title: Upright cone beam CT imaging using the onboard imager.
abstract: OBJECTIVE
  Many patients could benefit from being treated in an upright position. The objectives of this study were to determine whether cone beam computed tomography (CBCT) could be used to acquire upright images for treatment planning and to demonstrate whether reconstruction of upright images maintained accurate geometry and Hounsfield units (HUs).
  
  
  METHODS
  A TrueBeam linac was programmed in developer mode to take upright CBCT images. The gantry head was positioned at 0°, and the couch was rotated to 270°. The x-ray source and detector arms were extended to their lateral positions. The x-ray source and gantry remained stationary as fluoroscopic projections were taken and the couch was rotated from 270° to 90°. The x-ray tube current was normalized to deposit the same dose (measured using a calibrated Farmer ion chamber) as that received during a clinical helical CT scan to the center of a cylindrical, polyethylene phantom. To extend the field of view, two couch rotation scans were taken with the detector offset 15 cm superiorly and then 15 cm inferiorly. The images from these two scans were stitched together before reconstruction. Upright reconstructions were compared to reconstructions from simulation CT scans of the same phantoms. Two methods were investigated for correcting the HUs, including direct calibration and mapping the values from a simulation CT.
  
  
  RESULTS
  Overall geometry, spatial linearity, and high contrast resolution were maintained in upright reconstructions. Some artifacts were created and HU accuracy was compromised; however, these limitations could be removed by mapping the HUs from a simulation CT to the upright reconstruction for treatment planning.
  
  
  CONCLUSIONS
  The feasibility of using the TrueBeam linac to take upright CBCT images was demonstrated. This technique is straightforward to implement and could be of enormous benefit to patients with thoracic tumors or those who find a supine position difficult to endure.
authors.affiliation: Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030 and The University of Texas Graduate School of Biomedical Sciences at Houston, Houston, Texas 77030.
authors.forename: Xenia
authors.initials: X
authors.lastname: Fave
authors.affiliation: Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030.
authors.forename: Jinzhong
authors.initials: J
authors.lastname: Yang
authors.affiliation: Varian Medical Systems, Zug 6303, Switzerland.
authors.forename: Luis
authors.initials: L
authors.lastname: Carvalho
authors.affiliation: Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030 and The University of Texas Graduate School of Biomedical Sciences at Houston, Houston, Texas 77030.
authors.forename: Rachael
authors.initials: R
authors.lastname: Martin
authors.affiliation: Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030.
authors.forename: Tinsu
authors.initials: T
authors.lastname: Pan
authors.affiliation: Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030.
authors.forename: Peter
authors.initials: P
authors.lastname: Balter
authors.affiliation: Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030.
authors.forename: Laurence
authors.initials: L
authors.lastname: Court
mesh_terms: D016477:Artifacts; D002138:Calibration; D003198:Computer Simulation; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation* / Q000379:methods*; D005240:Feasibility Studies; D006801:Humans; D008176:Lung Volume Measurements / Q000379:methods; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D020959:Polyethylene; D011187:Posture; D011829:Radiation Dosage; D055618:Radiation Equipment and Supplies; D013902:Radiography, Thoracic / Q000295:instrumentation / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012399:Rotation
chemical_list: D020959:Polyethylene