A contrast scan is a medical imaging test, such as a CT (computed tomography) scan or MRI (magnetic resonance imaging), that uses a special dye called a ‘contrast agent’ to make certain areas of the body easier to see. The contrast helps highlight blood vessels, organs, or abnormal tissues, providing clearer and more detailed images.

Why is it used?

The contrast agent shows the blood flow through the arteries and veins, blockages, bleeding or abnormal growths, and detailed organ structure, for instance of the brain, liver, or kidneys.

In short, contrast helps to highlight differences between normal and abnormal tissue, improving diagnosis and treatment planning.

How is it administered?

The contrast agent is usually injected into a vein, but in some cases, it can be swallowed or given as a rectal enema, depending on the area being examined. It temporarily changes the way radiation or magnetic fields interact with the body’s internal structures.

Is there an iodine allergy risk in a contrast scan?

This is a common concern, but it’s a bit misunderstood.

People often believe they are allergic to iodine because they may have reacted to contrast dye in the past or to shellfish, which contain iodine. However, iodine itself is not an allergen. According to radiologists and allergists, the body doesn’t mount an allergic immune response to iodine as it’s a basic element, essential to human health, particularly for thyroid function.

What causes allergic reactions in contrast scans?

The culprits are usually one of the other compounds, not iodine. Most contrast agents used in CT scans are iodinated contrast agents; however, reactions tend to be linked to the chemical structure of the compound, not its iodine content.

Reactions may range from mild (nausea, itching, flushing) to more serious (difficulty breathing or anaphylactoid reactions), which mimic allergies but do not involve the immune system in the same way.

These reactions are typically caused by:

• Concentration of the contrast agent.
• Molecular structure (ionic vs non-ionic).
• Patient-specific factors such as a history of allergies, asthma, or previous reaction to contrast.

Note: For MRI scans, a different type of contrast is used, which is gadolinium-based, and, while allergic reactions are possible, they are extremely rare.

Advancements in the type of contrast agent used have significantly reduced the rate of reactions in patients. Still, patients are routinely asked if they had any previous allergic reactions to contrast agents as part of the safety checks before a contrast scan. Radiology departments and their staff are also equipped and well trained to respond swiftly and competently when a patient does have a reaction.

In conclusion

To confirm: It’s not the iodine, it’s the other ingredients in the dye, and the body’s unique response to them. That is why patients are asked about previous contrast reactions, asthma, or other allergies before being given the contrast injection. Whether you are asked or not, it’s always best to inform the radiology team if you have had any previous allergic contrast reactions.

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• This article is part of a series aimed at decoding radiology jargon for patients. Visit our news section for more.
• The article is intended to create awareness and none of the information it contains should replace the need for seeking personal advice from a qualified medical practitioner.
• Apart from selected screening examinations, all radiology procedures are performed upon referral from a qualified medical professional.