Equis ISSN 2398-2977

Radiography: contrast media

Contributor(s): Sarah Freeman, Justin Goggin

Introduction

  • 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

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Dennis R & Herrtage M E (1989) Low osmolar contrast media - a review. Vet Radiol 30, 2-12.


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