Study on Data Readout and Smear Correction of Full-frame CCD Detector for CT Scanning

Abstract

X-ray detectors made by optical fiber coupled with full-frame CCD are widely used in highprecision X-ray CT scanning of laboratory. Because of the strong penetration of Xrays, the conventional shutter can’t realize ray shielding, so the fullframe CCD detector usually works in the shutterfree mode which can’t avoid generating image smears. Due to image smears, the image observation is seriously affected and the dynamic range of effective data of the CCD detector is reduced. The smears of projection images of fullframe CCD detector must be corrected. In order to correct smears of projection images in CT scanning application, the characteristics of image smear of fullframe CDD in standard readout mode and charge flush readout mode were studied in this paper. And the applicability of the widely used smear correction method using line pixels data of the optical black region to the smear correction of Xray radiation imaging was studied too. It is found that the application of dark pixels correction method to correct the radiation projection image may bring thin strip artifacts. The results come from two main reasons. Firstly, the fluctuation of pixels data can’t be effectively suppressed by calculating the mean value, because the number of lines of pixels data of the optical black region is too small. Secondly, the fluctuation of line pixels data of the optical black region will increase due to the radiation interference during integration. An improved smear correction method using nonexposed line pixels data was proposed in this paper to avoid the two shortcomings above. Using this method, the image smear is corrected by using the nonexposed line pixels data which aren’t exist during integration, so the radiation interference during integration is well solved. In addition, the stability of correction data is improved by calculating the mean value of more nonexposed line pixels data because the number of nonexposed line pixels data can be customized on demand. Further study of dynamic exposure CT scanning imaging shows that the correlation between the smear charges and the exposure charges of the readout image has changed due to the change of the position of the measured sample during exposure and image readout. Therefore, the image smears of dynamic exposure can’t be suppressed by conventional image correction. In order to solve the problem of correcting the smear in dynamic exposure imaging, a ROI fast readout mode based on radiation shielding was proposed, which can effectively suppress the generation of image smear by shortening the transfer time of the integral charge. The experimental results show that the smears of DR image are effectively suppressed, and the quality of the projection image is improved, when applying smear correction method proposed in this paper to static exposure CT scanning. And when applying the ROI fast readout mode proposed in this paper to dynamic exposure CT scanning, the smears of DR image are suppressed, the dynamic range of effective data of the detector is improved, and the problem of image smear correction for dynamic exposure imaging is effectively solved.

Keywords

• full-frame ccd
• non-exposed line pixels data
• region of interest
• interline transfer smear