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:
- Persist for sufficient length of time.
- Easily and totally excreted or eliminated from body.
- 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
This article is available in full to registered subscribers
Sign up now to purchase a 30 day trial, or