Computed Tomography (CT) Scan

January 15, 2026

This page was reviewed under our medical and editorial policy by Henry Krebs, M.D., associate clinical professor, Department of Diagnostic Radiology, City of Hope^® Cancer Center Atlanta 

A computed tomography (CT) scan is a type of imaging test used to assist in cancer diagnosis, staging and treatment.

A cancer care team may order a CT scan to:

• Screen for certain types of cancer
• Guide a needle during a biopsy, a procedure to remove a tissue sample to be studied in a laboratory for cancer markers
• Guide the administration of certain types of treatments
• View the size, shape and location of a tumor to help determine the cancer’s stage, which is the amount and location of cancer in the body
• Use the image data collected to create a treatment plan or understand how well treatment is working
• Monitor for cancer after treatment is finished

What Is a CT Scan?

A CT scan (also referred to as a CAT scan) is an imaging test that uses a special X-ray machine to scan a patient’s body with a thin beam of electromagnetic waves (a type of radiation). An array of radiation detectors rotate around the patient and determine how much how much radiation passed through that body part to create a volume of information.

These volumes are then processed by a computer to create images of the patient's anatomy, which are layered on top of each other to form a 3D view of a specific area of the body. These images may include views of the chest, abdomen, head, neck, limbs, pelvis or even the inside of organs, like the colon and the lungs.

For example, CT scans may be used to screen for lung and colorectal cancer, for planning surgery to remove a cancerous tumor, or to guide microwave ablation (high-energy radio waves that generate heat to destroy cancer cells) or brachytherapy (internal radiation therapy using implants). A doctor may also order a total body CT scan that takes images from head to toe to assist in pinpointing the cancer’s stage.

CT Scan With Contrast

A contrast agent is a liquid material sometimes given to a patient before and during a CT scan. It helps differentiate tissues, blood vessels and organs from other body structures during the scan. This assists in getting more diagnostic pictures.

Contrast agents commonly used with CT scans include compounds that contain barium-sulfate and iodine because these substances change the way X-rays interact with the body. They may be taken:

• Orally by the patient in several forms, such as powders, tablets, liquids or pastes
• Injected into a vein by a member of the medical team (intravenous or IV administration)
• Inserted rectally using an enema

Oral contrast agents help highlight organs of the gastrointestinal (GI) tract, such as the esophagus, stomach and small bowel.

IV injections may highlight any number of internal organs and structures like the lungs, bladder, spleen, muscles, bones or blood vessels, as well as the GI tract organs.

Typically, an enema containing a contrast agent is given if a CT scan is ordered of the lower GI tract (the colon and rectum).

Sometimes, patients report that intravenous contrast agents leave a slightly metallic taste in their mouth. A sensation of warmth or even feeling like they wet their pants may also occur with IV injection, but it goes away quickly.

CAT Scan Versus CT Scan

A CT scan and a CAT scan are the same test. CT scans actually have a few different names, including:

• Computed tomography scan, abbreviated as CT scan
• Computerized tomography scan, abbreviated as CT scan
• Computerized axial tomography, abbreviated as CAT scan
• Spiral or helical CT, because modern CT scanners take a series of continuous images using a fast, spiral (helical) technique

CT Scan Versus MRI

Rather than using small amounts of radiation to create images, a magnetic resonance imaging (MRI) scan uses powerful magnets. If certain details of soft tissues or other body parts are necessary, a doctor may order an MRI scan instead of a CT scan. For instance, if a doctor needs details about soft tissues in a person’s brain, ligaments, tendons or knee or shoulder joints, an MRI scan may be performed.

One benefit of CT scans over MRIs is that they tend to be less sensitive to movements by the patient during the procedure. They may also be used on patients who have implants, which is not always the case with MRI scans because the magnets may interfere with the implants’ functioning abilities.

Does a CT Scan Show Cancer?

A CT scan may show tumors or nodules, including their size, location and shape. However, while the test is used to assist doctors in making a cancer diagnosis, it cannot definitively show that those tumors or nodules are cancer. Follow-up tests, particularly biopsies, are used in tandem with CT scans to help doctors diagnose cancer, stage the disease and understand if the cancer has returned after treatment.

How Is a CT Scan Performed?

Before a CT scan begins, patients should be sure to discuss their health history with their doctor, including if they are pregnant, breastfeeding or have any known allergies to contrast agents, seafood or iodine. Patients should also let the radiologist or technologist operating the scanner know if they have any implants, such as a heart pacemaker, so that care may be taken when scanning the area where they are located.

CT scans may take place in an outpatient clinic or hospital. Step by step, what follows is what to expect.

1. The patient changes into a hospital gown and removes all jewelry and any other items containing metal, such as underwire bras, hearing aids and hair clips, which may interfere with CT images.
2. If necessary, a contrast agent will be given by a member of the medical team, either orally, rectally or through an IV line.
3. A radiology technologist will help the patient into the correct position — on his or her back, side or stomach — on a flat exam table, which may have straps, cradles or special devices to help hold different body parts in place.
4. The technologist goes to a room where the CT scan controls are located (the patient and tech are still able to communicate via speakers and a call button located inside the scanner).
5. Based on control commands from the technologist, the exam table slides back and forth inside an opening in the CT scan machine. The machine (a big doughnut-shaped piece of equipment) rotates around the table and directs small X-ray beams from different angles at the area(s) being scanned.
6. During the scan, the technologist may ask the patient to hold his or her breath briefly at certain points and to lie as still as possible.
7. Those undergoing a colon scan may have air pumped into the colon during the scan to give a clearer picture of its inner surfaces.
8. Depending on the part of the body being scanned, the patient may be advised not to eat after midnight or have any food or water four hours before the procedure. Some patients may be asked to take a laxative or have an enema prior to the scan to clean out the lower GI tract in order to create clearer pictures.

How Long Does a CT Scan Take?

CT scans are typically fast procedures, lasting 5 to 15 minutes, and the entire process at the clinic or hospital may take up to an hour. Keep in mind that the time a CT scan takes may vary depending on which body part is being scanned, the type of contrast agent used (if any) and how long it takes to get the patient into position. 

CT Scan Side Effects

Some people experience side effects from the contrast agent used in the CT scan. These may include:

• Face swelling or itching that lasts up to an hour
• Shortness of breath
• Nausea
• Wheezing or coughing
• Rash

It may be helpful to drink plenty of water after a CT scan to help move the contrast agent out of the body faster and reduce potential side effects.

Patients should let the doctor know right away if they are experiencing any side effects. Though it is rare, they are sometimes due to a serious, potentially life-threatening allergic reaction.

People may also experience kidney problems after a CT scan, although this is also rare, and if they do occur, it is more likely to be in patients with preexisting kidney issues. In these cases, doctors will give the patient medications or IV fluids to clear out the contrast agent and help stabilize the kidneys.

How Long Do CT Scan Results Take?

While getting the results of CT scans ordered for emergency care typically takes just a few hours, patients who have a regularly (non-emergency) scheduled CT scan may have to wait longer. In general, it takes one to two days for the results of a non-emergency CT scan to be reported to the doctor or uploaded into the patient’s portal.

The patient’s care team may answer any questions about the procedure while patients wait, including when the results are expected, and, once they are in, what the findings may mean. One of the care team’s specialty doctors may also provide information on any follow-up tests or treatment options that they recommend based on the CT scan results.