A positron emission tomography (PET) scan is a nuclear imaging technique that allows a physician to view a patient’s organs and how they are functioning. This minimally invasive imaging test produces three-dimensional images of the chemical activity within the heart, brain, breasts, ovaries and other organs. This activity is displayed as colors that are more intense or less intense, depending upon the status of the organ or tissue.

PET scans are ordered by physicians to help with the diagnosis and management of cancers, certain brain disorders and heart disease. They are especially helpful in:

• Determining the presence and extent of coronary artery disease (CAD) or damage to the heart after a heart attack.
  
  
• Revealing the existence and monitoring the spread of cancer.
  
  
• Pinpointing neurological conditions such as clinical depression and Alzheimer’s disease by monitoring the brain’s activity.

PET scanning is not as readily available as more conventional nuclear imaging because of its greater cost and the need for a cyclotron, a machine used to create high-speed beams of atomic or subatomic particles. However, most large medical facilities have the ability to administer the test. Before the test, patients are encouraged to:

• Wear comfortable, loose clothing
  
  
• Refrain from eating or drinking, typically for at least six hours before the test
  
  
• Discuss current medications with their physician

During the test, patients may feel the prick of a needle 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 minimal, roughly the same as patients are exposed to during a CAT scan (computed axial tomography). The test takes at least an hour to complete. Afterward, patients may drive themselves home and resume their usual activities. They are typically instructed to drink plenty of water to flush the radioactive material from their body.

A positron emission tomography (PET) scan is a minimally invasive radionuclide imaging technique that produces three-dimensional images of the living heart, brain, breasts, ovaries or other organs at work. The scan works by creating computer images of the chemical changes that take place inside tissue. These images can provide information on blood flow, oxygen consumption, blood sugar metabolism and concentrations of various compounds in selected organs.

PET scans are used in the diagnosis and management of many diseases and disorders. These include:

• Cancer. PET scans can reveal and monitor the existence and spread of cancer, including breast cancer and women’s reproductive cancers. PET scans identify the presence of a cancerous tumor because malignant tissue tends to use more energy and absorb more of a radioactive tracer than normal tissue. Thus, these areas appear as more intense colors on a PET scan image and are referred to as “hot spots.” PET scans are particularly effective in detecting tumors larger than 8 millimeters. PET scans can also help determine when cancer has spread to the lymph nodes even before enlargement of the lymph nodes is evident.
  
  
• Heart disorders. PET scans can reveal areas of reduced blood flow in the heart, and indicate early signs of coronary artery disease. They can also reveal any damaged or dead muscle that results after a heart attack. The scan can help a physician distinguish between dead heart tissue that no longer functions and poorly functioning heart tissue that can be repaired through forms of heart surgery, including angioplasty or coronary artery bypass surgery.
  
  
• Neurological disorders. PET scans can reveal brain activity that may be indicative of certain neurological disorders, such as:
  
  
• Depression. Depressed people appear to have less activity in certain brain regions than people who are not depressed.  
  
  
• Alzheimer’s disease. PET scans can detect subtle changes in brain activity that may indicate the presence of Alzheimer’s disease before a patient is aware of any symptoms.
  
  
• Other brain disorders. PET scans may help physicians pinpoint the source of abnormal brain activity associated with epileptic seizures, help test brain function after a stroke and assess progressive degenerative conditions such as Parkinson’s disease. Memory disorders and mental health conditions such as schizophrenia can be monitored.

PET scans require the injection of a radioactive material, which is not dangerous, unless the patient:

• Is pregnant or breastfeeding
• Has severe narrowing (stenosis) of the aortic valve
• Has severe heart failure

The chemicals used are mostly natural body compounds such as glucose (blood sugar), which are labeled or “tagged” with a small amount of radioactive material. For this reason, the test results of diabetic individuals may be affected by their glucose levels.

A PET scan usually begins with the injection of a radioactive tracer into the patient’s bloodstream. This tracer attaches itself to a bioavailable substance, such as glucose. Once the tracer is in the patient’s body, it begins to decay. At the same time, the glucose or other substance is absorbed into the muscle or organ being targeted.

As the tracer decays it emits positively charged particles called positrons, which are destroyed by negatively charged electrons. This process creates a type of energy called gamma rays. These gamma rays are detected by machines on or around the patient called photomultiplier-scintillator detectors. The information that is detected and recorded by these machines is then analyzed and reconstructed by a computer to produce clear three-dimensional images of the brain, heart or other targeted area of the body that needs to be examined. By studying the reabsorption pattern of the trace element, physicians are able to assess the functioning of an organ, such as the heart muscle or the spread (metastasis) of cancer.

A PET scan takes place in a medical facility with a PET scanner. In some smaller hospitals or rural areas, the PET scanner may be in a mobile unit that visits the hospital on a scheduled basis. Patients should wear comfortable, loose-fitting clothing on the day of the test. They might need to change into a hospital gown during the examination. Generally, they should refrain from eating and drinking at least six hours prior to the test.

Patients who are having their hearts scanned should not consume caffeine for at least 24 hours before the test. Patients with diabetes will need to discuss dietary guidelines for the day of the test with their physician in order to moderate their blood sugar (glucose) levels. Additionally, their physician may give specific instructions about reducing or stopping certain medications before the test.

Just before the test, the radiology technician or physician will describe the test and discuss the patient’s medical history. The technician will note areas of concern, such as ovarian cancer or breast cancer, when conducting the test. A very small amount of a radioactive compound will be combined with sugar, water or ammonia and injected into the patient through an intravenous (I.V.) line. The compound travels throughout the body and the PET scan creates images showing how the material is processed.

The scanner itself has a hole in the middle, and resembles a large doughnut. It contains multiple rings of detectors that record the emission of energy from the radioactive substance. This information is fed into a computer that creates color images of the body.

In many cases, an initial scan is made before the radioactive compound is injected. The patient will lie down on an examination table with the arms extended over the 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. It takes about 30 minutes for the substance to travel through the body and be absorbed into the body tissues. Areas of the body that take up greater amounts of the radioactive compound appear as brighter, or “hot” spots on the PET scan. Areas of lower uptake of radioactive material show up as darker, “cold” spots on the PET scan. The second scan also lasts about 10 to 30 minutes.

During a PET scan, the technician and patient are able to communicate with each other through a microphone and speakers in the machine. The two-way communication system allows the technician to provide instructions to the patient as well as information regarding the time element. Often, it is uncomfortable for the patient to stay in the same position for the necessary length of time. The patient will be able to inform the technician of any problems encountered during the procedure. In some machines, music can be played for the patient to help with relaxation. 

PET scans can be used to diagnose heart conditions. Areas of the heart that are viable will take in more of the tracer and appear as “hot spots,” while areas of the heart that are no longer viable will take in less of the tracer and show as “cold spots.” The PET scan may be performed in a manner similar to a standard stress test. In such testing, the patient is given a PET scan while still at rest. The patient then exercises on a treadmill or receives a medication that stresses the heart. Following the exercise or medication, another PET scan is taken.

PET scans can also help detect and determine the stage of cancers, including breast cancer and women’s reproductive cancers. Cancer cells absorb sugar faster than normal cells, so a buildup of the radioactive sugar in certain cells indicates a possible tumor. A PET scan may be administered when cancer is first diagnosed to determine where the cancer is located and if the original tumor has spread to other parts of the body. It also may be administered following treatment to monitor the status of the cancer. A PET scan is a fairly common test for patients who have been diagnosed with cancer.

When testing for a possible tumor, the patient is injected with the radioactive sugar, which is allowed to circulate in the body for 45 minutes. The sugar will accumulate in any tumor that is present. The patient then lies down on a table that moves through the PET scanner six or seven times over a 45-minute period. The PET scanner identifies the radiation and feeds the information to a computer, which generates images that are interpreted by a radiologist.

Depending on the organ or tissue being examined, some patients may undergo additional tests using other dyes or chemicals. In such instances, testing may continue for up to three to four hours.

After the test, the patient can get up from the exam table and, if wearing a hospital gown, change back into street clothes. There are usually no restrictions on daily routine, but it is recommended that the patient drink plenty of fluids to flush the tracer from the body.

Results of the PET scan will be sent to a patient’s physicians following the test. In cancer patients, the results may be sent to numerous physicians, such as the medical oncologist, radiation oncologist and surgeon. The test can help these physicians formulate a treatment plan or evaluate the effectiveness of treatment already received by the patient. In some cases, a PET scan may be used to monitor the patient’s condition over time for any sign of cancer recurrence. 

It may take one to three days for a physician to fully interpret the report and deliver the results to the patient, although preliminary information may be available within minutes. The physicians will review the results with the patient and determine if any additional testing or treatment is necessary.

The amount of radiation used in PET scans is so low that it poses no health risks to most patients. Pregnant women and those who are nursing, however, are generally discouraged from undergoing the procedure. This helps protect the fetus or infant (in the case of breastfeeding) from exposure to potentially harmful radiation. Also, young children may have a higher level of vulnerability to the radiation.

Patients should also inform their physician of all prescription or over-the-counter medications they are taking, as these have the potential to interfere with test results. Use of vitamins and herbal supplements also should be reported to the physician.

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to positron emission tomography (PET scan):

1. Why am I having a PET scan instead of another imaging test?
2. How should I prepare for my PET scan?
3. Do I need to change my diet, medication regimen or other routines prior to the test?
4. Can I have this test if I’m pregnant or breastfeeding?
5. What are the risks associated with this test?
6. Where can I have a PET scan performed?
7. How long is the test expected to take?
8. When and from whom will I receive the results?
9. What can be done if I become anxious during the test?
10. What are my restrictions following a PET scan?
11. What additional tests might be needed?
12. How often will I need to have PET scans?