Quick answer
What is a CT scan?
A CT scan, or computed tomography scan, combines X-ray images taken from different angles to build detailed cross-sectional pictures of the body. It is used to detect cancers, strokes, injuries and blood clots, sometimes with an iodine contrast drink or injection. It uses more radiation than a plain X-ray, so it is only used when the benefit is clear.
How a CT scan works
During a CT scan, an X-ray tube rotates around the patient, taking images from many different angles. A computer then combines these individual slices into detailed three-dimensional pictures of the body’s internal structures, giving doctors far more detail than a standard X-ray alone.
What a CT scan is used for
A CT scan is commonly used to assess stroke and trauma, to stage cancer, to check for a pulmonary embolism, and to investigate suspected appendicitis. It can also be used to guide biopsies, helping doctors take tissue samples from precise locations inside the body.
Contrast precautions for a CT scan
Before contrast is given, staff usually check kidney function and ask about any known allergies, and sometimes ask patients taking metformin to pause it around the time of the scan. Drinking plenty of fluids after a contrast scan helps the body clear the dye.
Radiation awareness with CT scanning
The radiation dose used in a CT scan is kept as low as reasonably possible, following what is known as the ALARA principle. A CT scan is only carried out when the result is likely to change how a condition is diagnosed or managed, so that the benefit justifies the radiation exposure involved.
Common questions about the CT scan
- What is a CT scan with contrast?
- A CT scan with contrast uses an iodine-based dye, usually given into a vein, to highlight blood vessels and tumours more clearly on the images. You will usually be asked to drink plenty of fluids afterwards to help clear the contrast, unless you have been told to restrict fluids for another reason.
- What happens if I am allergic to CT contrast dye?
- You should tell hospital staff about any known allergy to contrast dye before your scan. They can then arrange premedication to reduce the risk of a reaction, or use an alternative imaging method if that is safer for you.
- Can I have CT contrast if I have kidney disease?
- Your kidney function, measured by eGFR, will usually be checked before contrast is given, because contrast dye can affect the kidneys. Staff weigh this risk against the benefit of the scan when deciding whether to proceed.
- What is the difference between a CT scan and an MRI scan?
- A CT scan is generally faster and is often preferred for looking at bone, bleeding, and the lungs. An MRI scan tends to give better detail of soft tissues, so the choice between them depends on what your doctor needs to see.