

Geometric parameters that define the system geometry are crucial for image reconstruction in X-ray computed tomography (CT). This paper considers the problem of determining geometric parameters for an offset flat-panel cone beam CT (CBCT) system, a recently introduced modality with a large field of view, with assumption of unstable mechanism. To find the geometric parameters for each projection view, we use projection matrix method and design a dedicated phantom that is partially visible in all projection views. The phantom consists of balls distributed symmetrically in a cylinder to ensure the use of phantom in all views. To compensate for imperfect placement of balls in phantom and the non-symmetry of ball shapes, and to prevent the errors in estimated geometric parameters when using in reconstruction, instead of decomposing the projection matrix into actual geometric parameters which are then manually corrected before using in reconstruction, we directly use the projection matrix and its pseudo inverse for projection and backprojection in reconstruction. The experiments illustrate the efficacy of the proposed method with a real offset flat-panel CBCT system in dental imaging.