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Positron Emission Tomography - Computed Tomography (PET/CT)

Positron emission tomography (PET) uses small amounts of radioactive materials called radiotracers or radiopharmaceuticals, a special camera and a computer to evaluate organ and tissue functions. By identifying changes at the cellular level, PET may detect the early onset of disease before other imaging tests can.

Tell your doctor if there is any possibility you are pregnant or you are breastfeeding. Your doctor will tell you how to prepare based on the type of your exam. Discuss any recent illnesses, medical conditions, medications you are taking and allergies – especially to contrast material. Your doctor will likely tell you not to eat anything and to drink only water for several hours before your scan. Leave jewelry at home and wear loose, comfortable clothing. You may wear a gown during the exam.

What is Positron Emission Tomography – Computed Tomography (PET/CT) Scanning?

Positron emission tomography, also called PET imaging or a PET scan, is a type of nuclear medicine imaging.

Nuclear medicine imaging uses small amounts of radioactive material to diagnose, evaluate or treat a variety of diseases. These include many types of cancers, heart disease, gastrointestinal, endocrine or neurological disorders and other abnormalities. Because nuclear medicine exams can pinpoint molecular activity, they have the potential to identify disease in its earliest stages. They can also show whether a patient is responding to treatment.

Nuclear medicine imaging procedures are noninvasive. With the exception of intravenous injections, they are usually painless. These tests use radioactive materials called radiopharmaceuticals or radiotracers to help doctors diagnose and evaluate medical conditions.

Radiotracers are molecules linked to, or "labeled" with, a small amount of radioactive material that can be detected on the PET scan. Radiotracers accumulate in tumors or regions of inflammation. They can also bind to specific proteins in the body. The most commonly used radiotracer is F-18 fluorodeoxyglucose, or FDG, a molecule similar to glucose. Cancer cells are more metabolically active and may absorb glucose at a higher rate. This higher rate can be seen on PET scans. This allows your doctor to identify disease before it may be seen on other imaging tests. FDG is just one of many radiotracers in use or in development.

Depending on the type of exam, the radiotracer is injected, swallowed or inhaled as a gas. It eventually accumulates in the area of the body under examination. A special camera or imaging device detects radioactive emissions from the radiotracer. The camera or device produces pictures and provides molecular information.

Many centers superimpose nuclear medicine images with computed tomography (CT) or magnetic resonance imaging (MRI) to produce special views. This is known as image fusion or co-registration. These views allow the doctor to correlate and interpret information from two different exams on one image This leads to more precise information and accurate diagnoses. Single photon emission computed tomography/computed tomography (SPECT/CT) and positron emission tomography/computed tomography (PET/CT) units can perform both exams at the same time. PET/MRI is an emerging imaging technology. However, it is not universally available at this time.

A PET scan measures important body functions, such as metabolism. It helps doctors evaluate how well organs and tissues are functioning.

CT imaging uses special x-ray equipment, and in some cases a contrast material, to produce multiple images of the inside of the body. A radiologist views and interprets these images on a computer monitor. CT imaging provides excellent anatomic information.

Combined PET/CT scanners perform almost all PET scans today. These combined scans help pinpoint abnormal metabolic activity and may provide more accurate diagnoses than the two scans performed separately.

What are some common uses of the procedure?

Doctors perform PET and PET/CT scans to:

  • detect cancer and/or make a diagnosis.
  • determine whether a cancer has spread in the body.
  • assess the effectiveness of treatment.
  • determine if a cancer has returned after treatment.
  • evaluate prognosis.
  • assess tissue metabolism and viability.
  • determine the effects of a heart attack myocardial infarction on areas of the heart.
  • identify areas of the heart muscle that would benefit from angioplasty or coronary artery bypass surgery (in combination with a myocardial perfusion scan).
  • evaluate brain abnormalities, such as tumors, memory disorders, seizures and other central nervous system disorders.
  • map normal human brain and heart function.

How should I prepare for a PET and PET/CT scan?

You may wear a gown during the exam or be allowed to wear your own clothing.

Women should always tell their doctor and technologist if there is any possibility that they are pregnant or they are breastfeeding. See the Safety in X-ray, Interventional Radiology and Nuclear Medicine Procedures page for more information about pregnancy and breastfeeding related to nuclear medicine imaging.

Tell the doctor and the technologist performing your exam about any medications you are taking, including vitamins and herbal supplements. List any allergies, recent illnesses and other medical conditions.

You will receive specific instructions based on the type of your PET scan. Diabetic patients will receive special instructions to prepare for this exam.

If you are breastfeeding at the time of the exam, ask your radiologist or doctor how to proceed. It may help to pump breast milk ahead of time and keep it on hand for use until the PET radiotracer and CT contrast material are no longer in your body.

Leave metal objects including jewelry, eyeglasses, dentures and hairpins at home as they may affect the CT images. You may need to remove hearing aids and removable dental work.

Generally, your doctor will tell you not to eat anything for several hours before a whole body PET/CT scan. Eating may alter the distribution of the PET tracer in your body and can lead to a suboptimal scan. This could require you to repeat the scan on another day, so following instructions regarding eating is very important. You should not drink any liquids containing sugars or calories for several hours before the scan. Instead, you are encouraged to drink water. If you are diabetic, your doctor may give you special instructions. Tell your doctor about all the medications you are taking. List any allergies, especially to contrast materials or iodine.

Your doctor will check for any conditions you may have that could increase the risk of receiving intravenous contrast material.

What does the equipment look like?

A PET scanner is a large machine with a round, donut-shaped hole in the middle. It looks similar to a CT or MRI unit. Multiple rings of detectors inside the machine record the energy emissions from the radiotracer in your body.

The CT scanner is typically a large, donut-shaped machine with a short tunnel in the center. You will lie on a narrow examination table that slides in and out of this short tunnel. Rotating around you, the x-ray tube and electronic x-ray detectors are located opposite each other in a ring, called a gantry. The computer workstation that processes the imaging information is located in a separate control room. This is where the technologist operates the scanner and monitors your exam in direct visual contact. The technologist will be able to hear and talk to you using a speaker and microphone.

Combined PET/CT scanners look similar to both the PET and CT scanners.

A computer helps create the images from the data obtained by the gamma camera.

How does the procedure work?

Ordinary x-ray exams create an image by passing x-rays through the body. Nuclear medicine exams use a radioactive material called a radiopharmaceutical or radiotracer. This material is injected into the bloodstream, swallowed or inhaled as a gas. The material accumulates in the area of your body under examination, where it gives off a small amount of energy in the form of gamma rays. Special cameras detect this energy and, with the help of a computer, create pictures that offer details on the structure and function of organs and tissues.

PET scans only use radiotracer injections.

Unlike other imaging techniques, nuclear medicine exams focus on processes within the body, such as rates of metabolism or levels of various other chemical activity. Areas of greater intensity, called "hot spots," indicate where large amounts of the radiotracer have accumulated and where there is a high level of chemical or metabolic activity. Less intense areas, or "cold spots," indicate a smaller concentration of radiotracer and less activity.

For more information on how a CT scan works, see Computed Tomography.

How is the procedure performed?

Nuclear medicine imaging is performed on outpatients and hospitalized patients.

You will lie on an examination table. If necessary, a nurse or technologist will insert an intravenous (IV) catheter into a vein in your hand or arm.

PET scans only use radiotracer injections.

The radiotracer typically takes about 30-60 minutes to travel through your body and be absorbed by the area under examination. You will be asked to rest quietly and to avoid movement and talking.

You may be asked to drink some contrast material that will localize in the intestines and help the radiologist interpreting the exam.

You will be moved into the PET/CT scanner to begin imaging. You will need to remain still during imaging. The CT exam takes place first, followed by the PET scan. On occasion, a second CT scan with intravenous contrast will follow the PET scan. For more information on how a CT scan is performed, see Computed Tomography. The CT scan takes less than two minutes. The PET scan takes 20-30 minutes.

Total scanning time is usually about 30 minutes.

Depending on which area is under examination, additional tests involving other tracers or drugs may be used. This could lengthen the procedure time to three hours. For example, if you are examined for heart disease, you may undergo a PET scan both before and after exercising or before and after receiving IV medication that increases blood flow to the heart.

When the examination is complete, you may be asked to wait until the technologist checks the images in case more images are needed. Sometimes, more images are obtained to clarify or better visualize certain areas or structures. The need for more images does not necessarily mean there was a problem with the exam or that something abnormal was found. It should not cause you concern.

If you had an intravenous (IV) line inserted for the procedure, it will usually be removed unless you are scheduled for another procedure that same day that requires an IV line.

What will I experience during and after the procedure?

Except for intravenous injections, most nuclear medicine procedures are painless. They are rarely associated with significant discomfort or side effects.

When the radiotracer is given intravenously, you will feel a slight pin prick when the needle is inserted into your vein for the intravenous line. You may feel a cold sensation moving up your arm when the radiotracer is injected. Generally, there are no other side effects.

PET scans only use radiotracer injections.

With some procedures, a catheter may be placed into your bladder. This may cause temporary discomfort.

It is important to remain still during the exam. Nuclear imaging itself causes no pain. However, having to remain still or to stay in one particular position during imaging may cause discomfort.

If you have a fear of closed spaces, you may feel anxious during the exam.

Unless your doctor tells you otherwise, you may resume your normal activities after your exam. A technologist, nurse or doctor will provide you with any necessary special instructions before you leave.

The small amount of radiotracer in your body will lose its radioactivity over time through the natural process of radioactive decay. It may also pass out of your body through your urine or stool during the first few hours or days following the test. Drink plenty of water to help flush the radioactive material out of your body.

For more information on what you will experience during and after a CT scan, see Computed Tomography.

Who interprets the results and how do I get them?

A radiologist or other doctor specially trained in nuclear medicine will interpret the images and send a report to your referring physician.

If your doctor has ordered a diagnostic CT, a radiologist with specialized training in interpreting CT exams will send a report to your referring physician.

What are the benefits vs. risks?


  • Nuclear medicine examinations provide unique information—including details on the function and anatomy of body structures—that is often unattainable using other imaging procedures.
  • Nuclear medicine scans provide the most useful diagnostic or treatment information for many diseases.
  • A nuclear medicine scan is less expensive and may yield more precise information than exploratory surgery.
  • By identifying changes in the body at the cellular level, PET imaging may detect the early onset of disease before it is evident on other imaging tests such as CT or MRI.
  • For additional benefits of CT exams, see Computed Tomography (CT).

The benefits of a combined PET/CT scan include:

  • greater detail with a higher level of accuracy; because both scans are performed at the same time without the patient having to change positions, there is less room for error.
  • greater convenience for the patient who undergoes CT and PET at one time rather than two different times.


  • Because only a small dose of radiotracer is used, nuclear medicine exams have a relatively low radiation exposure. This is acceptable for diagnostic exams. Thus, the radiation risk is very low when compared with the potential benefits.
  • Nuclear medicine diagnostic procedures have been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
  • Treatment risks are always weighed against the potential benefits for nuclear medicine therapeutic procedures. Your doctor will inform you of all significant risks prior to the treatment and give you an opportunity to ask questions.
  • Allergic reactions to radiotracers are extremely rare and usually mild. Always tell the nuclear medicine personnel of any allergies you may have or other problems that may have occurred during a previous nuclear medicine exam.
  • Injection of the radiotracer may cause slight pain and redness. This should rapidly resolve.
  • Women should always tell their doctor and radiology technologist if there is any possibility that they are pregnant or they are breastfeeding. See the Safety in X-ray, Interventional Radiology and Nuclear Medicine Procedures page for more information about pregnancy, breastfeeding and nuclear medicine exams.
  • For risks of CT exams, see Computed Tomography (CT).

What are the limitations of Positron Emission Tomography – Computed Tomography (PET/CT)?

Nuclear medicine procedures can be time consuming. It can take several hours to days for the radiotracer to accumulate in the area of interest, and imaging may take up to several hours to perform. In some cases, newer equipment can substantially shorten the procedure time.

The image resolution of nuclear medicine images may not be as high as that of CT or MRI. However, nuclear medicine scans are more sensitive for a variety of indications, and the functional information they yield is often unobtainable by other imaging techniques.

Altered blood sugar or blood insulin levels may adversely affect the test results of diabetic patients or patients who have eaten a few hours prior to the exam.

The radiotracer decays quickly and is effective for only a short time. Therefore, it is important for you to be on time for your appointment and to receive the radioactive material at the scheduled time. Late arrival for an appointment may require you to reschedule the procedure.

A very obese person may not fit into the opening of a conventional PET/CT unit.

This page was reviewed on August, 01, 2019


Radiologist preparing a patient for a computed tomography (CT) exam. View full size with caption

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