Metal artifact reduction in x-ray computed tomography (CT) by constrained optimization

• Published in: Medical physics (Lancaster) Vol. 38; no. 2; pp. 701 - 711
• Main Authors: Zhang, Xiaomeng, Wang, Jing, Xing, Lei
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01.02.2011
• Subjects: ALGORITHMS; Artifacts and distortion; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Computed tomography; computerised tomography; COMPUTERIZED TOMOGRAPHY; gradient methods; IMAGE PROCESSING; Image Processing, Computer-Assisted - methods; image reconstruction; image segmentation; Interpolation; ITERATIVE METHODS; Medical image artifacts; Medical image noise; medical image processing; Medical image reconstruction; Medical imaging; Medical X‐ray imaging; metal artifact; METALS; minimisation; MINIMIZATION; Modulation transfer functions; NOISE; Nonmetals; Numerical optimization; optimization; Phantoms, Imaging; Quality Control; Radiation Imaging Physics; RADIATION SOURCE IMPLANTS; RADIOLOGY AND NUCLEAR MEDICINE; Reconstruction; Segmentation; Tomography, X-Ray Computed - methods; X RADIATION; X‐ray imaging; computed tomography; metal artifact; image reconstruction; optimization
• ISSN: 0094-2405; 2473-4209; 0094-2405
• DOI: 10.1118/1.3533711

Purpose: The streak artifacts caused by metal implants have long been recognized as a problem that limits various applications of CT imaging. In this work, the authors propose an iterative metal artifact reduction algorithm based on constrained optimization. Methods: After the shape and location of metal objects in the image domain is determined automatically by the binary metal identification algorithm and the segmentation of “metal shadows” in projection domain is done, constrained optimization is used for image reconstruction. It minimizes a predefined function that reflectsa priori knowledge of the image, subject to the constraint that the estimated projection data are within a specified tolerance of the available metal-shadow-excluded projection data, with image non-negativity enforced. The minimization problem is solved through the alternation of projection-onto-convex-sets and the steepest gradient descent of the objective function. The constrained optimization algorithm is evaluated with a penalized smoothness objective. Results: The study shows that the proposed method is capable of significantly reducing metal artifacts, suppressing noise, and improving soft-tissue visibility. It outperforms the FBP-type methods and ART and EM methods and yields artifacts-free images. Conclusions: Constrained optimization is an effective way to deal with CT reconstruction with embedded metal objects. Although the method is presented in the context of metal artifacts, it is applicable to general “missing data” image reconstruction problems.