title: Study of the scan uniformity from an i-CAT cone beam computed tomography dental imaging system.
abstract: OBJECTIVE
  As part of an ongoing programme to improve diagnosis and treatment planning relevant to implant placement, orthodontic treatment and dentomaxillofacial surgery, a study has been made of the spatial accuracy and density response of an i-CAT, a cone beam CT (CBCT) dental imaging system supplied by Imaging Sciences International Inc.
  
  
  METHODS
  Custom-made phantoms using acrylic sheet and water were used for measurements on spatial accuracy, density response and noise. The measurements were made over a period of several months on a clinical machine rather than on a machine dedicated to research.
  
  
  RESULTS
  Measurements on a precision grid showed the spatial accuracy to be universally within the tolerance of +/-1 pixel. The density response and the noise in the data were found to depend strongly on the mass in the slice being scanned.
  
  
  CONCLUSIONS
  The density response was subject to two effects. The first effect changes the whole slice uniformly and linearly depends on the total mass in the slice. The second effect exists when there is mass outside the field of view, dubbed the "exo-mass" effect. This effect lowers the measured CT number rapidly at the scan edge furthest from the exo-mass and raises it on the adjacent edge. The noise also depended quasi-linearly on the mass in the slice. Some general performance rules were drafted to describe these effects and a preliminary correction algorithm was constructed.
authors.affiliation: Dental School, Dental Health and Biological Sciences, Cardiff University, Heath Park, Cardiff CF14 4XY, Wales, UK. bryantJA@cardiff.ac.uk
authors.forename: J A
authors.initials: JA
authors.lastname: Bryant
authors.forename: N A
authors.initials: NA
authors.lastname: Drage
authors.forename: S
authors.initials: S
authors.lastname: Richmond
mesh_terms: D015502:Absorptiometry, Photon; D000465:Algorithms; D016477:Artifacts; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation*; D006801:Humans; D019047:Phantoms, Imaging; D010865:Pilot Projects; D011861:Radiography, Dental / Q000295:instrumentation*; D012542:Scattering, Radiation