Family doctor

What is it?

Computed tomography (CT), also known as Computed Axial Tomography or CAT Scanning, uses x-rays and an advanced computer system to obtain cross-sectional images of the body.

What is it used for?

CT is used for scanning most parts of the body, including the brain, chest, abdomen, pelvis, and sometimes the bones, joints and spine.

Who can have CT?

Because x-rays are used, scanning in pregnancy is generally avoided. Otherwise, anyone can safely undergo CT.

How much radiation exposure is there?

An average CT scan of the head results in a dose equivalent of approximately 1-3mSv. To put this into perspective, the average background radiation dose in New Zealand is 2mSv per year. This is the dose that the average person receives from the environment each year. In some populated parts of the world the background radiation is more than 10 times higher, with no detectable increase in cancer incidence. The dose received from a CT scan is considered to be safe.

Before the scan

You may be asked to fast for up to 4 hours prior to your scan. For scans of the abdomen and pelvis, the bowel needs to be opacified prior to the scan. This is done by drinking a barium or iodine-based dye prior to the scan. This is usually taken at 10 minutes and 1- 2 hours before the scan, and sometimes also 12 hours before the scan. No other special preparation is usually needed, but for some pelvic scans a small enema may be given immediately before the scan, and for some gynecological scans a tampon may be required.

What happens during the scan?

In the scan room, you will lie on a table that subsequently moves slowly through a hole in the center of the scanner. You may require an intravenous injection of an iodine-based "dye", or contrast agent during the scan. This circulates through your bloodstream, highlighting many organs and making abnormalities more conspicuous.

The dye is rapidly removed from the body by the kidneys. It is common to notice a warm sensation or a metallic taste in the mouth; these sensations pass quickly. Nausea, vomiting and allergic reactions to the dye are uncommon. People with impaired kidney function may not be able to receive the dye injection, and diabetics taking metphormin (Glucophage) should have had a recent blood test for kidney function.

The dye is injected into a vein, usually at the elbow or wrist. It may be given as a hand injection by the doctor prior to the scan, or by an injection pump during the scan. During the scan the x-ray tube rotates around you; you will not see this as it is hidden by the scanner housing. You may be required to hold your breath during the scan. A typical scan takes 20 seconds to 2 minutes, depending on the type of scan you are having, although you should allow up to 30 minutes to include the setup time and time to print and check the images afterwards.

After the scan

The images will be printed and the radiologist will study them and issue a report. The report is usually available to the referring doctor within 24 hours, or immediately in urgent cases.