Contrast agents are an essential part of modern medical imaging. This guide highlights essential principles any doctor should know about contrast – how it works, when to use it, and how to manage risks – so you can make safe clinical decisions and answer patients’ questions with confidence.
Contents
What is contrast?
Contrast refers to a group of chemical agents used in imaging to aid in the characterisation of pathology. In simple terms, contrast is a “digital highlighter”. It makes certain tissues stand out (it “contrasts” them against other tissues), helping differentiate between structures of a similar density that would otherwise look very similar on a non-contrast scan.
Contrast agents can be used for every type of imaging modality. In this article, we cover contrast use in X-rays (including fluoroscopy and angiography), CT and MRI.
- X-ray: Uses barium or iodinated contrast. These are both radio-opaque – they absorb X-rays and appear bright white.
- CT: Usually uses iodinated contrast, most commonly injected intravenously. This is the most used contrast imaging for patients on general wards and in A+E.
- MRI: Usually uses gadolinium-based contrast agents, which work by changing the magnetic properties of water molecules, causing some tissues to “light up”.
Think of a plain scan as a sketch and a contrast scan as the same sketch but filled with colour.
When do we use contrast?
X-ray
- Barium is used orally or rectally in fluoroscopy to examine the GI tract.
- Iodine is often used intravenously in angiography to assess blood vessels.
CT
- Primarily used to evaluate blood vessels, solid organs, and suspected masses.
- If you need to see abnormalities of blood flow (e.g. clot or active bleeding), or want to look at the liver, kidneys, spleen, pancreas or bowel, you’ll probably need IV contrast.
MRI
- Useful in identifying tumours, infection and inflammation, particularly in the brain, spine, heart and soft tissues.
- It may also be used to assess blood vessel abnormalities (MR angiography), joints (MR arthrography) and the heart.
- Bear in mind that the decision to use MRI contrast is context-specific and is typically made by a radiologist or a senior clinician.
Things to consider before requesting a contrast scan
Most safety issues related to contrast media arise from intravenous use. Key considerations include:
1. Renal Function
IV iodinated contrast is cleared by the kidneys. In patients with impaired renal function, contrast can occasionally cause contrast-induced nephropathy (CIN).
- Check the (recent) eGFR
Generally, inpatients should have an up-to-date eGFR before receiving IV contrast.
When vetting a contrast CT scan, the radiologist will always ask for a recent eGFR so ensure you have this handy when you speak to them.
The exception to this is the most urgent of cases (e.g. polytrauma) where the risk of CIN is often outweighed by the imminent risk of death from undiagnosed pathology. In these cases, it may be appropriate to proceed without knowing the eGFR.
Hospitals generally tend to use an arbitrary eGFR cut-off of <30ml/minute, below which IV contrast is relatively contraindicated. However, this will vary between hospitals.
Speak to your senior or the on-call radiologist to discuss the risk profile and possible alternatives e.g. postponing the scan until renal function improves, using a non-contrast study or a different modality.
2. Allergy and Adverse Reactions
Rarely, contrast can cause allergic or adverse reactions. Iodinated contrast media are more likely to cause adverse reactions than gadolinium – although modern iodinated contrast media are much safer than in the past, and serious reactions are now relatively rare.
- Non-allergic reactions: Transient flushing or altered taste is normal and not an allergy.
- Mild/moderate allergic reactions: Urticaria, pruritus, “itchy” throat, bronchospasm, facial oedema, with stable observations.
- Severe allergic reactions: Anaphylaxis – almost all of these occur within 20 minutes of injection. This represents the main absolute contraindication to contrast.
Other than previous true anaphylaxis to contrast, most adverse reactions are relative contraindications. The risks and benefits should be weighed up by a senior clinician.
Sometimes, patients can be pre-medicated with steroids to manage the risk of adverse reactions. Speak to a radiologist if you think this might be useful for your patient.
3. Pregnancy
Radiation and contrast (which crosses the placenta) are generally avoided in pregnancy. However, depending on the clinical urgency, the benefit to the mother may outweigh the risk to the fetus. Always involve your seniors in this decision.
4. Metformin
If a patient has poor kidney function and is taking metformin, the drug may be withheld for 48 hours after contrast administration to reduce the risk of lactic acidosis (although there is a lack of evidence surrounding this issue).
5. IV Access
Ensure the patient has a working cannula – ideally 20G (pink) or larger. A small or displaced cannula increases the risk of contrast extravasation.
Common Scans That Use Contrast
| Scan | Why Contrast is Helpful | Clinical Indication |
| CT Abdomen/Pelvis | Differentiates between normal bowel, solid organs, and abscesses | Suspected appendicitis, abscess, or bowel pathology |
| CTA Head + Neck | Maps arteries of the brain and neck | Acute stroke protocols |
| CTPA | Makes blood in pulmonary arteries white to highlight dark clots | Suspected PE |
| CT Aortogram | Highlights the aorta to identify tears or flaps | Suspected aortic dissection or aneurysm |
| Trauma CT | Identifies active extravasation (where contrast is leaking from a vessel) | Major trauma |
Common Scans That Do NOT Use Contrast
In some cases, contrast can hide pathology, and therefore a non-contrast scan is preferred. The following are common non-contrast CT scans used on general wards and in A+E:
Non-Contrast CT Head:
- First-line investigation for suspected acute intracranial haemorrhage or acute stroke.
- Fresh blood appears hyperdense (bright white) on CT. IV contrast will make blood vessels white too, which can obscure the haemorrhage.
CT KUB:
- Used for suspected renal or ureteric stones, which usually appear hyperdense (bright/white) on CT.
- IV contrast is avoided because its hyperdensity can obscure the stone.
High-Resolution CT Chest:
- Used to assess lung parenchyma
- Contrast adds no value to an HRCT and can obscure fine parenchymal detail by increasing vascular attenuation.
References
- Standards for intravascular contrast agent administration to adult patients. 3rd ed. London: The Royal College of Radiologists; 2015 (Updated 2022).
- Contrast medium, Radiopaedia; 2025: https://radiopaedia.org/articles/contrast-medium?lang=gb
- Guidance on the use of iodine-based contrast media in patients taking metformin. London: The Royal College of Radiologists; 2014.
- Acute kidney injury: prevention, detection and management. NICE guideline [NG148]. London: NICE; 2019.
- Pregnancy and breastfeeding: post-procedure advice for patients. London: The Royal College of Radiologists; 2019.
Written by Dr Haris Hanif & edited by Dr Olivia Parish
