title: Megavoltage planar and cone-beam imaging with low-Z targets: dependence of image quality improvement on beam energy and patient separation.
abstract: OBJECTIVE
  The purpose of this study is to investigate the improvement of megavoltage planar and cone-beam CT (CBCT) image quality with the use of low atomic number (Z) external targets in the linear accelerator.
  
  
  METHODS
  In this investigation, two experimental megavoltage imaging beams were generated by using either 3.5 or 7.0 MeV electrons incident on aluminum targets installed above the level of the carousel in a linear accelerator (2100EX, Varian Medical, Inc., Palo Alto, CA). Images were acquired using an amorphous silicon detector panel. Contrast-to-noise ratio (CNR) in planar and CBCT images was measured as a function of dose and a comparison was made between the imaging beams and the standard 6 MV therapy beam. Phantoms of variable diameter were used to examine the loss of contrast due to beam hardening. Porcine imaging was conducted to examine qualitatively the advantages of the low-Z target approach in CBCT.
  
  
  RESULTS
  In CBCT imaging CNR increases by factors as high as 2.4 and 4.3 for the 7.0 and 3.5 MeV/Al beams, respectively, compared to images acquired with 6 MV. Similar factors of improvement are observed in planar imaging. For the imaging beams, beam hardening causes a significant loss of the contrast advantage with increasing phantom diameter; however, for the 3.5 MeV/Al beam and a phantom diameter of 25 cm, a contrast advantage remains, with increases of contrast by factors of 1.5 and 3.4 over 6 MV for bone and lung inhale regions, respectively. The spatial resolution is improved slightly in CBCT images for the imaging beams. CBCT images of a porcine cranium demonstrate qualitatively the advantages of the low-Z target approach, showing greater contrast between tissues and improved visibility of fine detail.
  
  
  CONCLUSIONS
  The use of low-Z external targets in the linear accelerator improves megavoltage planar and CBCT image quality significantly. CNR may be increased by a factor of 4 or greater. Improvement of the spatial resolution is also apparent.
authors.affiliation: Department of Radiation Oncology, Dalhousie University, 5820 University Avenue, Halifax, Nova Scotia B3H 1V7, Canada. james.robar@cdha.nshealth.ca
authors.forename: James L
authors.initials: JL
authors.lastname: Robar
authors.forename: Tanner
authors.initials: T
authors.lastname: Connell
authors.forename: Weihong
authors.initials: W
authors.lastname: Huang
authors.forename: Robin G
authors.initials: RG
authors.lastname: Kelly
mesh_terms: D000535:Aluminum / Q000737:chemistry; D000818:Animals; D001842:Bone and Bones / Q000000981:diagnostic imaging; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D003952:Diagnostic Imaging / Q000379:methods*; D004583:Electrons; D006257:Head / Q000000981:diagnostic imaging; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008168:Lung / Q000000981:diagnostic imaging; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons; D011829:Radiation Dosage; D017655:Silicon Compounds / Q000737:chemistry; D013552:Swine; D014867:Water / Q000737:chemistry
chemical_list: D017655:Silicon Compounds; D014867:Water; D000535:Aluminum