Barium and iodinated contrast media

See BSAVA Guide to Procedures in Small Animal Practice

MRI 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) 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 intravenous administration of gadoxetic acid have been reported in humans. Many contrast agents are hyper-osmolar and irritant if extravasation occurs (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), and therefore should be given through an intravenous catheter. Anaphylaxis occurs rarely (0.001% - 0.01% in human studies). 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. May cause fetal abnormalities in rabbits.

Drug interactions: May have interactions with Class 1a and 3 antiarrhythmics. Gadobenic acid may compete for cannalicular multispecific organic anionic transporter sites. Caution should be used if administering anthracyclines, vinca alkyloids 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, e.g. serum iron determination using complexometric methods, transient increase in liver enzymes, 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 poor contrast enhancement with standard dose or for detection of metastases and if using low-field scanner. Enhancement visible up to 45–60 minutes post-administration.