title: Strict X-ray beam collimation for facial bones examination can increase lens exposure.
abstract: OBJECTIVE
  It is well accepted that collimation is a cost-effective dose-reducing tool for X-ray examinations. This phantom-based study investigated the impact of X-ray beam collimation on radiation dose to the lenses of the eyes and thyroid along with the effect on image quality in facial bone radiography.
  
  
  METHODS
  A three-view series (occipitomental, occipitomental 30 and lateral) was investigated, and radiation doses to the lenses and thyroid were measured using an Unfors dosemeter. Images were assessed by six experienced observers using a visual grading analysis and a total of 5400 observations were made.
  
  
  RESULTS
  Strict collimation significantly (p<0.0001) reduced the radiation dose to the lenses of the eyes and thyroid when using a fixed projection-specific exposure. With a variable exposure technique (fixed exit dose, to simulate the behaviour of an automatic exposure control), while strict collimation was again shown to reduce thyroid dose, higher lens doses were demonstrated when compared with larger fields of exposure. Image quality was found to significantly improve using strict collimation, with observer preference being demonstrated using visual grading characteristic curves.
  
  
  CONCLUSIONS
  The complexities of optimising radiographic techniques have been shown and the data presented emphasise the importance of examining dose-reducing strategies in a comprehensive way.
authors.affiliation: Discipline of Medical Radiation Sciences, Faculty of Health Sciences, University of Sydney, Sydney, NSW, Australia.
authors.forename: R
authors.initials: R
authors.lastname: Powys
authors.forename: J
authors.initials: J
authors.lastname: Robinson
authors.forename: P L
authors.initials: PL
authors.lastname: Kench
authors.forename: J
authors.initials: J
authors.lastname: Ryan
authors.forename: P C
authors.initials: PC
authors.lastname: Brennan
mesh_terms: D000704:Analysis of Variance; D005147:Facial Bones / Q000000981:diagnostic imaging*; D006801:Humans; D007908:Lens, Crystalline / Q000528:radiation effects*; D058958:Organs at Risk / Q000528:radiation effects*; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011859:Radiography; D012015:Reference Standards; D013961:Thyroid Gland / Q000528:radiation effects; D014965:X-Rays