Research On Method For Removing Ring Artifacts In CT Images

CT technology not only is able to show that structure relation and defect status of object interior details accurately and clearly, but also can quantitatively give the radiation density data and material composition. So it is widely applied in medical illness diagnoses and industrial defect recognition and property measurement. Because of the advantages of rapid scanning, the third generations scanning mode has become dominating in MCT and ICT fields. As one of the important problems of the third generations scanning mode, ring artifacts not only effect the image quality, but also hamper subsequent processing, such as image noise processing, image segmentation, and so on. Finally, image recognition ability and measurement accuracy is reduced. This problem is particularly outstanding in ICT imaging system. Therefore, ring artifact removing is the key step to pre-correction of CT images.According to the characteristics that the ring artifacts of CT image show vertical directional lines in the sinogram of CT images, two kinds of ring artifact correction algorithms were proposed based on the sinogram.The first ring artifact correction method was based on projection profile high-pass filter. Firstly, the S-L filter was adopted to filter original projection data and enhance artifact information. Secondly, the filtered projection data was processed by projection integral, and then the projection profile was processed by differential to further increase the difference between artifacts edge data and work piece contour data. Thirdly, the differenced projection data was mean filtered according to the high-frequency sampling interpolation times, and artifacts'location was automatically searched according to threshold selected by gross error criterion. Finally, through analyzing the reasons why the ring artifacts could not be corrected by traditional linear interpolation, the linear extrapolation interpolation was proposed to correction the projection data of ring artifacts.The second ring artifact correction method was based on the improved Canny algorithm. Firstly, the S-L filter was adopted to filter original projection data and enhance artifact information before using Gaussian filter. Then the gradient was acquired by the filter, which composed by 2d first-order partial derivative to reduce the noise affection. Thirdly, the angle thresholds are established for limiting gradient direction, particularly excluding diagonal direction edge detection and achieve vertical direction edge detection. Fourthly, a gradient threshold and an angle threshold were used to detect and connect marginal point of the artifacts. Finally, through analyzing the problems that the traditional linear interpolation algorithm is employed to correct the projection data, the subsection B-spline fitting method is proposed to implement multiple continuous ring artifacts correction.Based on the actual CT images calibration experiment, the result indicates that the two proposed correction algorithms is able to remove the ring artifacts effectively and keep image edges and resolution unchanged. On one hand, the first correction algorithm is simple, and effective to remove single dispersed ring artifacts. But it is limited for multiple continuous ring artifacts. On the other hand, the second correction algorithm is able to reduce the multiple continuous ring artifacts, yet not effective to detect the ring artifacts which the maximum connecting length is smaller than chain length threshold. The two proposed methods have different characteristics, complement each other in some aspects,but can not totally replace.