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Contrast Agents in Veterinary Radiology

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Liz is a licensed veterinary medical technologist. She acquired a B.S. in veterinary medical technology from Lincoln Memorial University.

There are three main categories of contrast agents used in veterinary medicine: iodinated, barium sulfate, and negative contrast. Iodinated contrasts are further subdivided into ionic and non-ionic. Negative contrasts consist of room air, CO2 (carbon dioxide), and NO (nitric oxide).

Ionic Iodinated Contrasts

Examples of ionic iodinated contrast agents include Conray, Hypague, and Renografin. They can be used in most soft tissue cases, but are contraindicated in central nervous system studies. These substances are salts, and therefore in a solution, exist as a positive and a negative particle. There are various mixtures available, and some may come with sodium as the sole cation, meglumine as the sole cation, or as a mix of the two. Agents that contain both sodium and meglumine salts may be the best for multipurpose use. Concentrations are expressed as percentages that refer to the salt concentration. The dose given to the animal is based upon the amount of iodine the solution contains, which should be indicated on the label. For intravenous use, generally 400mg/pound is given, with a maximum dose of 35g. In intravenous cases, full strength contrast is generally used. In other cases such as cystograms, arthrograms, or fistulograms, the contrast is often diluted to 25% to 50% for two reasons; the diluted contrast is less irritating to the tissues, and to improve visualization as the contrast will not be so opaque.

Non-Ionic Iodinated Contrasts

Examples of non-ionic iodinated contrast agents include Iohexol and Iopamidol. They are similar to ionic agents, except that they do not dissociate in a solution, and are therefore less osmotic. In addition to soft tissue cases, non-ionic agents can also be used for myelography; however, these agents are more expensive than their ionic cousins. The myelography dose for non-ionic contrast is 0.3mg/kg (with a maximum dose 0.45ml/kg), with a gastrointestinal dose of 10ml/kg after diluting 50:50 with water.

Radiograph of diffuse accumulation of Iohexol in the kidneys of a cat with CNE.

Radiograph of diffuse accumulation of Iohexol in the kidneys of a cat with CNE.

Barium Sulfate Contrast

Barium sulfate contrast agents may come as a suspension or a paste. They are used only for gastrointestinal studies, are inert, and are not absorbed by the GI tract. Barium U.S.P. is not recommended as it requires mixing, does not form a uniform solution, and tends to settle before administration is complete. Paste is best for esophagrams as it coats the esophagus and remains there for some time. Suspension is best for all-purpose use and can be purchased at 74% weight/volume. It is best to dilute to 37% weight/volume by mixing 50:50 with water. The suggested dose here is 6ml/lb.

Negative Contrast Agents

Negative contrast agents can be used in the gastrointestinal tract, the bladder, the joints, and in body cavities. Room air is not recommended in the bladder or joints, (especially if they are inflamed) due to the risk of air embolism. For this same reason, CO2 or NO are recommended for such studies as they are readily soluble in blood, therefore decreasing the risk of embolism.

Side Effects of Contrast

In general, more side effects are associated with ionic rather than non-ionic iodinated contrast agents. Common side effects of iodinated contrast agents include anaphylaxis, vomiting or nausea, pain at the injection site, bradycardia, hypotension, cardia arrhythmias, nephrotoxicity, diarrhea, or sometimes sudden death. Each patient must be assessed individually before contrast administration. When using iodinated contrast agents, it is advisable to place an intravenous catheter, monitor parameters closely, have an ECG machine available, and have a crash cart nearby. Certain risk factors may increase the possibility of these side effects, such as dehydration, allergies or hypersensitivity to the contrast agent, cardiac disease, diabetes, multiple myeloma, hyperproteinemia, phaeochromocytoma, and renal disease.

There are minimal side effects associated with barium administration, besides mild constipation. Barium should not be administered if a gastrointestinal perforation is suspected, as free barium in the peritoneal cavity may cause granulomas. In these cases, iodinated contrast agents should be used.

Radiologic Contrast Dosages and Side Effects

Contrast TypeDosageSide Effects

Ionic Iodinated

For IV use, generally 400mg/lb. Maximum dose- 35g. In other cases, dilute 25-50%.

Anaphylaxis, vomiting or nausea, pain at the injection site, bradycardia, hypotension, cardia arrhythmias, nephrotoxicity, diarrhea, sudden death.

Non-Iodinated

Myelography dose- 0.3mg/kg, maximum dose- 0.45ml/kg. GI dose- 10ml/kg after 50:50 dilution with water.

Same as ionic agents, but with reduced risk.

Barium Sulfate

6 mL/lb after diluting to 37% weight/volume.

Mild constipation. Granulomas if GI perforation occurs.

Negative Contrast

To effect; case-by-case basis.

Air embolism.

General Guidelines for Contrast Studies

  • Have the area of interest as clean and free of debris as possible.
  • Always acquire survey films prior to the contrast study, and have all necessary equipment gathered prior to starting the study.
  • Educate yourself concerning any risk factors that may affect the patient.
  • Properly label all films with the time of acquisition, and keep the cassettes, table, and the animal free of contrast material.
  • Notify the veterinarian of any concerns.

Sources

  • Notes from University of Tennessee College of Veterinary Medicine radiology externship course
  • Personal experience as a veterinary technologist

© 2018 Liz Hardin

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