Canis ISSN: 2398-2942

Radiography: contrast media

Contributor(s): Justin Goggin


  • Contrast media are agents used to improve visualization of organs within tissue of similar radiographic opacity.
  • Contast techniques provide detail of organ size, shape, position and internal detail.
  • In some instances subjective assessment of organ function is possible.
  • The ideal contrast agent should be:
    • Inert.
    • Non-toxic.
    • Persist for sufficient length of time.
    • Easily and totally excreted or eliminated from body.
    • Cheap.
    • Have different X-ray absorptive power from tissue of interest.
  • There are disadvantages of and risks associated with all contrast media.

Principles of contrast

  • Contrast on a radiograph is the difference in optical density between areas of the radiograph.
  • The density produced on a radiograph at 50-70 kV is proportional to the atomic number squared of the tissue under examination.
  • Contrast media may be divided intopositive(radiopaque) andnegative(radiolucent) contrast agents.
  • Positive contrast agents have a higher atomic number than tissue, eg:
    • Barium = 56.
    • Iodine = 53.
    • Bone = 14.0.
    • Soft tissue = 7.4.
    • Fat = 5.9.
    • (Lead = 82).
  • Negative contrast agents are relatively radiolucent due to low atomic number and electron density.
  • Before performing any contrast study survey radiographs must be taken to identify lesions that may be masked by contrast administration, eg radio-opaque foreign bodies which may be masked by barium administration.

Types of contrast agent

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Further Reading


Refereed papers