title: Dose distribution for dental cone beam CT and its implication for defining a dose index.
abstract: OBJECTIVE
  To characterize the dose distribution for a range of cone beam CT (CBCT) units, investigating different field of view sizes, central and off-axis geometries, full or partial rotations of the X-ray tube and different clinically applied beam qualities. The implications of the dose distributions on the definition and practicality of a CBCT dose index were assessed.
  
  
  METHODS
  Dose measurements on CBCT devices were performed by scanning cylindrical head-size water and polymethyl methacrylate phantoms, using thermoluminescent dosemeters, a small-volume ion chamber and radiochromic films.
  
  
  RESULTS
  It was found that the dose distribution can be asymmetrical for dental CBCT exposures throughout a homogeneous phantom, owing to an asymmetrical positioning of the isocentre and/or partial rotation of the X-ray source. Furthermore, the scatter tail along the z-axis was found to have a distinct shape, generally resulting in a strong drop (90%) in absorbed dose outside the primary beam.
  
  
  CONCLUSIONS
  There is no optimal dose index available owing to the complicated exposure geometry of CBCT and the practical aspects of quality control measurements. Practical validation of different possible dose indices is needed, as well as the definition of conversion factors to patient dose.
authors: Department of Oral Health Sciences, Leuven, Belgium. ruben.pauwels@med.kuleuven.be
authors: R
authors: R
authors: Pauwels
authors: C
authors: C
authors: Theodorakou
authors: A
authors: A
authors: Walker
authors: H
authors: H
authors: Bosmans
authors: R
authors: R
authors: Jacobs
authors: K
authors: K
authors: Horner
authors: R
authors: R
authors: Bogaerts
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D004867:Equipment Design; D005373:Film Dosimetry / Q000295:instrumentation / Q000379:methods; D006801:Humans; D019047:Phantoms, Imaging; D019904:Polymethyl Methacrylate; D011786:Quality Control; D011829:Radiation Dosage; D011834:Radiation Monitoring / Q000295:instrumentation / Q000379:methods; D011861:Radiography, Dental / Q000379:methods*; D012542:Scattering, Radiation; D013819:Thermoluminescent Dosimetry / Q000295:instrumentation / Q000379:methods; D014867:Water
chemical_list: D014867:Water; D019904:Polymethyl Methacrylate