Calibration method of center of rotation under the displaced detector scanning for industrial CT

For computed tomography, projection center of rotation (COR) is a significant reconstruction parameter which needs to be precisely measured before an image reconstruction. Otherwise, serious flame-shaped artifacts will arise in the CT images. In particular, when a displaced detector scanning mode is used for the inspection of a large object, the line connecting the X-ray focus and the COR is not perpendicular to the detector, making the calibration of the COR more difficult. In this paper, a new calibration method of the COR based on the symmetrical property of a sinogram is proposed. According to this method, the possible COR is enumerated among a range and the original sinogram is transformed onto a virtual detector. Then all the variances between the sum of the projection data on the left half side of the virtual detector and that on the right half side of the virtual detector are calculated. Finally, the accurate COR is determined by seeking out the minimum value of the variances. The proposed method does not need any dedicated phantom to complete the calibration, but rather directly makes use of the sinogram of the scanned sample to search the COR. Because all the projection data collected by a detector unit is summed up to perform the calibration, the algorithm is not sensitive to random noise, which has been proved by the simulated data with high-level noise and the experimental data