Contributors: Justin Goggin
Species: Canine | Classification: Miscellaneous
- 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
Negative contrast agents
Ionic water soluble
- Oxygen, air, carbon dioxide and nitrous oxide.
- Negative contrast agents can be used for:
- Do not give as much mucosal detail as positive or double contrast studies.
- Fatal air embolus has been reported following pneumocystography and urethrography.
Nitrous oxide and carbon dioxide are probably safer due to higher solubility in serum
- Oxygen is a fire risk.
- Readily available.
Positive contrast agents
- Provide contrast from tissues by virtue of high atomic number.
- Give better contrast and mucosal detail than negative contrast agents.
- There are two readily available types of contrast agent, ie iodine-based and barium-based.
- Used primarily for contrast examination of the gastrointestinal tract as not metabolized, soluble or sterile.
- Historically used for cystography and bronchography although now superseded by safer iodine based water-soluble agents.
- Available in several forms:
- Colloidal suspension gives good mucosal detail.
- No osmotic effect therefore radiopacity persists.
- Relatively palatable.
- Aspiration pneumonia
- If leaks into body cavities or organs may persist indefinitely and can cause granulomatous reactions.
Iodine based preparations
- Divided into four types.
Ionic water soluble
- Imaging of:
- Dissociate in solution and unsuitable for myelography as ionic charge is neurotoxic.
Must use non-ionic water-soluble contrast agents for myelography
- High osmotic pressure 5-7 times body fluids which can cause adverse reactions:
- Circulatory effects, eg hypotension.
- Palpitations and ECG changes.
- Extravascular irritation.
- Contraindicated if cardiovascular or renal insufficiency.
- Renal toxicity.
Many of these potential side effects can be avoided by performing studies under general anesthesia,and assuring the patient is adequately hydrated prior to the procedure.
- Water soluble and intravenous administration possible.
- Excreted by kidney therefore can be used for urographic studies.
Non-ionic water soluble
- Latest agents are iso-osmolar with plasma.
- Advantages- a number of advantages over ionic water-soluble compounds:
- No ionic charge therefore can be used for myelography Radiography: myelography.
- Lower osmolarity therefore safer for cardiographic, renal studies and in neonates.
- Fewer side effects.
- More expensive.
- Iodinized oils which do not mix with water.
- Mainly of historical interest. Rarely used clinically.
- Used for imaging:
- Lymphatic system.
- Sinus tracts.
- Salivary glands.
- Viscous agents are no longer available but were used in bronchography.
Agents excreted in the bile
- Chylocystopaquescan be administered intravenously.
- Contrast agents with high protein-binding capacity, eg meglumine ioglycomate are excreted via the biliary system.
- Used to image:
- Gall bladder.
- Bile ducts.
Oral preparations are not suitable for animal use as have variable absorption and excretion patterns