- What are imaging tests?
- Who does imaging tests and who interprets them?
- Types of imaging tests
- Computed tomography (CT) scan
- Magnetic resonance imaging (MRI)
- X-rays and other radiographic tests
- Nuclear medicine scans
- Categories of some common imaging tests
- Questions about radiation risk from imaging tests
- Factors that determine which imaging tests are used in different types of cancer
- To learn more
Computed tomography (CT) scan
Also called CT scan, CAT scan, and spiral or helical CT.
What does it show?
CT scans show a slice, or cross-section, of the body. The image shows your bones, organs, and soft tissues more clearly than standard x-rays. Because the picture is made by a computer, it can be enlarged to make it easier to see and interpret.
Since the late 1970s, CT scans have been very useful in helping doctors find cancer. CT scans can show a tumor’s shape, size, and location, and even the blood vessels that feed the tumor – all without having to cut into the patient.
Doctors often use CT scans to help them guide a needle to remove a tissue sample. This is called a CT-guided biopsy. They can also be used to guide needles into tumors for some types of cancer treatments, such as radiofrequency ablation (RFA), which uses heat to destroy a tumor.
By comparing CT scans done over time, doctors can see how a tumor is responding to treatment or find out if the cancer has come back after treatment.
How does it work?
In a way, CT scans are like standard x-ray tests (see the “X-rays and other radiographic tests” section). But an x-ray test aims a broad beam of radiation from only one angle. A CT scan uses a pencil-thin beam to create a series of pictures taken from different angles. Each angle produces a slightly different view of the organs and soft tissues. The information from each angle is fed into a computer, which calculates how the images overlap. The computer then creates a black and white picture that shows a slice of a certain area of the body – much like looking at a single slice from a loaf of bread.
The picture can be made clearer by the use of special contrast materials which can be swallowed as a liquid, put into a vein, or put into the intestines through the rectum as an enema. Because body tissues absorb these materials differently, the CT image will show greater contrast between types of tissues. This allows things like tumors to be seen more clearly.
Today, spiral CT (also known as helical CT) is the most common type of CT used. It’s a faster machine that uses less radiation than the original CT scanner. As technology advances, CT scans are getting even better – faster and able to scan very thin slices.
By placing CT image slices on top of each other, the machine can create a 3-dimensional (3-D) image, which provides even more information about certain cancers. The 3-D image can be rotated on a computer screen to look at different views.
Doctors are now taking CT technology one step further in a technique called virtual endoscopy. They can look at the inside surfaces of organs such as the lungs (virtual bronchoscopy) or colon (virtual colonoscopy or CT colonography) without actually having to put scopes into the body. They can arrange the 3-D images to create a black and white view on the computer screen, which looks a lot like it would if they were doing an actual endoscopy.
How do I get ready for the test?
CT scans are most often done on an outpatient basis, so you do not have to be in a hospital to get one.
In some cases, your doctor may tell you not to eat or drink overnight or for several hours before the test. Or you might need to use a laxative or an enema to clean out your bowel and remove material that could get in the way of seeing inside the belly and intestines. Depending on the part of the body being studied, you also may need to drink contrast liquid or get a contrast enema before the test. If you are going to get contrast material, you might have an intravenous (IV) catheter put into a vein in your arm or hand.
You may be asked to undress, put on a robe, and remove underwire bras, jewelry, piercings, or any other metal objects that may get in the way of the image. You may be asked remove dentures, hearing aids, hair clips, and so on, as they can affect the CT pictures.
Also let the technologist know if you have a pacemaker, infusion port, or other implanted medical device. This will not keep you from getting a CT scan, but extra care can be taken if that area is to be scanned.
What is it like having the test?
A radiology technologist does the CT scan. The scanner is a large, doughnut-shaped machine. You lie on a thin, flat table that slides back and forth inside the hole in the middle of the scanner. As the table moves into the opening, an x-ray tube rotates within the scanner, sending out many tiny x-ray beams at precise angles. These beams quickly pass through your body and are detected on the other side of the scanner. You may hear buzzing and clicking as the scanner switches on and off.
During a CT head scan, your head will be held still in a special device.
You will be alone in the exam room during the CT scan, but the technologist will be able to see, hear, and talk to you at all times.
A CT is painless but you may find it uncomfortable to hold still in certain positions for minutes at a time. You may also be asked to hold your breath for a short time, since chest movement can affect the image. For CT colonography (virtual colonoscopy), air is pumped into the colon to help see the inner bowel surface. This can be uncomfortable.
If you are to be given contrast material in a vein, you will probably have a scan first, then get the contrast dye, and then have a second scan done. When the contrast is given, you may get a feeling of warmth that spreads through your body. Some people say that this can feel like they “wet their pants.” This is only a feeling, and it goes away quickly. You might also get a bitter or metallic taste in your mouth.
How long does it take?
A CT scan can take anywhere from 10 to 30 minutes, depending on what’s being scanned. It depends on how much of your body your doctors want to look at and whether contrast dye is used. It often takes more time to get you into position and give the contrast dye than to take the pictures. After the test, you may be asked to wait while the results are looked at to see if more pictures are needed.
What are the possible complications and side effects?
Some people react to the contrast dye. Possible reactions include:
- Shortness of breath
- Itching or facial swelling that can last up to an hour
These symptoms usually are mild and they most often go away on their own, but they can sometimes signal a more serious reaction that needs to be treated. Be sure to let your radiology technologist and your health care team know if you notice any changes after getting the contrast dye.
In rare cases, people can have a severe allergic reaction that causes low blood pressure or trouble breathing and requires treatment right away.
Before getting contrast dye, be sure to let your health care team know if you’ve ever had a reaction to contrast dye, seafood, or iodine in the past. This is important because it may put you at risk for reacting to the contrast dye used in CT scans. If there’s a risk that you might have an allergic response, you may be given a test dose of the contrast material first. When someone has had a severe reaction in the past, they may need to take drugs (usually a steroid, like prednisone) to prevent another reaction. Sometimes these drugs need to be started the day before the scan.
The IV contrast dye can also cause kidney problems. This is rare, and it’s more common in someone whose kidneys already don’t work well. If you need a scan with contrast dye, your doctor will first do a blood test to check your kidney function. You may also get extra fluids in an IV or medicines to help your kidneys get rid of the dye safely.
What else should I know about this test?
- Although a CT scan is sometimes described as a “slice” or a “cross-section,” no cutting is involved.
- The amount of radiation you get during a CT scan is a good deal more than that with a standard x-ray.
- People who are very overweight may have trouble fitting into the CT scanner.
- Tell your doctor if you have any allergies or are sensitive to iodine, seafood, or contrast dyes.
- Tell your doctor if you could be pregnant or are breastfeeding.
- CT scans can cost up to 10 times as much as a standard x-ray.
Last Medical Review: 03/16/2015
Last Revised: 09/15/2015