title: Low-dose megavoltage cone-beam CT for radiation therapy.
abstract: OBJECTIVE
  The objective of this work was to demonstrate the feasibility of acquiring low-exposure megavoltage cone-beam CT (MV CBCT) three-dimensional (3D) image data of sufficient quality to register the CBCT images to kilovoltage planning CT images for patient alignment and dose verification purposes.
  
  
  METHODS
  A standard clinical 6-MV Primus linear accelerator, operating in arc therapy mode, and an amorphous-silicon (a-Si) flat-panel electronic portal-imaging device (EPID) were employed. The dose-pulse rate of 6-MV Primus accelerator beam was windowed to expose an a-Si flat panel by using only 0.02 to 0.08 monitor unit (MUs) per image. A triggered image-acquisition mode was designed to produce a high signal-to-noise ratio without pulsing artifacts. Several data sets were acquired for an anthropomorphic head phantom and frozen sheep and pig cadaver head, as well as for a head-and-neck cancer patient on intensity-modulated radiotherapy (IMRT). For each CBCT image, a set of 90 to 180 projection images incremented by 1 degree to 2 degrees was acquired. The two-dimensional (2D) projection images were then synthesized into a 3D image by use of cone-beam CT reconstruction. The resulting MV CBCT image set was used to visualize the 3D bony anatomy and some soft-tissue details. The 3D image registration with the kV planning CT was performed either automatically by application of a maximization of mutual information (MMI) algorithm or manually by aligning multiple 1D slices.
  
  
  RESULTS
  Low-noise 3D MV CBCT images without pulsing artifacts were acquired with a total delivered dose that ranged from 5 to 15 cGy. Acquisition times, including image readout, were on the order of 90 seconds for 180 projection images taken through a continuous gantry rotation of 180 degrees. The processing time of the data required an additional 90 seconds for the reconstruction of a 256(3) cube with 1.0-mm voxel size. Implanted gold markers (1 mm x 3 mm) were easily visible or all exposure levels without artifacts. In general, the presence of high Z materials such as tooth fillings or implanted markers did not result in visible streak artifacts. The registration of structures such as the spinal canal and the nasopharynx in the MV CBCT and kV CT data sets was possible with millimeter and degree accuracy as assessed by displacement simulations and subsequent visual evaluation.
  
  
  CONCLUSIONS
  We believe that the quality of these images, along with the rapid acquisition and reconstruction times, demonstrates that MV CBCT performed by use of a standard linear accelerator equipped with a flat-panel imager can be applied clinically for patient alignment.
authors.affiliation: Department of Radiation Oncology, University of California San Francisco, CA 94143-1708, USA. pouliot@radonc17.ucsf.edu
authors.forename: Jean
authors.initials: J
authors.lastname: Pouliot
authors.forename: Ali
authors.initials: A
authors.lastname: Bani-Hashemi
authors.forename: Josephine
authors.initials: J
authors.lastname: Chen
authors.forename: Michelle
authors.initials: M
authors.lastname: Svatos
authors.forename: Farhad
authors.initials: F
authors.lastname: Ghelmansarai
authors.forename: Matthias
authors.initials: M
authors.lastname: Mitschke
authors.forename: Michele
authors.initials: M
authors.lastname: Aubin
authors.forename: Ping
authors.initials: P
authors.lastname: Xia
authors.forename: Olivier
authors.initials: O
authors.lastname: Morin
authors.forename: Kara
authors.initials: K
authors.lastname: Bucci
authors.forename: Mack
authors.initials: M
authors.lastname: Roach
authors.forename: Paco
authors.initials: P
authors.lastname: Hernandez
authors.forename: Zirao
authors.initials: Z
authors.lastname: Zheng
authors.forename: Dimitre
authors.initials: D
authors.lastname: Hristov
authors.forename: Lynn
authors.initials: L
authors.lastname: Verhey
mesh_terms: D000818:Animals; D005240:Feasibility Studies; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D006801:Humans; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012756:Sheep; D013552:Swine; D014057:Tomography, X-Ray Computed