Removal Of Ring Artifacts In Cone-beam CT Imaging Based On The Projections

Image-Guiding technology is an important part of precise radiotherapy.In order to reduce the set-up error,Cone-Beam computed tomography(CBCT)is often used to scan the patients,and the CBCT images are matched with planning CT(pCT)images before irradiation.Due to short-circuit,open-circuit and inaccurate response,bad pixels maybe exist in the flat-panel detector,subsequently result in ring artifacts in the reconstructed CBCT images.Ring artifacts affect the CBCT image quality,thus the accuracy of image registration is seriously affected,which may cause large difficulties in patient set-up correction.It is necessary to correct the bad pixels in image reconstruction.In order to remove the ring artifacts in the CBCT images,the interpolation based on the template method,the morphological method and the adaptive median filter method were used to correct the bad pixels in the raw projections.The interpolation based on the template method uses the detector output under different exposures to identify and mark the position of the bad pixels,and then interpolating the bad pixels in the raw projections according to the bad pixel map;The morphological method is based on the open operation and the close operation to correct the abnormal pixels in the raw projections;The adaptive median filter method is filtering based on the local information characteristics of the raw projections.The corrections were performed for Catphan phantom,head and neck patient and thorax and abdomen patient to validate these methods.The corrected projections were reconstructed and compared with the results of the correction of ring artifacts based on the reconstructed CBCT images.The three methods could correct the bad pixels in the raw projections and remove the ring artifacts in the CBCT images effectively.In the view of the correction effect,the interpolation based on the template method was superior to the morphological method and the adaptive median filter method.From the image resolution,the interpolation based on the template method and the adaptive median filter method was better than the morphological method;To the efficiency of the correction,the morphological method(0.108 s/frame)was superior to the interpolation based on the template method(0.185s/frame)and the adaptive median filter method(0.95 s/frame).Compared with the results of ring artifacts corrected based on the reconstructed CBCT images,the correction based on the raw projections was better,the area of correction has higher resolution and the gray level was continuous.According to the demand of the clinical application,the correction based on the post-reconstruction required additional correction time,and the correction based on the pre-reconstruction could be operated during the scanning,especially the interpolation based on the template method and the morphological method could correct the raw projections without increasing the time of the image-guiding process.The correction based on the raw projections could remove the ring artifacts in the CBCT image effectively.The effect of correction is good and has some clinical application value.