title: Foreign bodies: radiopaque compared to what?
abstract: BACKGROUND
  The term "radiopaque" is commonly used when discussing foreign bodies and lists of radiopaque and non-radiopaque materials are sometimes created, but radiopacity is not a binary concept. A more fundamental understanding of radiopacity is necessary for the work-up of foreign bodies.
  
  
  OBJECTIVE
  To demonstrate that "radiopaque" is a relative term.
  
  
  METHODS
  Twenty foreign bodies of various sizes, shapes and materials were placed in a basin. Radiographs were obtained of the objects with 0, 2.5, 5 and 10 cm depth of water in the basin to simulate the appearance of a foreign body in human tissue. The progressively changing appearance of the objects was observed.
  
  
  RESULTS
  All of the objects were radiographically visible in the empty basin. Different objects demonstrated different appearances when immersed in progressively deeper levels of water, illustrating the effect of the surrounding water as well as the effects of object shape, size, orientation and composition.
  
  
  CONCLUSIONS
  The concept of radiopacity seems simple and the term "radiopaque" is sometimes used to describe an all or none phenomenon. However, radiopacity is a comparative concept implying relative X-ray attenuation. Accurate understanding of the radiographic appearance of foreign bodies requires one to keep in mind the concepts of relative X-ray attenuation and contrast resolution.
authors.affiliation: Department of Radiology, The Children's Hospital of Philadelphia, Philadelphia, PA 19104, USA. halversonm@email.chop.edu
authors.forename: Mark
authors.initials: M
authors.lastname: Halverson
authors.forename: Sabah
authors.initials: S
authors.lastname: Servaes
mesh_terms: D002675:Child, Preschool; D005188:False Negative Reactions; D005547:Foreign Bodies / Q000000981:diagnostic imaging*; D006801:Humans; D008297:Male; D019047:Phantoms, Imaging; D011859:Radiography; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity