3-D correction of ring and radiant artifacts in flat panel detector based cone beam volume CT imaging

Abstract

The use of °at-panel detectors (FPDs) is becoming increasingly popular in cone beam volume and multi-slice CT imaging. But due to the presence of the defective and insu±cient calibrated detector elements in the FPD, the diagnostic quality of the FPD-based CT images in both CT systems is degraded by ring and radiant artifacts. In this thesis, a detail analysis including the classi¯cation, detection and correction of these ring artifacts is presented. Unlike conventional approaches, we emphasize here on the separate detection and correction schemes for each type of rings for their e®ective removal. Two di®erent ring and radiant artifact correction algorithms are presented here: one is applicable for the fan or parallel beam geometry based multi-slice CT and the other for the cone beam geometry based CT. For the correction of the ring artifacts in multi-slice CT, an artifact strength based sinogram-processing method is presented and a mathematical index is used to isolate the defective and mis-calibrated pixels from the good ones. For the correction of artifacts resulting from the defective detector elements, 2D variable window moving average and weighted moving average ¯lters are proposed. On the other hand, the normalization correction scheme is adopted to correct the responses of the mis- calibrated detector elements. Next, a novel approach is presented to eliminate the ring and radiant artifacts of a 3-D cone beam volume CT image using the proposed sinogram based ring artifact removal method. Finally, an e®ective and fast ring artifact correction technique speci¯cally for 3-D cone beam volume CT image is proposed. Some template images are derived from the responses of the detector pixels using their statistical properties and then, an e®ective non-causal derivative based detection algorithm in 2-D space is presented for the detection of defective and mis- calibrated detector elements separately. An image inpainting based 3-D correction scheme is proposed for the estimation of responses of defective pixels and the responses of the mis-calibrated pixels are corrected using the normalization technique. A simpli¯cation of the proposed method is also suggested for its real time implementation. The overall experimental results demonstrate the superiority of the proposed two correction methods (applicable for the multi-slice and cone beam CTs) over the other methods reported in the literature.