Metal Artifact Reduction in Computed Tomographic (CT) Images for Radiotherapy Treatment Planning

• Author / Creator

  Paudel, Moti R

• High density/high atomic number metallic objects create shading and streaking metal artifacts in the CT image that can cause inaccurate delineation of anatomical structures or inaccurate radiation dose calculation.
  We developed techniques for reducing metal artifacts in both megavoltage CT (MVCT) and kilovoltage CT (kVCT) images. We remodelled the iterative maximum polychromatic algorithm for CT (IMPACT) by adding a model for pair/triplet production and incorporating the energy dependent response of the detectors and successfully applied it to two MVCT systems. In the corrected image of a phantom, the error in the measured electron density of a plexiglass background was <1%. The mean deviation of measured electron density (0.295 1.695 relative to water) for a range of materials was <3%. For the kVCT beam, a thickness ≥13 mm of steel plate resulted in photon starvation at the detector. The modifications, similar to those for MVCT, made to kVCT in the IMPACT algorithm did not improve its performance due to photon starvation.
  An algorithm (MVCT-NMAR) was developed that uses prior information from MVCT images to correct artifacts in kVCT. The MVCT-NMAR greatly reduced the metal artifacts in kVCT without deforming structures and did not introduce secondary artifacts except for a few faint streaks. The radiation doses calculated on those corrected images were closer to the doses in a reference image due to the more accurate CT numbers. These improvements were significant when compared to the commercial metal artifact correction method (OMAR algorithm in Philips CT scanner).
  The MVCT-NMAR algorithm was further improved to remove remaining fine streakings and applied to the images of five patients. The technique greatly reduced the metal artifacts and avoided secondary artifacts. Those were significant improvements over the commercial OMAR correction method and depended upon accurate registration of the MVCT and kVCT images. Large dose reduction outside the planning target volume was observed for a prostate cancer patient when these images were used without the requirement that beams avoid passing through prostheses in TomoTherapyTM treatment plans. Thus the use of MVCT NMAR corrected images in radiotherapy treatment planning may raise the quality of cancer treatments for patients having metallic implants.

• Subjects / Keywords

  • MVCT
  • Computed Tomography
  • Radiotherapy Treatment Planning
  • KVCT
  • Metal Artifact Reduction
  • Metal Artifacts

• Graduation date

  Spring 2014

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R30Z71379

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.