Barium contrast media

Barium contrast media appear radiopaque on a radiograph due to barium sulphate having a higher atomic number than soft tissues. Barium sulphate is inert with no osmotic potential and is not absorbed or acted upon by alimentary secretions.

There is a wide variety of barium sulphate preparations used in veterinary radiography and some of the preparations available at the time of writing are shown in the table below. None of these preparations is authorized for veterinary use and most are POM.

TABLE: Preparations of barium sulphate used in veterinary radiography.

Options:

• Barium paste is primarily used to highlight abnormalities of oesophageal mucosa.
• A barium meal (barium paste added to food) is best for identifying functional abnormalities.

Use:

• Barium contrast media are used for studies of the GI tract.
• Barium-impregnated polyethylene spheres (BIPS) may be used to assess intestinal transit times and gastric emptying rate, as well as to detect obstructions.
• As with all contrast studies, plain radiographs should be obtained prior to the administration of barium.
• Barium paste is used for oesophageal studies as it adheres to the mucosa. Barium can also be mixed with food to evaluate the oesophagus and may demonstrate strictures or dilatations not seen with liquid barium or paste.
• Liquid barium may be used to evaluate any part of the GI tract and is used alone for evaluation of the stomach and intestines. Flavoured preparations designed for human use may be unpalatable for some animals.

Safety and handling:

• Skin irritation may occur with skin contact.
• Take care not to spill barium on the patient’s coat; any spills should be carefully cleaned off.

Contraindications and complications:

• Barium is insoluble and should not be used outside the GI tract.
• A barium study should not be performed <24 hours before endoscopy.
• Leakage of barium into body cavities may lead to granulomatous reactions or adhesions.
• If perforation of the GI tract is suspected, low-osmolar water-soluble contrast media, rather than barium media, should be used.
• Care should be taken when administering barium to dysphagic animals, distressed patients, or those with swallowing disorders or laryngeal paralysis, as aspiration of barium into the bronchi and lungs may occur and can result in aspiration pneumonia.
• If inappropriately sized BIPS are used, mechanical obstruction of the GI tract can occur (seen primarily in cats and very small dogs).
• May cause constipation, transient diarrhoea and abdominal pain.

Iodinated contrast media

Iodinated contrast media appear radiopaque on a radiograph due to iodine having a higher atomic number than soft tissues. Water-soluble iodinated contrast media are available in ionic and non-ionic forms, and a wide range is available. The more commonly used forms are detailed in the table below. None is authorized for veterinary use and most are POM.

TABLE: Commonly used iodinated contrast media

Use:

• Iodinated contrast agents are used for:
  • Intravascular studies (e.g. upper urinary tract) and angiography of the cardiovascular and portal systems
  • Lower urinary tract
  • Joints (arthrography)
  • Salivary glands (sialography)
  • Lacrimal sac and duct
  • Investigation of peripheral sinuses and fistulae
  • Myelography
  • Lymphangiography.
• Iodinated contrast media can be used to evaluate the GI tract where perforation and leakage are potentially present and hence the use of barium is contraindicated. In practice, alternative methods (e.g. positional radiography, ultrasonography or CT) to demonstrate leakage should be considered first.
• In veterinary practice, ionic agents are mainly used for cardiovascular and urinary tract studies. Ionic agents should not be used for myelography.
• In veterinary practice, non-ionic agents are mainly used for myelography and arthrography. They are also used in cardiovascular and urinary tract studies, where the status of the patient dictates their use and ionic media are contraindicated.

Contraindications:

• Known or suspected hypersensitivity to iodinated contrast media.
• Should be used with care in animals with renal failure. Iodinated contrast media are primarily excreted via the kidneys and pre-existing renal disease predisposes to contrast media-induced renal failure.
• Anuria.
• Dehydration.
• Hypovolaemia.
• Hypotension.

Adverse reactions: Contrast reactions can be divided into non-idiosyncratic reactions, which are dose dependent, and idiosyncratic reactions, which are independent of the dose administered.

Non-idiosyncratic reactions: Associated with the hyperosmolality and ionicity of the iodine-containing contrast agents and, therefore, are more common and severe with ionic agents than non-ionic agents. Reactions may be acute (occur within 5–10 minutes following administration) or delayed and include:

• Contrast-induced nephrotoxicity
• Cardiovascular effects including bradycardia, hypertension and hypotension
• Nausea and vomiting
• Neurotoxicity
• Irritation following extravasation
• Bronchospasm and skin reactions may occur in patients with known allergic disease.

Idiosyncratic reactions: Represent anaphylactic or allergic-type reactions, as antibodies to contrast agents have not been demonstrated. Reactions are evident immediately following i.v. administration and present as:

• Severe respiratory signs associated with laryngeal oedema, bronchospasm and pulmonary oedema
• Cardiovascular collapse (hypotension, decreased cardiac output)
• Urticaria.

Drug interactions:

• Iodinated contrast media decrease thyroid uptake of iodine and preclude therapeutic radioiodine therapy for 2 months following administration.
• Hypersensitivity reactions may be aggravated in patients receiving beta blocker medication.
• Metformin can cause lactic acidosis in patients with renal impairment.

Magnetic resonance imaging contrast media

Several gadolinium chelates are used for magnetic resonance imaging (MRI) contrast studies. None of them is authorized for veterinary use and all are POM.

Action: Gadolinium is a paramagnetic agent and exerts its effects due to seven unpaired electrons, which cause a shortening of T1 and T2 relaxation times of adjacent tissues. This results in increased signal intensity on T1-weighted MR images. Unbound gadolinium is highly toxic and so is chelated to reduce toxicity. Gadolinium chelates do not cross the normal blood–brain barrier due to their large molecular size.

Use:

• During MRI examination to identify areas of abnormal vascularization or increased interstitial fluid, delineate masses and demonstrate disruption of the blood–brain barrier and areas of inflammation.
• Gadodiamide, gadobutrol, gadoteric acid and gadobenic acid are also used for contrast-enhanced magnetic resonance angiography (MRA).
• The low concentration form of gadopentetic acid (2 mmol/l) is authorized for intra-articular use for MR arthrography in humans.
• Gadobenic acid and gadoxetic acid are also transported across hepatocyte cell membranes (gadoxetic acid via organic anionic-acid transporting peptide 1) and are used in the characterization of liver lesions.

Safety and handling: Contact with skin and eyes may cause mild irritation.

Contraindications:

Use with caution in cardiac disease, pre-existing renal disease and neonates. Contraindicated in severe renal impairment.

Adverse reactions: Nephrogenic systemic fibrosis (most commonly associated with gadodiamide but also gadopentetic acid and gadoversetamide) has been reported in humans but not in animals. Increase in QT interval and other arrhythmias have also been reported, and cardiac monitoring is recommended in the event of accidental overdosage.

Transient episodes of shortness of breath following i.v. administration of gadoxetic acid have been reported in humans. Many contrast agents are hyperosmolar and irritant if extravasation occurs and therefore should be given through an i.v. catheter (although studies in animals have shown gadopentetic acid and gadoteridol to be less toxic to subcutaneous tissues than an equal volume of iodinated contrast media). Anaphylaxis occurs rarely (0.001%–0.01% in human studies). MRI contrast agents may cause a transient increase in serum bilirubin if there is pre-existing hepatic disease. Retained gadolinium deposits in certain areas of the brain have been reported recently in human patients – the clinical significance of these deposits is unknown at present.

Drug interactions: MRI contrast agents may have interactions with Class 1a and 3 antiarrhythmics. Gadobenic acid may compete for canalicular multispecific organic anionic transporter sites. Caution should be used if administering anthracyclines, vinca alkaloids and other drugs using this transporter. Anionic drugs excreted in bile (e.g. rifampicin) may reduce hepatic uptake of gadoxetic acid, reducing contrast enhancement. May affect some laboratory results, such as serum iron determination using complexometric methods, may cause a transient increase in liver enzymes, and some linear non-ionic gadolinium-based contrast media (gadodiamide, gadoversetamide) may cause false reduction in serum calcium measurement.

Doses and further advice on use: Should be used routinely for MRI examinations of the brain. Use for MRI of other body regions if abnormal vascularization, inflammation or neoplasia is suspected, for postsurgical evaluation or if MRI study is normal despite significant clinical signs.

Post-contrast images should ideally be obtained within 30 minutes of contrast administration. Total doses should not exceed 0.3 mmol/kg (varies with product).

Dogs: 0.1 mmol/kg i.v. (all except gadoxetic acid). Give as bolus if performing MRA, dynamic contrast or liver studies: 0.025 mmol/kg i.v. bolus of gadoxetic acid for liver studies; 0.05 mmol/kg of gadobenic acid for liver studies. Repeat doses (not gadoxetic acid) of up to 0.3 mmol/kg total dose may be helpful in some cases if contrast enhancement is poor with a standard dose or for detection of metastases if using low-field scanner. Enhancement is visible up to 45–60 minutes post administration.

TABLE: MRI contrast media.