Computed Tomography (CT) Scan

Approved by the Cancer.Net Editorial Board, 02/2020

Doctors use a computed tomography (CT) scan, also called a CAT scan, to find cancer. They may also use it to learn more about the cancer after they find it. The scan lets them:

  • Learn the cancer’s stage. Knowing this helps you and your doctor choose the best treatment options. It also helps doctors predict how well you will recover.

  • Find the right place for a biopsy.

  • Plan radiation therapy for cancer treatment.

  • See how well treatment is working during the treatment period.

  • Check for new cancer growth after treatment ends during follow-up care.

How does a CT scan work?

A CT scan takes pictures of the inside of the body using x-rays taken from many angles. A computer combines these pictures into a detailed, 3-dimensional image. This image will show abnormal areas and any tumors.

Some people receive a special dye called a contrast medium before the scan. When injected in your vein, the contrast dye travels through the bloodstream and helps create a clearer picture of specific parts of your body. It may also be given as a liquid to swallow, depending on what part of your body needs to be scanned.

Areas commonly scanned for cancer include the head, neck, chest, abdomen, pelvis, or limbs. A "total body" CT scan generally includes at least the chest, abdomen, and pelvis. Doctors often use this is for cancer staging.

The benefits of having a CT scan usually outweigh the risks. During a CT scan, you will be exposed to a small amount of radiation. This low dose of radiation has not been shown to cause harm. For children or for people who need multiple CT scans and x-rays, there may be a small potential increased risk of cancer in the future. In many cases, doctors will use low-dose CT scans for children or limit the area that needs to be scanned. If you are having multiple CT scans and x-rays, ask your doctor about tests that lessen your exposure to radiation.

What is an integrated PET-CT scan?

Your doctor might suggest an integrated PET-CT scan. This combines pictures from a CT scan and another scan called Positron Emission Tomography (PET). The machine does both scans at once. When this test is recommended, your doctor can learn more from the 2 scans together than from either test alone.

Who does my CT scan?

You can have a CT scan done at the radiology or radiation oncology center of a hospital. You can also do it at an outpatient imaging center. A radiologic technologist performs the scan. A diagnostic radiologist reads the scan and decides what it means.

How can I prepare for my CT scan?

When you schedule your CT scan, the hospital or center staff will tell you how to prepare.

What to eat. The staff may tell you to drink only clear liquids starting at midnight the night before your appointment. They may also tell you not to eat or drink for at least 4 hours before the scan. For scans of some parts of the body, it is okay to eat and drink at any time prior. Ask your health care team for specific instructions for your test to be sure.

What to wear. Wear loose fitting clothing without metal zippers or buttons. You will need to remove any clothing that includes metal and could affect the scan. This includes belts, earrings, shirts with snaps or zippers, bras, and glasses. If your clothing cannot be worn during the scan, you can wear a hospital gown.

If you need contrast medium for the scan. You may need a contrast medium during the scan. If so, the doctor may ask you to have a blood test that checks your kidney function. You can have the blood test done any time up to a few weeks before the scan.

Personal medical history or concerns. Be ready to talk about these topics with the health care team:

  • All medications you are taking.

  • Any medical problems you have, such as diabetes.

  • Any drug or food allergies you have.

  • Any allergic reactions to iodine you may have had in the past.

  • Whether you should take your usual medications on the day of the scan.

  • If you are breastfeeding or could be pregnant. A CT scan could put the baby at risk.

  • Any concerns you have about the test.

Insurance, costs, and consent. If you are concerned about the cost of your CT scan, find out beforehand what your insurance provider will cover. Ask how much of the cost you will have to pay. Once you get to the doctor's office or hospital, the staff will ask you to sign a consent form. This form states that you understand the benefits and risks of the scan and agree to have it.

What happens during a CT scan?

Depending on which part of your body the scan will focus on, you may receive a contrast medium. If you get it by injection, you may feel heat or itching at the injection site. Or, you may have a metallic taste in your mouth. Both feelings should stop after a few minutes. If you have a more severe reaction, like trouble breathing, tell the technologist right away.

The technologist will help position you on an exam table. The table may have straps, pillows, or a special cradle for your head to hold you in place. You will likely lie on your back, but the technologist may ask you to lie on your side or stomach. This will depend on which part of your body needs to be scanned.

Sometimes a CT scan is used to plan for radiation therapy. In this case, your body position will be very important for the scan. The technologist may use a special device like a mask or a body cast to keep your body still during the scan.

The CT scan machine looks like a large donut. The exam table will slide back and forth through the large hole in the center of the machine. The scanner rotates around you. At first, the table will move through the scanner quickly. This helps the technologist confirm that your body is in the right position. After that, the table will move more slowly.

CT scans are not painful. But you will need to lie still for the entire scan, which may become unpleasant. Since the scanner is shaped like a donut, you will not be enclosed in the scanner at any time. You will hear whirring or clicking sounds from the machine. Some machines are louder than others.

During the scan, the technologist will be in a nearby control room. They will be able to see you through a window or a video camera. And, you will be able to talk with them through an intercom system.

The technologist may ask you to hold your breath during part of the scan. That is because the body's movement from breathing can blur the pictures. The technologist may also raise, lower, or tilt the exam table to create the correct angle for the x-rays. Ask them to tell you when the table will move.

Your visit will usually last up to 1 hour. The scan itself takes only 10 to 15 minutes or less. Newer scanners, including spiral or helical CT scanners, are even faster. If a larger part of your body is scanned, the test may last longer. The technologist should be able to give you a time estimate before you begin.

When the scan is done, the technologist may ask you to stay on the exam table for a few minutes. During this time, a radiologist will look at the images to see if the technologist needs to take more.

After the CT scan

You can go back to your normal activities, such as driving, right after your CT scan. If you received contrast medium, you should drink a lot of water to flush it out of your body.

Questions to ask your health care team

Before having a CT scan, you may want to ask your health care team these questions:

  • What will happen during the CT scan?

  • Who will do the CT scan?

  • How long will the CT scan take?

  • Will there be any discomfort?

  • Can I bring my own music to listen to during the CT scan?

  • What are the risks and benefits of having a CT scan?

  • Do I need to bring any of my other radiologic images with me to the CT scan?

  • Does the recommended imaging center usually perform CT scans?

  • Will I receive a contrast medium before the scan? If so, how will it be given?

  • May I eat or drink before the CT scan? Should I take my usual medications that day?

  • Does the center have an emergency response plan in case I have an allergic reaction to the contrast medium?

  • Will I need to avoid any activities after the CT scan?

  • When will I learn the results of the CT scan?

  • How will I get the results of the CT scan?

  • Will I need any other tests?

Related Resources

Cancer.Net Podcast: Coping with "Scanxiety"

Preparing for your PET-CT Scan

More Information

National Cancer Institute: Computed Tomography (CT) Scans and Cancer Computed Tomography (CT) - Body