Distortion correction, geometric calibration, and volume reconstruction for an isocentric C-Arm X-Ray system

In order to facilitate cone-beam CT using a mobile x-ray C-arm, accurate geometric calibration must be carried out. Such calibration requires a precise correction of x-ray image intensifier (II) distortion. Regional distortion correction (DC) was performed for the x-ray II using a planar grid phantom. The DC reduced the RMS error of known locations (or points) spanning the II field of view from 1.71 pixels to 0.63 pixels. For each projection, a six-ball calibration phantom was then used to determine the behavior of 9 geometric-calibration parameters during a 360° scan of the phantom, using a direct calibration method in the literature. These parameters were subsequently used in a modified Feldkamp (FDK) filtered back projection algorithm to achieve 3D volume reconstruction. Without the correction of II distortion the geometric calibration parameters showed the addition of incorrect structured behavior attributed to distortion while the reconstructed image was observed to be noisier when compared to the calibration and reconstruction when DC was incorporated. Further, a control was implemented using a distortion free digital flat panel detector. The reconstruction using the control was seen to have improved quality but was comparable to the quality of the distortion corrected x-ray II reconstruction.