CT With Contrast: What to Expect

What is CT contrast?

Contrast medium is a substance used to make particular structures more visible on medical imaging.

For most contrast-enhanced CT examinations, the contrast contains iodine.

Iodine absorbs X-rays strongly.

When iodinated contrast travels through arteries, veins, organs and soft tissues, it changes the way those structures appear on CT.

This allows radiologists to see differences that may be difficult or impossible to identify on an unenhanced scan.

Why do I need contrast?

The answer depends on what doctors are trying to investigate.

Contrast-enhanced CT can help assess:

• —cancer
• —infection
• —inflammation
• —internal bleeding
• —blood vessels
• —pulmonary embolism
• —aortic disease
• —abdominal organs
• —bowel disease
• —trauma
• —complications after surgery.

For example, contrast can help a radiologist determine whether:

• —a mass is solid or cystic
• —a tumour is involving nearby structures
• —an artery is blocked
• —there is active internal bleeding
• —an organ is inflamed
• —a blood clot is present.

Does every CT scan need contrast?

No.

Some CT examinations are deliberately performed without contrast.

Examples can include:

• —coronary artery calcium scoring
• —some kidney-stone CT scans
• —certain CT brain examinations
• —some lung examinations
• —some bone examinations.

Other studies require contrast because the diagnostic question cannot be answered adequately without it.

The decision depends on:

• —what part of the body is being scanned
• —what doctors need to know.

How is contrast given?

For most contrast-enhanced CT scans, a small cannula is inserted into a vein, usually in your arm or hand.

The contrast is then injected using an automatic injector.

The injection may happen relatively quickly because the timing of contrast within the blood vessels and organs is important.

The CT scanner then captures images at a carefully selected time after the injection.

Why does timing matter?

Contrast travels through the circulation in predictable phases.

Radiologists can choose when to scan depending on what they need to assess.

For example:

• —Arterial phase — emphasises arteries and highly vascular structures.
• —Portal venous phase — commonly used for abdominal and cancer imaging.
• —Delayed phase — may provide additional information about certain organs, tumours or the urinary tract.

Some CT examinations therefore involve more than one set of images after contrast.

What will the injection feel like?

This is one of the most common things patients worry about.

When contrast enters the bloodstream you may experience:

• —Warmth — a sudden wave of warmth through your chest, abdomen, pelvis or body.
• —Metallic taste — some people briefly notice an unusual or metallic taste.
• —Feeling as though you have passed urine — this is extremely common. You almost certainly have not actually passed urine; it is simply a temporary warm sensation around the pelvis.

These sensations normally disappear very quickly.

Will the contrast injection hurt?

You may feel the cannula being inserted, or slight pressure or coolness during injection.

The injection itself should not usually be painful.

If you experience significant pain, burning or swelling around the cannula, tell the radiographer immediately.

What is contrast extravasation?

Occasionally, contrast leaks from the vein into the surrounding soft tissues instead of remaining completely within the bloodstream.

This is called contrast extravasation.

It may cause:

• —swelling
• —tightness
• —discomfort
• —tenderness.

Most cases are minor and settle with conservative treatment.

Very large-volume or severe extravasation is uncommon.

Tell the radiographer immediately if the injection site becomes painful or swollen.

Is iodinated contrast safe?

For most patients, yes.

Modern non-ionic iodinated contrast media are widely used and have a strong overall safety record.

Possible complications include:

• —allergic-like or hypersensitivity reactions
• —contrast extravasation
• —kidney problems in susceptible patients
• —thyroid effects in selected patients.

Serious complications are uncommon.

The radiology team assesses whether the diagnostic benefit of giving contrast outweighs the potential risk.

Can I be allergic to CT contrast?

Yes.

Immediate hypersensitivity reactions can occur.

Most are mild.

Possible symptoms include:

• —itching
• —rash
• —hives
• —sneezing
• —nausea.

More serious reactions may cause:

• —facial or throat swelling
• —breathing difficulty
• —low blood pressure
• —severe anaphylaxis.

Serious reactions are rare, but radiology departments that administer intravenous contrast are equipped to recognise and treat them.

What is the biggest risk factor for another contrast reaction?

One of the most important risk factors is having previously experienced a hypersensitivity reaction to iodinated contrast itself.

If this has happened, tell the radiology team:

• —what happened
• —approximately when
• —how severe it was
• —whether treatment was needed
• —if you know which contrast agent was used.

Where appropriate, the department may use a different contrast agent or make other modifications to your examination.

I have a shellfish allergy — can I have CT contrast?

Usually, yes.

This is a common medical myth.

Shellfish allergy is usually caused by proteins such as tropomyosin.

It is not caused by iodine.

Iodine itself is an essential element in the human body and is not an allergen.

Research and professional guidance do not support treating shellfish allergy as a specific contraindication to iodinated contrast.

So:

• —Shellfish allergy does not mean you are “allergic to iodine”.

What about an allergy to Betadine or iodine antiseptic?

Again, that does not automatically mean you will react to CT contrast.

Reactions to:

• —povidone-iodine
• —antiseptic preparations
• —seafood

involve different substances and mechanisms.

The phrase “iodine allergy” should therefore be avoided because it can create unnecessary confusion.

However, always tell the healthcare team about genuine previous allergic reactions so they can assess them properly.

What if I reacted to contrast before?

Do not simply assume you can never receive contrast again.

The team may consider:

• —whether contrast is essential
• —what happened during the previous reaction
• —which agent was used
• —whether an alternative iodinated agent could be used
• —whether specialist allergy assessment is useful
• —whether another imaging test could answer the question.

Recent evidence increasingly supports switching to a different iodinated contrast agent as an important strategy for reducing recurrent reactions in appropriately selected patients.

Routine steroid premedication for every previous reaction is increasingly questioned because the evidence for its effectiveness is limited.

Your radiology team will follow its local contrast-safety policy.

Can contrast affect my kidneys?

This needs a little explanation.

For many years, iodinated contrast was thought to cause acute kidney injury relatively frequently.

Modern controlled studies suggest that the kidney risk attributable specifically to intravenous CT contrast has often been overestimated, because people requiring contrast-enhanced CT are often unwell for other reasons that can also affect kidney function.

Doctors therefore distinguish between:

• —Contrast-associated acute kidney injury — kidney function worsens after contrast administration, but contrast may or may not actually be the cause.
• —Contrast-induced acute kidney injury — contrast itself is believed to have directly caused the deterioration.

The second is more difficult to prove and appears substantially less common than historically assumed.

Who is at greater kidney risk?

Kidney function deserves particular consideration in people with:

• —severe chronic kidney disease
• —acute kidney injury
• —rapidly deteriorating kidney function
• —certain combinations of medical illness and dehydration.

Many modern guidelines use an eGFR around 30 mL/min/1.73 m² as an important threshold at which the balance of risk and benefit deserves closer consideration.

That does not mean everyone below that value is forbidden from receiving contrast.

Sometimes the diagnostic benefit is far more important than the potential kidney risk.

Will I need a kidney blood test?

Not everyone does.

Your department may ask about:

• —known kidney disease
• —diabetes
• —previous kidney problems
• —medication.

If risk factors are present, an eGFR may be checked before an elective contrast-enhanced CT.

RCR quality standards require imaging services to have processes for identifying patients at increased renal risk and obtaining kidney-function measurements when appropriate.

Emergency imaging is different.

Emergency imaging is different

If contrast CT is urgently needed to diagnose a potentially serious condition, current RCR and Royal College of Emergency Medicine guidance says that waiting for kidney-function blood results should not routinely delay emergency contrast CT when that delay could harm the patient.

What is eGFR?

eGFR stands for estimated glomerular filtration rate.

It is calculated from a blood test and gives an estimate of how effectively your kidneys are filtering blood.

A lower eGFR generally indicates reduced kidney function.

But the number always needs to be interpreted in clinical context.

What if I only have one kidney?

Having one kidney does not automatically mean you cannot receive contrast.

The important issue is how well that kidney functions.

Some people with one kidney have completely normal kidney function.

Tell the radiology team about your medical history so they can decide whether a blood test is required.

I take metformin — do I need to stop it?

Do not stop metformin yourself simply because you are having a CT scan.

Most people taking metformin do not automatically need to stop it for routine contrast-enhanced CT.

The important issue is whether there is severe renal impairment, acute kidney injury or deteriorating kidney function.

Policies vary between hospitals.

In higher-risk situations, you may be given specific instructions to temporarily stop metformin and restart it after kidney function has been reassessed.

Tell the radiology team that you take metformin and follow their instructions.

Current evidence suggests that continuing metformin around contrast exposure in appropriately selected patients does not itself appear to increase acute kidney injury or metabolic-acidosis risk.

Should I drink lots of water after contrast?

If you normally drink fluids freely, normal hydration afterwards is reasonable.

You do not need to force yourself to drink excessive amounts of water in an attempt to “flush out” the contrast.

If you have:

• —heart failure
• —kidney failure
• —a fluid restriction

follow your healthcare team's advice rather than deliberately increasing your fluid intake.

How does contrast leave the body?

For most people, intravenous iodinated contrast is removed largely through the kidneys and passed into the urine.

I have thyroid disease — does it matter?

Tell the radiology team if you have a thyroid condition.

Iodinated contrast contains a large iodine load.

Most people tolerate this without a problem.

However, in susceptible people it can occasionally trigger changes in thyroid function, including:

• —hyperthyroidism
• —hypothyroidism.

Risk is greater in some people with pre-existing thyroid disease.

This does not mean people with thyroid disorders cannot have contrast CT.

It means the clinical team may need to consider the circumstances more carefully.

What if I am due to have radioactive iodine treatment?

This is particularly important.

Iodinated contrast can temporarily interfere with the uptake of radioactive iodine by the thyroid.

If you are:

• —undergoing thyroid-cancer treatment
• —due to receive radioactive iodine
• —having specialist thyroid investigations

tell both the radiology and thyroid teams.

The timing of your CT may need to be considered.

What if I am pregnant?

Tell the radiology team if you are pregnant or think you may be pregnant.

CT uses ionising radiation, so the appropriateness of the scan depends on:

• —which part of the body is being examined
• —how urgent the clinical question is
• —whether the fetus would be exposed
• —whether another test could provide the same information.

Iodinated contrast can cross the placenta.

It may still be given during pregnancy when the diagnostic benefit justifies its use.

In urgent situations, delaying an important CT scan can itself carry significant risk.

The decision is therefore individual.

Can I breastfeed after CT contrast?

Yes.

Current Royal College of Radiologists and Society of Radiographers guidance states that patients who wish to continue breastfeeding after iodinated contrast should be able to do so.

Routine interruption of breastfeeding or “pumping and dumping” is not required after iodinated CT contrast.

Only a very small amount enters breast milk, and very little of that is absorbed by the baby's gut.

Do I need to stay in hospital afterwards?

Often only briefly.

Current NHS patient guidance says you may be asked to remain in the department for up to around 30 minutes after intravenous contrast so staff can ensure you are well before you leave.

Local practice varies.

Most outpatients can then go home.

Can reactions happen later?

Yes, although most important acute reactions occur relatively soon after injection.

Some people experience delayed symptoms, particularly skin reactions, hours or days later.

These may include:

• —rash
• —itching.

If you develop a significant reaction after leaving hospital, seek appropriate medical advice.

For severe symptoms such as difficulty breathing, facial or throat swelling or collapse, seek emergency help immediately.

Does contrast stay in my body forever?

No.

For people with normal renal function, most iodinated contrast is cleared from the bloodstream through the kidneys and excreted in the urine.

It does not permanently dye your organs.

Does CT contrast contain radioactive material?

No.

The contrast itself is not radioactive.

The radiation involved in CT comes from the X-rays produced by the scanner.

Contrast simply changes how strongly different structures absorb those X-rays.

Can I refuse contrast?

Yes.

You should be able to ask why contrast is being recommended and discuss any concerns.

Useful questions include:

• —Why does this particular CT need contrast?
• —What would the scan show without it?
• —Do I have any increased risk from contrast?
• —Do I need kidney-function testing?
• —I reacted previously — what precautions are needed?
• —Is there an alternative imaging test?

However, declining contrast can sometimes significantly reduce the usefulness of the examination.

For some clinical questions, a non-contrast CT may simply not provide the information your doctor needs.

Why did my previous CT not use contrast?

Because CT protocols are designed around specific clinical questions.

For example:

• —Kidney stones — often assessed using a non-contrast CT.
• —Pulmonary embolism — requires carefully timed intravenous contrast to show the pulmonary arteries.
• —Cancer staging — often requires intravenous contrast to assess organs, tumours and lymph nodes.
• —Coronary calcium score — usually performed without intravenous contrast.
• —CT coronary angiography — requires intravenous contrast to show the coronary arteries.

So:

The same person can appropriately have one CT without contrast and another CT with contrast.

Five things to remember

• —The warm flush and “I feel like I've peed” sensation are common and usually disappear within seconds.
• —Shellfish allergy does not mean you are allergic to CT contrast.
• —The most important allergy history is a previous reaction to iodinated contrast itself.
• —Kidney risk is particularly relevant in severe kidney disease or acute kidney injury — not simply because somebody has ever had an abnormal kidney blood test.
• —Breastfeeding does not normally need to be interrupted after iodinated contrast.

Questions to ask before your scan

If you are worried about CT contrast, consider asking:

• —Why is contrast necessary?
• —Have my kidney results been checked if needed?
• —I take metformin — should I do anything differently?
• —I have thyroid disease — is that relevant?
• —I reacted to contrast before — what happens this time?
• —I am pregnant or breastfeeding — does that change the plan?

The key message

Iodinated CT contrast allows radiologists to answer clinical questions that a non-contrast scan sometimes cannot answer.

Modern contrast agents are generally very safe.

The decision to use contrast is based on balancing:

• —the small potential risks of the injection
• —against the risk of missing or delaying an important diagnosis without it.

For the vast majority of appropriately selected patients, the diagnostic benefit is substantially greater than the risk.