CT scans offer quick and high-quality imaging of the body, also known as computerised tomography or computed tomography. Sometimes, an iodine-based contrast agent is needed to enhance the images of certain parts of your body. This simple guide will help you to understand what a contrast agent is, why you might need one, and how they work.

Generally, CT scans are suitable for most patients and the potential risks from radiation are low, and carefully considered. There are some negative risks associated with CT scans with contrast media, compared to scans without. These can include mild side effects, allergic reactions, and even some kidney issues for those with mild baseline kidney disease. The safety of contrast agents will also be covered.

What is contrast dye for a CT scan, and why do I need it?

Intravenous contrast, also called a contrast agent, contrast media, or contrast dye, is a colourless liquid that is given to you by injection during a medical scan procedure. It helps to highlight and define specific areas of your body from the surrounding tissues. This can help your radiologist (a specially trained doctor who interprets medical images) to diagnose illnesses, as contrast agents make certain internal organs, blood vessels and tissues show up more clearly.

Not all medical imaging scans require contrast agents, but CT imaging tends to require it more often than MRI. It depends on the parts of the body being scanned and the purpose of the imaging exam. Your clinician will be able to help you understand whether you will need to have a contrast injection.

What contrast medium is given for CT scans?

Iodinated contrast materials are used for CT scans. Iodine is a substance that blocks X-rays from passing through your body as easily. This means that when iodine is in your blood vessels, organs and tissues, these areas will appear brighter in your CT scan images.

Intravenous contrast simply means you will be given a contrast media injection, usually into your arm. Oral contrast means you will be asked to drink the contrast material.

The contrast material for CT scans differs to MRI scans, which tend to use Gadolinium-based contrast agents.

Who will administer my contrast agent?

During your scan, you will be asked to lie on a flat bed, which enters into the CT scanner. You will usually need to have one set of images taken without contrast material, and another set taken after it has been administered. This helps the radiologist to compare the images and spot any anomalies.

Usually, your radiographer or radiologist will set up your IV, and administer the contrast agent. They might set up the IV before your first set of images, even if it will be administered during the second set. They will tell you when they are ready to administer the contrast media, so that you will know when to expect it.

Is an iodine contrast agent safe?

The majority of scans do not have any adverse effects due to contrast media exposure. However, it is important to note that some risk factors could cause complications in the following cases:

If you are pregnant
- In some cases, the benefits of having a scan can outweigh the risks to an unborn baby. But, as CT scans use radiation, and contrast agents can have side effects, it is important to disclose whether you're pregnant, or think you might be. It could be that your clinician advises you to wait to have a scan until after your baby is born.
- It is also worth noting that the Royal College of Radiologists advises that breast feeding after having iodine based contrast media is safe, and no additional precautions are needed. You can continue breast feeding as normal.
If you have kidney problems
- People with impaired kidney function or severe chronic kidney disease are at risk of acute renal failure occurring. This is because contrast material is removed from the body by the kidneys. People with normal kidney function are able to get rid of the contrast dye more easily without side effects.
If you have asthma
- Sometimes, contrast agents can cause shortness of breath or wheezing, which you may be more susceptible to if you have asthma. Let your clinician know if you have asthma.
If you have allergies
- In rare cases, contrast agents can cause a severe allergic reaction. If you're allergic to any foods, drugs, preservatives or even animals, you may have a slightly increased risk of a reaction. Make sure you let your clinician know if you suffer from allergies, or have had previous allergic reactions to contrast media.

What are examples of CT scan dye side effects?

There are some common contrast reactions that are nor dangerous and pass quickly after the scan has been completed. In rare cases, more serious reactions can occur.

Mild reactions or side effects can include:

A warming feeling
A metallic taste in your mouth
Feeling like you need to urinate
Nausea and headache
Skin flushing, a mild rash, or hives

Some more serious side effects can include:

Wheezing, difficulty breathing, or shortness of breath
Abnormal heart rhythms
Low blood pressure
Swelling of the throat
Allergic (anaphylactic) reactions - either moderate or severe reaction

If you do experience an adverse reaction, the medical team at your selected scanning centre will be on-hand to help you, and are highly trained to recognise and treat contrast reactions.

What contrast media preparation do I need to do?

If you have been told that your scan will involve administering IV contrast agent, you will be charged an additional £150 to cover the costs.

You will also be asked to provide recent blood test results, dated within the last 3 months, which show your kidney function, specifically EGFR and sufficient serum creatinine data. This is to ensure it is safe for you to have the contrast during your scan.

If you have not had a blood test within the last 3 months, you can get this arranged with your GP or a private clinic. Please send us the results and we will send this to the scanning company. Please note that your order will need to be paused until we have received your blood test results as a matter of your safety.

Before having your scan, you will be given special instructions about whether you should avoid eating, or if you can eat and drink normally, either by an appointment letter or phone call. If you can drink before your scan, it is recommended that you stay hydrated before and after your scan. If you drink plenty of water, it will help to flush out the contrast material from your system.

As with any CT scan, you might be asked to change into a hospital gown and remove metal objects such as watches and jewellery to ensure your CT images are clear and unobstructed.

How quickly does contrast material leave my body?

After your scan, you may be asked to wait in the scanning centre for 45 minutes, so that the medical team can monitor you and make sure you won't have an adverse or allergic reaction to the contrast material. If you want to drive after having contrast dye, you may be advised to wait one hour from the time you received the contrast medication.

After your imaging exam, your kidneys should flush the substance out of your system within a couple of days.

What should I do if I have severe/chronic kidney disease?

We ask for blood test results to check whether you might be at risk of kidney problems if you have an iodinated contrast media injection.

Sometimes, even if your results show reduced kidney function, having the scan will still outweigh the risks of making it worse. However, you should discuss this with your clinician, as an alternative imaging method such as MRI, or a non-contrast scan, may be suggested for your safety.

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A Guide To Contrast Media for CT Scans: When They’re Needed, and What To Expect