What is Positron Emission Tomography (PET) Scanning

Sample image obtained using a combination of PET and CT imaging technology.

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

Nuclear medicine is a subspecialty within the field of radiology that uses very small amounts of radioactive material to diagnose or treat disease and other abnormalities within the body.

Nuclear medicine imaging procedures are noninvasive and usually painless medical tests that help physicians diagnose medical conditions. These imaging scans use radioactive materials called a radiopharmaceutical or radiotracer.

Depending on the type of nuclear medicine exam you are undergoing, the radiotracer is injected into a vein, swallowed by mouth or inhaled as a gas and eventually collects in the area of your body being examined, where it gives off energy in the form of gamma rays. This energy is detected by a device called a gamma camera, a (positron emission tomography) PET scanner and/or probe. These devices work together with a computer to measure the amount of radiotracer absorbed by your body and to produce special pictures offering details on both the structure and function of organs and other internal body parts.

In some centers, nuclear medicine images can be superimposed with computed tomography (CT) or magnetic resonance imaging (MRI) to produce special views, a practice known as image fusion or co-registration. These views allow the information from two different studies to be correlated and interpreted on one image, leading to more precise information and accurate diagnoses.

A PET scan measures important body functions, such as blood flow, oxygen use, and sugar (glucose) metabolism, to help doctors evaluate how well organs and tissues are functioning.

What are some common uses of the procedure?

PET scans are performed to:

• detect cancer
• determine how much a cancer has spread in the body
• assess the effectiveness of a treatment plan, such as cancer therapy
• determine if a cancer has returned after treatment
• determine blood flow to the heart muscle
• determine the effects of a heart attack, or myocardial infarction, on areas of the heart
• identify areas of the heart muscle that would benefit from a procedure such as angioplasty or coronary artery bypass surgery (in combination with a myocardial perfusion scan).
• evaluate brain abnormalities, such as tumors, memory disorders and seizures and other central nervous system disorders
• to map normal human brain and heart function

How should I prepare?

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

Women should always inform their physician or technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. See the Safety page for more information about pregnancy and breastfeeding related to nuclear medicine imaging.

You should inform your physician of any medications you are taking as well as vitamins and herbal supplements and if you have any allergies. Also inform your doctor about recent illnesses or other medical conditions.

Jewelry and other accessories should be left at home if possible, or removed prior to the exam because they may interfere with the procedure.

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

What does the equipment look like?

A positron emission tomography (PET) scanner is a large machine with a round, doughnut shaped hole in the middle, similar to a CT unit. Within this machine are multiple rings of detectors that record the emission of energy from the radiotracer in your body.

A nearby computer aids in creating the images from the data obtained by the camera or scanner.

How does the procedure work?

With ordinary x-ray examinations, an image is made by passing x-rays through your body from an outside source. In contrast, nuclear medicine procedures use a radioactive material called a radiotracer, which is injected into your bloodstream, swallowed by mouth or inhaled as a gas. This radioactive material accumulates in the organ or area of your body being examined, where it gives off a small amount of energy in the form of gamma rays. A gamma camera, PET scanner, or probe detect this energy and with the help of a computer create pictures offering details on both the structure and function of organs and other parts of your body.

Unlike other imaging techniques, PET does not produce clear structural images but rather a rendering that reflects the level of chemical activity in your body. 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 activity. Less intense areas, or cold spots, indicate a smaller concentration of radiotracer and less chemical activity.

How is it performed?

Nuclear medicine imaging is usually performed on an outpatient basis, but is often performed on hospitalized patients as well.

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

Depending on the type of nuclear medicine exam you are undergoing, the dose of radiotracer is then injected intravenously, swallowed by mouth or inhaled as a gas.

It will take approximately 30 to 60 minutes for the radiotracer to travel through your body and to be absorbed by the organ or tissue being studied. You will be asked to rest quietly, avoiding movement and talking.

You will then be moved into the PET scanner and the imaging will begin. You will need to remain still during imaging.

Actual scanning time is approximately 45 minutes.

Depending on which organ or tissue is being examined, additional tests involving other tracers or drugs may be used, which could lengthen the procedure time to four hours. For example, if you are being examined for heart disease, you may undergo a PET scan both before and after exercising.

When the examination is completed, you may be asked to wait until the technologist checks the images in case additional images are needed.

If you had an intravenous line inserted for the procedure, it will be removed.

What will I experience during and after the procedure?

Most nuclear medicine procedures are painless.

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

When swallowed, the radiotracer has little or no taste. When inhaled, you should feel no differently than when breathing room air or holding your breath.

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

It is important that you remain still while the images are being recorded. Though nuclear imaging itself causes no pain, there may be some discomfort from having to remain still or to stay in one particular position during imaging.

If you are claustrophobic, you may feel some anxiety while you are being scanned.

Unless your physician tells you otherwise, you may resume your normal activities after your nuclear medicine scan.

Through the natural process of radioactive decay, the small amount of radiotracer in your body will lose its radioactivity over time. In many cases, the radioactivity will dissipate over the first 24 hours following the test and pass out of your body through your urine or stool. You may be instructed to take special precautions after urinating, to flush the toilet twice and to wash your hands thoroughly. You should also drink plenty of water to help flush the radioactive material out of your body.

Who interprets the results and how do I get them?

A radiologist who has specialized training in nuclear medicine will interpret the images and forward a report to your referring physician.

What are the benefits vs. risks?

Benefits

• The information provided by nuclear medicine examinations is unique and often unattainable using other imaging procedures.
• For many diseases, nuclear medicine scans yield the most useful information needed to make a diagnosis or to determine appropriate treatment, if any.
• Nuclear medicine is much less traumatic 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.

Risks

• Because the doses of radiotracer administered are small, diagnostic nuclear medicine procedures result in minimal radiation exposure. Thus, the radiation risk is very low compared with the potential benefits.
• Nuclear medicine has been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
• Allergic reactions to radiopharmaceuticals may occur but are extremely rare.
• Injection of the radiotracer may cause slight pain and redness which should rapidly resolve.
• Women should always inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. See the Safety page for more information about pregnancy, breastfeeding and nuclear medicine exams.

What are the limitations of Positron Emission Tomography?

Nuclear medicine procedures can be time-consuming. It can take hours to days for the radiotracer to accumulate in the part of the body under study and imaging may take up to several hours to perform, though new equipment is available that can substantially shorten the procedure time.

The resolution of structures of the body with nuclear medicine may not be as clear as with other imaging techniques, such as CT or MRI. However, the information gained from nuclear medicine is unequaled in other imaging techniques.

PET scanning can give false results if chemical balances within the body are not normal. Specifically, test results of diabetic patients or patients who have eaten within a few hours prior to the examination can be adversely affected because of altered blood sugar or blood insulin levels.

Because the radioactive substance decays quickly and is effective for only a short period of time, it is important for the patient to be on time for the appointment and to receive the radioactive substance at the scheduled time. Thus, late arrival for an appointment may require rescheduling the procedure for another day.