title: Generation of hybrid sinograms for the recovery of kV-CT images with metal artifacts for helical tomotherapy.
abstract: OBJECTIVE
  The overall goal of this study is to restore kilovoltage computed tomography (kV-CT) images which are disfigured by patients' metal prostheses. By generating a hybrid sinogram that is a combination of kV and megavoltage (MV) projection data, the authors suggest a novel metal artifact-reduction (MAR) method that retains the image quality to match that of kV-CT and simultaneously restores the information of metal prostheses lost due to photon starvation.
  
  
  METHODS
  CT projection data contain information about attenuation coefficients and the total length of the attenuation. By normalizing raw kV projections with their own total lengths of attenuation, mean attenuation projections were obtained. In the same manner, mean density projections of MV-CT were obtained by the normalization of MV projections resulting from the forward projection of density-calibrated MV-CT images with the geometric parameters of the kV-CT device. To generate the hybrid sinogram, metal-affected signals of the kV sinogram were identified and replaced by the corresponding signals of the MV sinogram following a density calibration step with kV data. Filtered backprojection was implemented to reconstruct the hybrid CT image. To validate the authors' approach, they simulated four different scenarios for three heads and one pelvis using metallic rod inserts within a cylindrical phantom. Five inserts describing human body elements were also included in the phantom. The authors compared the image qualities among the kV, MV, and hybrid CT images by measuring the contrast-to-noise ratio (CNR), the signal-to-noise ratio (SNR), the densities of all inserts, and the spatial resolution. In addition, the MAR performance was compared among three existing MAR methods and the authors' hybrid method. Finally, for clinical trials, the authors produced hybrid images of three patients having dental metal prostheses to compare their MAR performances with those of the kV, MV, and three existing MAR methods.
  
  
  RESULTS
  The authors compared the image quality and MAR performance of the hybrid method with those of other imaging modalities and the three MAR methods, respectively. The total measured mean of the CNR (SNR) values for the nonmetal inserts was determined to be 14.3 (35.3), 15.3 (37.8), and 25.5 (64.3) for the kV, MV, and hybrid images, respectively, and the spatial resolutions of the hybrid images were similar to those of the kV images. The measured densities of the metal and nonmetal inserts in the hybrid images were in good agreement with their true densities, except in cases of extremely low densities, such as air and lung. Using the hybrid method, major streak artifacts were suitably removed and no secondary artifacts were introduced in the resultant image. In clinical trials, the authors verified that kV and MV projections were successfully combined and turned into the resultant hybrid image with high image contrast, accurate metal information, and few metal artifacts. The hybrid method also outperformed the three existing MAR methods with regard to metal information restoration and secondary artifact prevention.
  
  
  CONCLUSIONS
  The authors have shown that the hybrid method can restore the overall image quality of kV-CT disfigured by severe metal artifacts and restore the information of metal prostheses lost due to photon starvation. The hybrid images may allow for the improved delineation of structures of interest and accurate dose calculations for radiation treatment planning for patients with metal prostheses.
authors.affiliation: Department of Radiation Oncology and Research Institute for Convergence of Biomedical Science and Technology, Pusan National University Yangsan Hospital, Yangsan 626-770, South Korea.
authors.forename: Hosang
authors.initials: H
authors.lastname: Jeon
authors.affiliation: Department of Radiation Oncology, Pusan National University Hospital, Busan 602-739, South Korea.
authors.forename: Dahl
authors.initials: D
authors.lastname: Park
authors.affiliation: School of Mechanical Engineering, Pusan National University, Busan 609-735, South Korea.
authors.forename: Hanbean
authors.initials: H
authors.lastname: Youn
authors.affiliation: Department of Radiation Oncology, Pusan National University Yangsan Hospital, Yangsan 626-770, South Korea.
authors.forename: Jiho
authors.initials: J
authors.lastname: Nam
authors.affiliation: Department of Radiation Oncology, Pusan National University Yangsan Hospital, Yangsan 626-770, South Korea.
authors.forename: Jayoung
authors.initials: J
authors.lastname: Lee
authors.affiliation: Department of Radiation Oncology, Pusan National University Hospital, Busan 602-739, South Korea.
authors.forename: Wontaek
authors.initials: W
authors.lastname: Kim
authors.affiliation: Department of Radiation Oncology, Pusan National University Hospital, Busan 602-739, South Korea.
authors.forename: Yongkan
authors.initials: Y
authors.lastname: Ki
authors.affiliation: Department of Radiation Oncology, Pusan National University Hospital, Busan 602-739, South Korea.
authors.forename: Yong Ho
authors.initials: YH
authors.lastname: Kim
authors.affiliation: Department of Radiation Oncology, Pusan National University Hospital, Busan 602-739, South Korea.
authors.forename: Ju Hye
authors.initials: JH
authors.lastname: Lee
authors.affiliation: Department of Radiation Oncology, Pusan National University Hospital, Busan 602-739, South Korea.
authors.forename: Dongwon
authors.initials: D
authors.lastname: Kim
authors.affiliation: School of Mechanical Engineering and the Center for Advanced Medical Engineering Research, Pusan National University, Busan 609-735, South Korea.
authors.forename: Ho Kyung
authors.initials: HK
authors.lastname: Kim
mesh_terms: D016477:Artifacts; D002138:Calibration; D003793:Dental Restoration, Permanent / Q000009:adverse effects; D006257:Head; D006801:Humans; D008670:Metals / Q000009:adverse effects; D008954:Models, Biological; D010388:Pelvis; D019047:Phantoms, Imaging; D019904:Polymethyl Methacrylate; D019736:Prostheses and Implants / Q000009:adverse effects; D018030:Silver Compounds / Q000009:adverse effects; D017971:Tin Compounds / Q000009:adverse effects; D014054:Tomography / Q000379:methods*
chemical_list: D008670:Metals; D018030:Silver Compounds; D017971:Tin Compounds; D019904:Polymethyl Methacrylate