Artifacts correction method for fan-beam CT with projections asymmetrically truncated on both sides

In fan-beam CT scanning, the scanned field of view is determined by the fan-beam angle and the effective length of a linear array detector. Therefore, when an object extends outside the scanned field of view, CT projection data acquired by the detector will not be complete and is truncated abruptly at the projection boundaries. Furthermore, due to mechanical misalignment of X-ray source, object and the linear array detector, the length of the truncated data on the left side is not equal to its length on the right side. This asymmetrical truncation on both sides will bring out dual bright-band artifacts in the reconstructed images. The purpose of this paper is to develop a method of eliminating these artifacts by projection extension techniques. We first extend the truncated projection unilaterally by using geometrical symmetry property of the fan-beam scanning. Through this method, the projection center of rotation is adjusted to the center of a virtual sinogram and the region of completely scanned field of view is enlarged. The projection values in the extended region equal their equivalent points in the raw sinogram. Thus, values of the supplemented projection data are not approximate and would not decrease the reconstruction accuracy. Next, “mirror extension” technique is preformed to avoid data discontinuity at the edges of a virtual sinogram. Extra data are supplemented to make the projection data on both boundaries attenuate to zero smoothly. Experimental results demonstrated that the dual bright-band artifacts were well eliminated and the correction method could be implemented within the convolution step of a filtered back-projection reconstruction with negligible computational expense.