title: A Method for Assessing Ground-Truth Accuracy of the 5DCT Technique.
abstract: OBJECTIVE
  To develop a technique that assesses the accuracy of the breathing phase-specific volume image generation process by patient-specific breathing motion model using the original free-breathing computed tomographic (CT) scans as ground truths.
  
  
  METHODS
  Sixteen lung cancer patients underwent a previously published protocol in which 25 free-breathing fast helical CT scans were acquired with a simultaneous breathing surrogate. A patient-specific motion model was constructed based on the tissue displacements determined by a state-of-the-art deformable image registration. The first image was arbitrarily selected as the reference image. The motion model was used, along with the free-breathing phase information of the original 25 image datasets, to generate a set of deformation vector fields that mapped the reference image to the 24 nonreference images. The high-pitch helically acquired original scans served as ground truths because they captured the instantaneous tissue positions during free breathing. Image similarity between the simulated and the original scans was assessed using deformable registration that evaluated the pointwise discordance throughout the lungs.
  
  
  RESULTS
  Qualitative comparisons using image overlays showed excellent agreement between the simulated images and the original images. Even large 2-cm diaphragm displacements were very well modeled, as was sliding motion across the lung-chest wall boundary. The mean error across the patient cohort was 1.15 ± 0.37 mm, and the mean 95th percentile error was 2.47 ± 0.78 mm.
  
  
  CONCLUSIONS
  The proposed ground truth-based technique provided voxel-by-voxel accuracy analysis that could identify organ-specific or tumor-specific motion modeling errors for treatment planning. Despite a large variety of breathing patterns and lung deformations during the free-breathing scanning session, the 5-dimensionl CT technique was able to accurately reproduce the original helical CT scans, suggesting its applicability to a wide range of patients.
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, California. Electronic address: tdou@mednet.ucla.edu.
authors.forename: Tai H
authors.initials: TH
authors.lastname: Dou
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, California.
authors.forename: David H
authors.initials: DH
authors.lastname: Thomas
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, California.
authors.forename: Dylan P
authors.initials: DP
authors.lastname: O'Connell
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, California.
authors.forename: James M
authors.initials: JM
authors.lastname: Lamb
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, California.
authors.forename: Percy
authors.initials: P
authors.lastname: Lee
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, California.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Low
mesh_terms: D000465:Algorithms; D016477:Artifacts; D002985:Clinical Protocols; D045853:Exhalation; D056973:Four-Dimensional Computed Tomography / Q000379:methods* / Q000592:standards; D006801:Humans; D001239:Inhalation; D008175:Lung Neoplasms / Q000000981:diagnostic imaging*; D009068:Movement; D011785:Quality Assurance, Health Care; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards; D015203:Reproducibility of Results; D012119:Respiration; D036542:Tomography, Spiral Computed / Q000379:methods* / Q000592:standards