title: Assessment of on-screen measurements, magnification, and calibration in digital radiography.
abstract: OBJECTIVE
  To compare calibration methods for digital radiography in terms of measurement accuracy and interobserver variability.
  
  
  METHODS
  Prospective study.
  
  
  METHODS
  Digital radiographic images of a 155-mm-long Steinmann pin.
  
  
  METHODS
  Measurement of pin length on digital radiographs was determined with a 25.4-mm-diameter calibration ball and commercially available software program via 3 calibration methods (ie, no calibration, autocalibration, and manual calibration). Digital radiographs of the calibration ball and pin were obtained with each placed at various vertical heights from the table (7 heights) and horizontal distances from the center of the beam (4 distances). Measurements of pin length on digital radiographs were made by 4 observers who were blinded to the orientation of the calibration ball and pin.
  
  
  RESULTS
  Pin lengths obtained by each calibration method were significantly different from each other and from the true value. Manual calibration was the most accurate. There was no significant interobserver variability in measurements. There was no significant change in measurements when the calibration ball was moved horizontally, but pin length measurements changed significantly when the ball was moved vertically (away from the table) with an approximate magnification error of 1% per centimeter of distance between the calibration ball and pin.
  
  
  CONCLUSIONS
  For digital radiography, manual calibration is recommended to achieve the most accurate measurements. Ideally, the calibration ball should be placed at the same vertical height as the object to be measured; however, if this cannot be achieved, the magnification error can be expected to be approximately 1% per centimeter of distance.
authors.affiliation: Aspen Meadow Veterinary Specialists, Longmont, CO 80501, USA. rread@aspenmeadowvet.com
authors.forename: Robyn L
authors.initials: RL
authors.lastname: Read
authors.forename: Colleen G
authors.initials: CG
authors.lastname: Duncan
authors.forename: Aaron D
authors.initials: AD
authors.lastname: Wallace
authors.forename: James A
authors.initials: JA
authors.lastname: Perry
authors.forename: Felix M
authors.initials: FM
authors.lastname: Duerr
mesh_terms: D000818:Animals; D001858:Bone Nails; D002138:Calibration; D011856:Radiographic Image Enhancement / Q000379:methods* / Q000592:standards*