Species: Feline   |   Classification: Miscellaneous

Introduction

  • Contrast media are agents used to improve visualization of organs within tissue of similar radiographic contrast.
  • Contast techniques provide detail of organ size, shape, position and internal detail.
  • In some instances assessment of organ function may be permitted.
  • The ideal contrast agent should be:
    • Inert.
    • Non-toxic.
    • Persist for sufficient length of time.
    • Easily and totally excreted from body.
    • Cheap.
    • Have different 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 proportion to the atomic number squared of the tissue under examination.
  • Contrast media may be divided intopositiveandnegativecontrast 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 specific gravity.
  • Before performing any contrast study plain radiographs must be taken to identify lesions that may be masked by contrast administration, eg radioopaque foreign bodies which may be masked by barium administration.

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.

    Carbon dioxide is 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 or excreted.
  • Historically used for cystography and bronchography although now superceded by safer iodine based water soluble agents.
  • Available in several forms:
    • Colloidal suspension (used for GI examinations).
    • Paste (used for esophageal studies especially if looking for mucosal abnormalities).
    • BIPS (used to assess GI motility and for detection of GI obstruction).
    • Powder.
    • Enema kits.
  • Advantages:
    • Insoluble.
    • Inert.
    • Gives good mucosal detail.
    • No osmotic effect therefore radiodensity persists.
    • Inexpensive.
    • Relatively palatable.
  • Disadvantages:
    • Aspiration pneumonia if 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.
    • Non-ionic water soluble.
    • Oily agents.
    • Agents excreted in the bile.

Ionic water soluble

Uses

  • Imaging of:
    • Cardiovascular system.
    • Urinary system.
    • Joints.
    • Salivary glands.
    • Tear ducts.
    • Fistulas/sinuses.
    • Gastrointestinal tract.

Low osmolar ionic water soluble agents safer if poor cardiac or renal function.

  • 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.
    • Palpitations and ECG changes.
    • Nausea/vomiting.
    • Urticaria.
    • Anaphylaxis.
    • Extravascular irritation.
    • Contraindicated if cardiovascular or renal insufficiency.

Many of these potential side effects can be avoided by performing studies under general anesthesia.

  • 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.
  • 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 aborption and excretion patterns.