WE‐G‐207‐09: A Practical Bowtie Ring Artifact Correction Algorithm for Cone‐Beam CT

Abstract

Purpose:Shaded ring artifact in cone‐beam CT (CBCT) is caused by scatter contamination from bowtie‐modulated primary beam and presents a unique ring pattern in CBCT image. Previous shading correction methods, which suppress the artifact as conventional scatter correction in projection domain, are complicated due to the non‐trivial scatter estimation. In this work, we propose a practical and readily implementable algorithm to correct for the severe shaded ring in CBCT without relying on prior information and directly in image domain.Methods:Due to the correlation between the ring pattern and bowtie modulator, an initial bowtie mask was reconstructed from bowtie‐modulated air‐scan projections as if they were acquired from a flat‐field exposure penetrating the bowtie modulator. The shape of the bowtie mask matches well with that of the shaded ring in CBCT, while its intensity needs to be scaled to fully compensate for the shading ring. To find a correct scaling factor, we start from the anatomical knowledge that the same tissue has comparable CT number. This knowledge indicates a sharp peak in histogram of that specific tissue. One way to achieve this goal is to maximize the peak value of the histogram of that tissue. The above concept is formulated as a mathematical optimization problem which is solved using a standard Simplex method. The shaded ring artifact in CBCT is finally corrected for by adding a scaled bowtie mask.Results:The proposed method is evaluated on one pelvis patient. Severe shaded ring artifact is greatly suppressed. Our method reduces the CT number error from >200 HU to be ∼50 HU, and increases the spatial uniformity by 1.3 times.Conclusion:We propose a practical image‐domain algorithm for shaded ring artifact correction in CBCT. It is computationally efficient and does not rely on prior knowledge. It is thus attractive for clinical use.This work is supported by the National Science Foundation of China (NSFC Grant No. 81201091), National High Technology Research and Development Program of China (863 program, Grant No. 2015AA020917), and Fund Project for Excellent Abroad Scholar Personnel in Science and Technology.