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

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 (1), 2-12 VetMedResource.