Contrast
A dye given to make blood vessels and tissues show up more clearly on a scan.
What it means
A substance given by injection, by mouth or occasionally rectally, which makes certain structures stand out on a scan.
- Iodine-based contrast for CT
- Gadolinium for MRI
- Microbubbles for some ultrasound scans
An injection commonly produces a warm flush through the body and a metallic taste, and a feeling as though you have wet yourself. All of that is normal, brief, and worth knowing in advance because it is genuinely startling otherwise.
In practice
On scan appointment letters, with instructions about fasting and blood tests.
Reports will describe findings as "enhancing" or "non-enhancing", meaning whether an area took up the contrast — which helps distinguish what things are.
Why it matters
Tell them about kidney problems. Reduced kidney function affects whether and how contrast is given, and a recent blood test is often needed first.
Tell them about any previous reaction to contrast, and about asthma or significant allergies.
If you take metformin, ask — it is sometimes paused around a contrast CT depending on kidney function.
Drink plenty afterwards, which helps clear it.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026