title: Metal artifact reduction in x-ray computed tomography (CT) by constrained optimization.
abstract: OBJECTIVE
  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 reflects a 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.
authors: Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA.
authors: Xiaomeng
authors: X
authors: Zhang
authors: Jing
authors: J
authors: Wang
authors: Lei
authors: L
authors: Xing
mesh_terms: D000465:Algorithms; D016477:Artifacts; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008670:Metals; D019047:Phantoms, Imaging; D011786:Quality Control; D014057:Tomography, X-Ray Computed / Q000379:methods*
chemical_list: D008670:Metals