title: A novel method for megavoltage scatter correction in cone-beam CT acquired concurrent with rotational irradiation.
abstract: OBJECTIVE
  Acquisition of cone-beam CT (CBCT) concurrent with VMAT results in scatter of the megavoltage (MV) beam onto the kilovoltage (kV) detector deteriorating CBCT image quality. The aim of this paper is to develop a method to estimate and correct for MV scatter reaching the kV panel.
  
  
  METHODS
  The correction method is based on an alternating pulse sequence such that the kV source is enabled in only every other frame. MV scatter, noise and artifacts in the kV-unexposed frames are separated based on their spatial and temporal frequency characteristics and subsequently subtracted from the exposed frames. The method was tested on a phantom and one lung cancer patient scanned on an Elekta Synergy system simultaneous with a 9 Gy VMAT delivery.
  
  
  RESULTS
  Without correction, acquisition during VMAT leads to significant cupping and loss of bone contrast. The novel method removes the signal due to MV scatter. Compared to acquisition without MV beam, the reconstruction quality is identical except for some unavoidable noise due to the scattered MV radiation in the exposed frames. Correction is performed in-line with acquisition without introducing delays.
  
  
  CONCLUSIONS
  A novel, efficient and effective method was developed to correct for MV scatter in CBCT scans acquired concurrent with rotational radiotherapy.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Lennert
authors.initials: L
authors.lastname: Ploeger
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
mesh_terms: D000465:Algorithms; D016477:Artifacts; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D003287:Contrast Media; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D011857:Radiographic Image Interpretation, Computer-Assisted; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012542:Scattering, Radiation
chemical_list: D003287:Contrast Media