A positron emission tomography (PET) scan is a unique noninvasive nuclear imaging technique that produces three-dimensional images of the living heart, brain or other organs. PET scans are often used in the diagnosis and management of cancers, certain brain disorders and heart disease. It is especially helpful to determine the presence and extent of coronary artery disease (CAD) or damage to the heart after a heart attack.

If the test is conducted after mild exercise (a stress test), it is similar to other nuclear stress tests, except that it has two major advantages. First, the images are less likely to be distorted by parts of the patient’s body (large breasts, obesity etc.), so abnormal results are more reliable, especially when combined with conventional computed tomography (CT) scanning. Second, it is an excellent tool for determining whether portions of the heart muscle are still viable (living and functioning) after a heart attack or other event in which there is a lack of oxygen-rich blood to the heart muscle.

However, PET scanning is not as readily available as more conventional nuclear imaging because of its greater cost and the need for a cyclotron device, which produces necessary isotopes on site. In recent years, the cost of PET scanning has dropped due to increased availability of isotopes and scanners. Also, in some centers, PET scanners have been combined into the same unit with CT scanners, thus reducing the time needed to conduct the test.

Before the test, people are encouraged to:

During the test, they may feel the needle prick when an intravenous (I.V.) line is inserted in their arm to administer a small amount of radioactive material. Otherwise, the test is painless, and the amount of radioactivity is roughly the same as patients are exposed to during a CT scan. The test takes at least an hour to complete. Afterward, patients may drive themselves home and go about their usual activities, drinking plenty of water to flush the radioactive material from their body.

A positron emission tomography (PET) scan is a unique, noninvasive imaging technique that can produce three-dimensional images of the living heart, brain or other organs at work, as well as providing information on the biochemical function of organs. The PET scan can provide information on blood flow, oxygen consumption, glucose metabolism and concentrations of various compounds in selected organs. PET scans require the injection of a radioactive material, which is not dangerous unless the patient has one of the following conditions:

A PET scan begins with the injection of a radioactive tracer into the patient’s bloodstream. This tracer is usually carried on a bioavailable substance, such as glucose, water or ammonia. Because glucose is used, the test results of people with diabetes may be affected by their blood sugar levels. Once the radioactive tracer is in the patient’s body, it begins to decay, even as the carrier substance is absorbed into the target tissues.

As the tracer decays, it emits positrons, which travel a short distance before being annihilated with a naturally present electron. This reaction produces a pair of annihilation photons (similar to gamma rays) that travel in opposite directions. These photons are sensed by special detectors that are placed around the body in an array, similar to the set-up used in conventional computed tomography (CT scanning). Using advanced logorithms, a computer calculates where the interaction between the electron and positron occurred. In this way, the computer can generate a very accurate, three dimensional image of the organ function. Organs that are not functioning optimally, or cancer tumors, have different absorption rates for the carrier substance, meaning they will show up on the scan in a different color. This test is valuable because it allows physicians to get a glimpse of organs on a molecular biological level so they can see changes in tissues even before any anatomical changes have occurred.

PET scanning, however, has several drawbacks. First off, it is very expensive because the radioactive isotopes used are short-lived and must be produced by a cyclotron at or near the site of PET scanner. While the number of regional facilities producing isotopes has increased, and the number of PET cameras has increased, the test remains relatively uncommon compared to some other heart imaging tools (e.g., echocardiography and CT scanning). Also, although the amount of radioactive material used is very small, it is not advisable for people to have repeat PET scans.

The main clinical use of PET scanning in heart disease is the evaluation of cardiac viability. This measure determines if the heart muscle is still able to function properly, usually after a heart attack or among patients with advanced heart failure. Using the PET scan, the physician can determine whether:

• Those portions are permanently scarred and no longer viable. 
  
  
• Those portions still contain living, viable heart muscle that is not able to function to its fullest potential (e.g., stunned or hibernating). This weakened functioning is probably due to a lack of oxygen-rich blood that is unable to travel through severely narrowed or blocked coronary arteries. This information can help the physician decide whether to repair or "revascularize" the obstructions.

If some living tissue is still present, a catheter-based procedure or bypass surgery (which creates a detour around a clogged artery) may be done to improve blood flow to the heart muscle. By reestablishing normal blood flow, the heart muscle may show significant improvement in its ability to pump. However, if the heart muscle is no longer viable, then medical management may be more appropriate, with therapy aimed toward 1) allowing the remaining viable muscle to work as efficiently as possible, and 2) preventing/treating other obstructive coronary disease. In extreme circumstances, where the patient cannot function with the remaining viable muscle, options may include a mechanical heart or a heart transplant.

The PET scan is not the only study that can determine cardiac viability. The more common single-photon emission computer tomography (SPECT) test is also used for this purpose. Currently, studies have yet to show which test is superior, and most hospitals use the SPECT test because it is less expensive. In the future, clinical, comparative studies may determine which test is superior, leading to possibly greater use to PET testing.

Less common heart-related uses of the PET scan include evaluating the function of, and blood flow to, the heart, in order to detect any signs of coronary artery disease, often before the disease becomes clinically evident. Other non-cardiac uses include scanning the brain for injury after a head trauma or for the presence of a tumor, as well as experimental use in pharmacology to determine how drugs interact with their target organs.

Cardiac PET stress testing, another type of radionuclide imaging test, can determine the presence and extent of coronary artery disease. PET scans may be recommended when other non-invasive tests (e.g., echocardiogram) do not yield a definite diagnosis. PET scanning has been long considered the golden standard to detect viable tissues within the heart.

For a cardiac PET, patients should wear comfortable, loose-fitting clothing on the day of the test, because they will need to change into a hospital gown from the waist up during the examination. They should also stop eating and drinking about four hours prior to the test. Diabetic patients will need to discuss specific dietary guidelines for the day of the test with their physician in order to moderate their glucose levels. Additionally, their physician may give specific instructions about reducing or stopping certain medications before the test.

Just before the test, the technician or physician will describe the test and discuss the patient’s medical history. A very small amount of a radioactive compound will be injected into the patient through the I.V. The compound selected is usually comprised of a radioactive isotope carried on a natural delivery agent such as glucose, water or ammonia. After the injection, there is a waiting period while the delivery agent is absorbed into the target organs. This waiting period varies, depending on which target organs are being visualized. After the waiting period is over, the patient is placed into the scanner and the image is generated.

In many cases, an initial scan will be made before the injection of the radioactive compound. The patient will lie down on an examination table with his or her arms over their head. The patient will be made as comfortable as possible, although some individuals may feel slightly uncomfortable holding the required position. The initial scan takes 10 to 30 minutes.

After the first scan, the radioactive compound is administered through the I.V. line. In the case of a suspected heart attack, areas of the heart that are viable will take in more of the tracer, showing as brighter or “hot” spots on the PET scan. Areas of the heart that are no longer viable will take in less of the tracer, showing as darker or “cold” spots on the PET scan. The second scan also lasts about 10 to 30 minutes.

Alternatively, the PET scan may be performed in a manner similar to a standard stress test. In this situation, a patient will be given a PET scan while at rest. Then he or she will either exercise on a treadmill or, if unable to exercise, will be given a medication to “stress” the heart. Another PET scan will then be taken.

After the test, the patient can get up from the exam table and change back into his or her clothes. There are no restrictions on daily routine, but it is recommended that the patient drink plenty of fluids in order to flush the tracer from the body.

Results of the PET scan will be interpreted and sent to the physician. It may take one to three days to fully interpret, report and deliver the results.

Depending on the results of this test, additional tests or treatment may be recommended.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following PET scanning-related questions:

1. Will I need any sedatives during the test?
   
   
2. Does the scanning machine make any noise?
   
   
3. Will I be enclosed?
   
   
4. Is there any danger of radiation exposure?
   
   
5. How many PET scans can I have in a lifetime?
   
   
6. If I've had other nuclear imaging studies, am I at increased risk because of exposure to additional radiation?
   
   
7. Is it possible that I'll have a reaction to the tracer element used?
   
   
8. What is the likelihood of flawed test results?
   
   
9. Are there any alternative tests you can use if I'm uncomfortable being exposed to radiation?
   
   
10. How long will it take to get my test results?