What to Expect During a CT Scan

A CT scan is one of the most common imaging tests used in cancer care. Knowing what actually happens takes away a lot of the mystery. It is a noninvasive test, so nothing is put inside your body during the scan itself. It uses special x-ray equipment and a computer to build detailed pictures of the inside of your body, arranged as thin "slices."

Depending on the reason for your scan, you may be asked to change into a gown. You may also need to remove jewelry or metal items that could blur the pictures. Many scans use a contrast dye to make the images clearer. Contrast can be given by mouth, injected into a vein, or given by enema. NCI names iodine and barium as common contrast dyes. If you might be pregnant, tell the staff before your scan. NCI says CT radiation is believed to be too low to harm a growing fetus, but providers may adjust the dose or use another test.

You lie very still on a table. The table passes slowly through the center of a large, donut-shaped machine. You may hear whirring sounds as the machine works. You may also be asked to hold your breath for a few seconds at a time, so the pictures do not blur.

The scan itself is not painful. Lying still in one position can feel slightly uncomfortable, especially if the scan takes a while. Most CT scanners surround only part of your body at a time, so people rarely feel closed in.

If your scan uses contrast, it is there to make the pictures clearer. That helps your care team see certain areas more easily. Reactions to contrast are uncommon. NCI lists itching or hives as mild reactions. Serious ones include shortness of breath and swelling of the throat or other body parts. MedlinePlus says IV contrast may cause a slight burning feeling, a metallic taste, and a warm flush. Tell your team about any kidney problems, and if you take metformin for diabetes. Tell the technologist right away if you notice any of these signs, so they can help.

A CT scan is usually done as an outpatient test, so you do not stay overnight. NCI says the scan usually lasts a few minutes to half an hour. The time depends on which part of your body is being imaged, and whether contrast is used.

CT scans use a small amount of ionizing radiation to create their pictures. NCI says the added cancer risk from one CT scan is small, and that the benefits when checking for serious conditions greatly outweigh the possible risk. MedlinePlus notes that many scans over time may raise cancer risk. If you have questions about how many scans you have had, or are likely to need, your care team can talk that through with you.

Your care team will tell you what to do afterward. If you received contrast, ask whether you should do anything.

Ask ahead of time whether you will need contrast. Ask how long the scan is expected to take. Ask whether there is anything specific you should do to prepare. Knowing the plan in advance can make the whole appointment feel much more predictable.

If you had contrast dye, a small number of reactions begin after you have left.

• Call 911 or go to an emergency department if your lips, tongue or throat start to swell, your breathing tightens, or you feel faint in the hours after the injection. Do not drive yourself.
• Call your care team the same day if an itchy rash or hives spreads, or you keep vomiting once you are home.
• Call your care team within a day or two if you have kidney trouble or diabetes and notice you are passing much less urine than usual, or your ankles puff up.

Sources: NCI — CT Scans and Cancer and MedlinePlus (National Library of Medicine, NIH) — CT Scan.