title: Post-processing sets of tilted CT volumes as a method for metal artifact reduction.
abstract: BACKGROUND
  Metal implants, surgical clips and other foreign bodies may cause 'streaking' or 'star' artifacts in computed tomography (CT) reconstructions, for example in the vicinity of dental restorations or hip implants. The deteriorated image quality complicates contouring and has an adverse effect on quantitative planning in external beam therapy.
  
  
  METHODS
  The potential to reduce artifacts by acquisition of tilted CT reconstructions from different angles of the same object was investigated. While each of those reconstructions still contained artifacts, they were not necessarily in the same place in each CT. By combining such CTs with complementary information, a reconstructed volume with less or even without artifacts was obtained. The most straightforward way to combine the co-registered volumes was to calculate the mean or median per voxel. The method was tested with a calibration phantom featuring a titanium insert, and with a human skull featuring multiple dental restorations made from gold and steel. The performance of the method was compared to established metal artifact reduction (MAR) algorithms. Dose reduction was tested.
  
  
  RESULTS
  In a visual comparison, streaking artifacts were strongly reduced and details in the vicinity of metal foreign bodies became much more visible. In case of the calibration phantom, average bias in Hounsfield units was reduced by 94% and per-voxel-errors and noise were reduced by 83%. In case of the human skull, bias was reduced by 95% and noise was reduced by 94%. The performance of the method was visually superior and quantitatively compareable to established MAR algorithms. Dose reduction was viable.
  
  
  CONCLUSIONS
  A simple post-processing method for MAR was described which required one or more complementary scans but did not rely on any a priori information. The method was computationally inexpensive. Performance of the method was quantitatively comparable to established algorithms and visually superior in a direct comparison. Dose reduction was demonstrated, artifacts could be reduced without compromising total dose to the patient.
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Ludwig-Maximilians-University of Munich, Marchioninistrasse 15, 81377 Munich, Germany. hendrik.ballhausen@med.uni-muenchen.de.
authors.forename: Hendrik
authors.initials: H
authors.lastname: Ballhausen
authors.forename: Michael
authors.initials: M
authors.lastname: Reiner
authors.forename: Ute
authors.initials: U
authors.lastname: Ganswindt
authors.forename: Claus
authors.initials: C
authors.lastname: Belka
authors.forename: Matthias
authors.initials: M
authors.lastname: Söhn
mesh_terms: D000465:Algorithms; D016477:Artifacts; D054893:Cone-Beam Computed Tomography; D006801:Humans; D008670:Metals; D019047:Phantoms, Imaging; D019736:Prostheses and Implants; D011856:Radiographic Image Enhancement / Q000379:methods*; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*
chemical_list: D008670:Metals