PET/CT is a new imaging tool that is highly advanced. PET/CT combines 2 different types of imaging into 1 procedure. PET and CT together produce a more accurate picture of what is happening in the body than either PET or CT alone. PET/CT is a very good tool for detecting cancer and how far it has spread. It can help the doctor decide on the best treatment for a patient.

PET/CT also can show how the cancer is responding to treatment. This patient education page tells you how PET/CT works. It also tells what you need to do when you have a PET/CT scan procedure.

What is PET?

“PET” stands for positron (POZ-ih-tron) emission tomography (tuh-MOG-ruff-ee). PET creates an image (scan) of your body’s biochemical activity. PET shows the rate at which your body’s cells break down and use sugar (glucose). This activity is called metabolism (meh-TAB-uh-liz-im). Cancer cells metabolize sugar at a higher rate than normal cells do. A PET scan shows this abnormal cell activity. For your PET scan, a small amount of radioactive material is injected into your blood stream. This material is called a radioisotope (RAY-dee-oh-EYE-suh-tope). A PET scanner detects the radioisotope and then creates an image on the computer screen. PET exposes you to a very low level of radiation.

What is CT?

“CT” is short for computed tomography (tuh-MOG-ruff-ee). CT uses x-rays and a computer to make an image of sections of your body. A CT scan shows your body’s organs, bones, and tissues in greater detail than regular x-rays do. For your CT scan, you will receive a contrast enhancing agent by intravenous line (IV), which helps produce an even clearer image. CT exposes you to a small amount of radiation.

What does PET/CT do?

A PET image is color coded — different colors show various levels of cell activity. A CT scan shows the exact locations of the body’s organs and also can show abnormal growths. When a CT scan is laid over a PET scan, doctors can pinpoint the exact location of abnormal cell activity. They can also see the level and extent of that activity. Even when an abnormal growth is not yet visible on a CT scan, the PET scan can show the abnormal cell activity.

PET/CT Scans


How do I prepare for my PET/CT exam?

This section gives you general guidelines to prepare for your exam. Your doctor, nurse, or testing center will give you more detailed instructions.

  • Before the day of your exam, a staff nurse from the testing center will call you. Tell the nurse all the medicines you take. The nurse will instruct you about taking your medicines, especially insulin or other diabetes medicines. If you have diabetes, your blood sugar level must be below 150 mg/dl when you arrive at the testing center.
  • Do not eat or drink anything for at least 6 hours before the exam, except for plain water. If your doctor has told you to take your regular medicine, take it with plenty of water.
  • Wear loose-fitting, comfortable clothes to the test. Leave your watch, jewelry, and other valuables at home.
  • If you are pregnant or think you might be, or if you are breast-feeding, tell your doctor or technologist before the exam.
  • If you’ve ever had an allergic reaction to an enhancing agent or iodine, or if you have asthma, tell the doctor or nurse. The doctor may prescribe special medicine for you to take before the exam.
  • If you are a large person or have a fear of enclosed spaces (claustrophobia), tell your doctor before the day of your PET/CT exam. The doctor can prescribe medicine to help you relax.
  • Do not schedule any other test that uses radioactive material for the same day as your PET/CT exam.
  • Plan to be at the testing center for 2 to 2 1/2 hours on the day of your PET/CT exam. It is very important to arrive on time at the testing center.
  • If you cannot come to the testing center at your scheduled time, please call 24 hours in advance to cancel your test.

What happens before the exam?

You will change into a hospital gown and remove all metal objects that could interfere with the scan. An IV will be inserted in your arm. You will receive the radioisotope through the IV. Then you’ll sit quietly for an hour while it moves through your body.

What happens during the exam?

You’ll be asked to empty your bladder, and then you will go to the scanning room. The PET/CT scanner is a machine that does PET and CT scanning in the same procedure. It has a large, deep ring — like a donut standing on its side. A scanning table will move slowly through the ring.
The technologist will help you onto the table. You must be able to lie very still on the table for the entire test, which takes about 30 minutes.

Next you will receive the enhancing agent through your IV. A brief sensation may move up your arm. You also may get a warm, flushed feeling; a taste of salt or metal in your mouth; or nausea for a few minutes. This is normal, but you should tell the technologist about these or other reactions. You may also have nausea, shortness of breath, itching, or sneezing. If any of these occur, tell the doctor or technologist right away.

The technologist will go into a room behind a glass window. The technologist can see you at all times during scanning and may give you added instructions. You’ll be able to talk to the technologist through an intercom during the scan. You should lie as quietly as possible. The scan is painless; you should not feel anything. It takes about one-half hour.

What happens after the exam?

Most of the radioisotope will collect in your bladder. After the exam, you should drink plenty of fluids to flush it out of your body. You may resume your normal diet.

How do I get my results?

The doctor who ordered the exam for you will discuss the results with you. Check with your doctor about how and when to get your results.

Questions and concerns

Some anxiety before and during a test is normal. But a PET/CT exam should not be a fearful experience for you. Feel free to express any concerns about your PET/CT exam. Please ask the medical staff any questions you may have.


My test appointment

Date: ____________ Time: ____________

Place: _______________________________

Report to: ____________________________

Phone number: _______________________

Special instructions: _______________________________________________________



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