Automated geometric calibration and reconstruction in circular cone-beam tomography

We have developed a method of automatic geometric calibration and reconstruction for cone-beam tomography with circular orbits. The reconstruction uses the Feldkamp algorithm with small modifications to accommodate geometric system imperfections. Complete analytic calibration is performed to directly estimate the 9 geometric parameters for each scanner position, without iterative schemes or non-linear optimization techniques. All angular positions are calibrated independently using a common reference frame in the filed-of-view, defined by the calibration phantom. Our simulation studies used a 256 × 256 sq-mm detector with 512 × 512 pixels, an orbit radius of 300 mm and detector orbit radius of 150 mm. The calibration object consisted of six balls all of radius 3 mm and varying densities. All 180 projections were processed automatically, and from the estimated parameters, the best circular orbit was fit to obtain suitable reconstruction parameters. The calibration parameters consistently estimated the cone-vertex position to within 0.5 mm or better. The detector positions were found to within 1 mm and the detector normal vectors were obtained to about 0.1 degrees. The in-plane rotation of the detector was found to within 0.03 degrees. Reconstructions using the automated calibration approach produced errors far smaller than the usual circular cone-beam artefacts.