title: Quantifying the reproducibility of lung ventilation images between 4-Dimensional Cone Beam CT and 4-Dimensional CT.
abstract: OBJECTIVE
  Computed tomography ventilation imaging derived from four-dimensional cone beam CT (CTVI4DCBCT) can complement existing 4DCT-based methods (CTVI4DCT) to track lung function changes over a course of lung cancer radiation therapy. However, the accuracy of CTVI4DCBCTneeds to be assessed since anatomic 4DCBCT has demonstrably poor image quality and small field of view (FOV) compared to treatment planning 4DCT. We perform a direct comparison between short interval CTVI4DCBCTand CTVI4DCTpairs to understand the patient specific image quality factors affecting the intermodality CTVI reproducibility in the clinic.
  
  
  METHODS
  We analysed 51 pairs of 4DCBCT and 4DCT scans acquired within 1 day of each other for nine lung cancer patients. To assess the impact of image quality, CTVIs were derived from 4DCBCT scans reconstructed using both standard Feldkamp-Davis-Kress backprojection (CTVIFDK4DCBCT) and an iterative McKinnon-Bates Simultaneous Algebraic Reconstruction Technique (CTVIMKBSART4DCBCT). Also, the influence of FOV was assessed by deriving CTVIs from 4DCT scans that were cropped to a similar FOV as the 4DCBCT scans (CTVIcrop4DCT), or uncropped (CTVIuncrop4DCT). All CTVIs were derived by performing deformable image registration (DIR) between the exhale and inhale phases and evaluating the Jacobian determinant of deformation. Reproducibility between corresponding CTVI4DCBCTand CTVI4DCTpairs was quantified using the voxel-wise Spearman rank correlation and the Dice similarity coefficient (DSC) for ventilation defect regions (identified as the lower quartile of ventilation values). Mann-Whitney U-tests were applied to determine statistical significance of each reconstruction and cropping condition.
  
  
  RESULTS
  The (mean ± SD) Spearman correlation between CTVIFDK4DCBCT and CTVIuncrop4DCT was 0.60 ± 0.23 (range -0.03-0.88) and the DSC was 0.64 ± 0.12 (0.34-0.83). By comparison, correlations between CTVIMKBSART4DCBCT and CTVIuncrop4DCT showed a small but statistically significant improvement with = 0.64 ± 0.20 (range 0.06-0.90, P = 0.03) and DSC = 0.66 ± 0.13 (0.31-0.87, P = 0.02). Intermodal correlations were noted to decrease with an increasing fraction of lung truncation in 4DCBCT relative to 4DCT, albeit not significantly (Pearson correlation R = 0.58, P = 0.002).
  
  
  CONCLUSIONS
  This study demonstrates that DIR based CTVIs derived from 4DCBCT can exhibit reasonable to good voxel-level agreement with CTVIs derived from 4DCT. These correlations outperform previous cross-modality comparisons between 4DCT-based ventilation and nuclear medicine. The use of 4DCBCT scans with iterative reconstruction and minimal lung truncation is recommended to ensure better reproducibility between 4DCBCT- and 4DCT-based CTVIs.
authors.affiliation: Radiation Physics Laboratory, School of Medicine, University of Sydney, Sydney, NSW, 2006, Australia.
authors.forename: Henry C
authors.initials: HC
authors.lastname: Woodruff
authors.affiliation: Radiation Physics Laboratory, School of Medicine, University of Sydney, Sydney, NSW, 2006, Australia.
authors.forename: Chun-Chien
authors.initials: CC
authors.lastname: Shieh
authors.affiliation: Radiation Physics Laboratory, School of Medicine, University of Sydney, Sydney, NSW, 2006, Australia.|Department of Medical Physics, School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, NSW, 2300, Australia.|Radiation Oncology Centre, Seventh Day Adventist Hospital, Wahroonga, NSW, Australia.
authors.forename: Fiona
authors.initials: F
authors.lastname: Hegi-Johnson
authors.affiliation: Radiation Physics Laboratory, School of Medicine, University of Sydney, Sydney, NSW, 2006, Australia.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Keall
authors.affiliation: Radiation Physics Laboratory, School of Medicine, University of Sydney, Sydney, NSW, 2006, Australia.|Northern Sydney Cancer Center, Royal North Shore Hospital, Sydney, NSW, 2065, Australia.
authors.forename: John
authors.initials: J
authors.lastname: Kipritidis
mesh_terms: D054893:Cone-Beam Computed Tomography; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging*; D015203:Reproducibility of Results; D012119:Respiration
keywords: 4D cone beam CT; deformable image registration; functional imaging; lung radiation therapy; ventilation