Computed tomography

The CT scanner is a large machine with a hole in the center. Before the scan begins, you’ll need to remove jewelry from the area being scanned. (The metal in the jewelry can change the image of the scan.)

You will be asked to lie on a table that slides into the center of the machine. In some cases, you may need to lie on your stomach. Usually, you will lie on your back. The scan itself is painless.

Once you’re inside the scanner, the machine's X-ray beam rotates around you. You’ll need to be still during the exam. Any movement can blur the images. You may be asked to hold your breath for short periods of time. You will hear clicking and whirring noises.

Generally, scans take only a few minutes. The newest machines can scan your entire body, head to toe, in less than 30 seconds.

Some CT exams need to use a special dye called contrast. Contrast helps create a clearer image. You may need to swallow the contrast. Or it may be put into your body through an intravenous line (IV).

If you get contrast through an IV, you may get a slight metallic taste in your mouth. Some people also feel a warm flushing feeling. This is normal, and it usually go away in a few seconds.

If have a scan that needs to use contrast, you may have to stop eating and drinking four to six hours before the test.

A computer takes the scans and creates several images, called slices. These images can be stored, viewed on a monitor, or printed on film. The slices can be put together to make a 3D image of an organ.

A radiologist will look at the images on a computer. Radiologists are trained to look at CT scans and other types of images and help answer questions about your health. The radiologist will send a report to your doctor.

There are very strict rules for CT scans and other X-ray devices to make sure they use the least amount of radiation possible.

CT scans do create low levels of ionizing radiation. (Ionizing radiation at high levels can damage tissue.) But the chance of being harmed by one scan is very small.

Usually, a pregnant woman isn’t given a CT scan of the abdomen (the area around the navel). It could harm the unborn child. If you’re pregnant or think you might be, ask about having an ultrasound instead.

The contrast that you’re given by IV will have iodine in it. Your kidneys help filter the iodine out of your body. If you have kidney disease or diabetes, you should drink plenty of fluids after the test. Your doctor will also check you for kidney problems after the test.

If you have diabetes or are on kidney dialysis, ask your doctor about the pros and cons of having a CT scan.

A very small number of people can have an allergic reaction to IV contrast. They may have some sneezing and itching. It’s very rare, but the contrast can cause a life-threatening allergic reaction.

If you have any trouble during the test, tell the CT technologist right away. Scanners have an intercom, so the tech can hear you at all times.