Why do some CT scans need contrast?

If you have been told that your CT scan may involve “contrast,” you may wonder what it is and why it is needed.

Contrast is a substance used to make certain structures easier to see on the scan. For some examinations it is given as a drink; for others, an iodine-based contrast agent is injected into a vein.

Contrast helps certain structures, such as blood vessels and organs, stand out more clearly on a CT scan. Whether you need it depends on what your clinician is looking for and your individual circumstances; many CT scans do not need contrast at all.

Before an injection, the team will ask relevant safety questions. Tell them if you have previously reacted to contrast, have kidney disease, might be pregnant, or have concerns about the injection.

You may also be asked about your medicines and other aspects of your health. Answering these questions helps the team plan the examination safely.

You may feel a brief warmth or unusual taste when contrast is injected. The radiographer will explain what to expect and monitor you during the examination. Serious reactions are uncommon, but you should tell the team immediately if you feel unwell.

For most CT scans with contrast at Key Imaging, we ask you not to eat for four hours before your appointment. You may drink fluids.

If this would be difficult because of a medical condition, such as diabetes, please contact us beforehand so we can advise you.

Preparation can vary between scans, so follow the instructions supplied with your appointment.

Do not stop any medicines unless you have specifically been told to do so.

After the scan, the team will advise whether you need a short period of observation and when you may leave.

If you have questions about contrast, please ask. Understanding why it has been recommended is part of feeling comfortable with your care.