Contributors: Justin Goggin

 Species: Canine   |   Classification: Miscellaneous

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

Negative contrast agents
  • Oxygen, air, carbon dioxide and nitrous oxide.
  • Negative contrast agents can be used for:
  • Disadvantages:
    • 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.
  • Advantages:
    • Readily available.
    • Cheap.

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.

Barium

  • 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:
  • Advantages:
    • Insoluble.
    • Inert.
    • Colloidal suspension gives good mucosal detail.
    • No osmotic effect therefore radiopacity persists.
    • Inexpensive.
    • Relatively palatable.
  • Disadvantages:
    • Aspiration pneumonia
    or loss of lung function if large volume aspirated.
  • 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

Uses

  • 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.
    • Nausea/vomiting.
    • Urticaria.
    • Anaphylaxis.
    • 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.
  • Advantages:
    • 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.
  • Disadvantages:
    • More expensive.

Oily agents

  • 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.
    Rarely used clinically due to poor opacification of biliary system in icteric patients.
    Oral preparations are not suitable for animal use as have variable absorption and excretion patterns