title: Optimization of the x-ray monitoring angle for creating a correlation model between internal and external respiratory signals.
abstract: OBJECTIVE
  To perform dynamic tumor tracking irradiation with the Vero4DRT (MHI-TM2000), a correlation model [four dimensional (4D) model] between the displacement of infrared markers on the abdominal wall and the three-dimensional position of a tumor indicated by a minimum of three implanted gold markers is required. However, the gold markers cannot be detected successfully on fluoroscopic images under the following situations: (1) overlapping of the gold markers; and (2) a low intensity ratio of the gold marker to its surroundings. In the present study, the authors proposed a method to readily determine the optimal x-ray monitoring angle for creating a 4D model utilizing computed tomography (CT) images.
  
  
  METHODS
  The Vero4DRT mounting two orthogonal kV x-ray imaging subsystems can separately rotate the gantry along an O-shaped guide-lane and the O-ring along its vertical axis. The optimal x-ray monitoring angle was determined on CT images by minimizing the root-sum-square of water equivalent path lengths (WEPLs) on the orthogonal lines passing all of the gold markers while rotating the O-ring and the gantry. The x-ray monitoring angles at which the distances between the gold markers were within 5 mm at the isocenter level were excluded to prevent false detection of the gold markers in consideration of respiratory motions. First, the relationship between the WEPLs (unit: mm) and the intensity ratios of the gold markers was examined to assess the validity of our proposed method. Second, our proposed method was applied to the 4D-CT images at the end-expiration phase for 11 lung cancer patients who had four to five gold markers. To prove the necessity of the x-ray monitoring angle optimization, the intensity ratios of the least visible markers (minimum intensity ratios) that were estimated from the WEPLs were compared under the following conditions: the optimal x-ray monitoring angle and the angles used for setup verification. Additionally, the intra- and interfractional variations in the intensity ratio were examined from the optimal x-ray monitoring angle.
  
  
  RESULTS
  A negative strong correlation was observed between the WEPL (x) and the intensity ratio (y) (y = 6.57 exp[-0.0125x] + 1, R = -0.88 [95% confidence interval: -0.85 to -0.90], p < 0.01). Our proposed method effectively avoided having the x-ray beam pass through high-density structures, although there were large interpatient variations in the optimal x-ray monitoring angle because of the geometric arrangement between the gold markers and the anatomical structures. The minimum intensity ratios that were estimated from the WEPLs at the optimal x-ray monitoring angle ranged from 1.43 to 2.48, which was an average of 1.27 times (range, 1.02-1.66) higher than the angles used for setup verification. The maximum intra- and interfractional decreases in the intensity ratio were 0.23 and 0.17, respectively.
  
  
  CONCLUSIONS
  The authors demonstrated that the optimal x-ray monitoring angle for creating a 4D model can improve the visibility of gold markers.
authors.affiliation: Department of Radiation Oncology and Image-applied Therapy, Graduate School of Medicine, Kyoto University, Japan.
authors.forename: Mami
authors.initials: M
authors.lastname: Akimoto
authors.forename: Mitsuhiro
authors.initials: M
authors.lastname: Nakamura
authors.forename: Nobutaka
authors.initials: N
authors.lastname: Mukumoto
authors.forename: Masahiro
authors.initials: M
authors.lastname: Yamada
authors.forename: Nami
authors.initials: N
authors.lastname: Ueki
authors.forename: Yukinori
authors.initials: Y
authors.lastname: Matsuo
authors.forename: Akira
authors.initials: A
authors.lastname: Sawada
authors.forename: Takashi
authors.initials: T
authors.lastname: Mizowaki
authors.forename: Masaki
authors.initials: M
authors.lastname: Kokubo
authors.forename: Masahiro
authors.initials: M
authors.lastname: Hiraoka
mesh_terms: D057918:Fiducial Markers; D056973:Four-Dimensional Computed Tomography / Q000379:methods* / Q000592:standards; D006801:Humans; D008954:Models, Biological; D009068:Movement; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000532:radiotherapy; D061089:Radiotherapy, Image-Guided / Q000379:methods* / Q000592:standards; D012119:Respiration