A positron emission tomography (PET) scan is a specific type of imaging test that can help your doctor see how the tissues and organs inside your body are functioning. Unlike other scanning techniques, a PET scan doesn't produce clear structural images of organs. Instead, it shows images containing areas of more or less intense color to provide information about chemical activity within certain organs and tissues. This chemical activity may indicate areas of disease not detected by other scanning methods. In certain conditions, this may cause your doctor to alter treatment plans. PET scanning is important in determining neurological conditions, heart disease and the spread of cancer.

A PET scan is an effective way for your doctor to examine the chemical activity in certain parts of your body, particularly your brain and your heart. PET scanning may help detect abnormal areas in your body. In particular, a PET scan may be used to detect or monitor:

• Cancer. A PET scan can be particularly effective in identifying if you have cancer, if it has spread and if it has responded to treatment. Tumors often use more energy than healthy tissues do and may absorb more of a radioactive tracer, which allows the tumors to appear on the scan. In addition, PET scanning may be valuable in assessing the effectiveness of chemotherapy.

  PET scanning can also be used to determine the stage of a cancer. In the past, doctors could determine whether cancer had spread to the lymph nodes only if there was evidence of lymph node enlargement. With PET scanning, doctors may be able to detect cancer that has spread to a lymph node before enlargement occurs, allowing for a change to more appropriate treatment.

  However, PET scan test results need to be interpreted carefully because noncancerous conditions can resemble cancer. Only a biopsy can confirm the presence of cancer.

• Heart problems. Doctors use PET scanning to detect areas of decreased blood flow in the heart. A PET scan may reveal early coronary artery disease and damaged or dead heart muscle due to a heart attack. It can also differentiate nonfunctioning dead heart muscle from poorly functioning heart muscle that would benefit from a procedure, such as heart surgery, angioplasty or coronary artery bypass surgery, to increase blood flow to heart muscle. A PET scan can be particularly important in people who've previously had heart attacks.
• Depression. PET scanning allows doctors and scientists to compare brain activity in people experiencing periods of depression with brain activity in nondepressed people. Studies show that depressed people have less activity in certain brain regions than do people who aren't depressed. However, more research is needed to confirm this approach.

• Alzheimer's disease. A PET scan can detect mild physiological changes in the brain even when no signs or symptoms of Alzheimer's disease are evident and before severe damage to brain cells and memory loss occur. PET scanning can also provide visual images of activity in the brain when a person is asked to read, talk or listen to music. Such uses of a PET scan may help scientists to better understand the progression of Alzheimer's disease in different areas of the brain. PET scanning can also be a valuable tool in differentiating Alzheimer's disease from other types of dementia disorders, such as Parkinson's disease and Huntington's disease.

  Genetic testing and PET scanning have been combined to examine the brains of middle-aged to older adults carrying a known genetic risk factor for Alzheimer's disease. Doctors and scientists are also combining PET scans with magnetic resonance imaging (MRI) to create three-dimensional images of the brain, making it possible to measure the rate at which various regions of the brain use, deposit and metabolize certain chemicals.

• Other brain disorders. A PET scan can help locate the origin of abnormal brain activity associated with seizures due to epilepsy and assess brain function after a stroke. Doctors can also use a PET scan to detect memory disorders and certain mental health disorders, such as schizophrenia.

A PET scan is usually done on an outpatient basis. Your doctor will give you detailed instructions on how to prepare for your scan. Before undergoing the scan, be sure to tell your doctor about any prescription and over-the-counter medications you're taking, as well as any vitamins and herbal supplements. If you're taking certain medications or have certain diseases, such as diabetes, you'll receive specific instructions before your scan.

A general rule is to not eat anything for a minimum of six hours before the scan. If you'll be having your heart scanned, don't use caffeine for at least 24 hours before your test. Be sure to wear comfortable clothes to your appointment. You may be asked to change into a hospital gown for the test. If an area of your body near your bladder needs to be examined, a soft tube (catheter) may be inserted into your urethra to keep urine drained from your bladder during the scan.

If you're pregnant or think you might be pregnant, tell your doctor before undergoing a PET scan. The radioactive substance used during this procedure may expose the fetus to radiation. If you're breast-feeding your baby, tell your doctor. An infant can be exposed to radiation through your breast milk.

A PET scan measures vital functions such as blood flow, oxygen use and blood sugar (glucose) metabolism. This can help doctors distinguish abnormal from normal functioning organs and tissues.

A PET scan is a special form of nuclear (radionuclide) scanning. This type of diagnostic imaging involves the use of a small amount of radioactive substance (radiotracer) that's generally injected into your bloodstream. Different tissues in your body take up different radionuclides, and therefore the radioactive substance used during a PET scan depends on the organ your doctor wants to investigate. The radioactive substance gives off tiny amounts of energy (radiation) in the form of gamma rays. These rays are detected by a device called a PET scanner or gamma camera.

Information from a PET scanner or gamma camera is processed and converted into images. A PET scan portrays chemical activity in parts of your body as images, or colors, of intensity. Areas of more intense color, or high uptake of the radioactive substance, are called hot spots. Areas of less intense color, which indicate a low uptake of radioactive substance, are called cold spots. During a PET scan, the radioactive substance emits particles called positrons. The number of positrons emitted by an organ or area of tissue indicates how much of the radioactive substance the organ or tissue has taken up and, therefore, how chemically active it is.

A nurse or technologist takes you into a special examination room that houses the PET scanner, which has a hole in the middle and looks like a large doughnut. Within the PET machine are multiple rings of detectors that record the emission of energy from the radioactive substance in your body, creating an image of a part of your body.

First, you lie down on an examination table and receive the radioactive substance (radiotracer) by intravenous injection, which feels like a slight pinprick. In some research studies, you may inhale the radioactive substance as a gas. It takes about 30 to 60 minutes for the radiotracer to travel through your body and be absorbed by the organ or tissue to be imaged. In the meantime, you'll rest quietly and avoid movement or talking, as this may alter the distribution of the substance. You won't be able to feel the radioactive substance in your body.

While lying on the examination table, you're then moved into the hole of the machine. The images are displayed on a monitor of a nearby computer. You're made as comfortable as possible before you're positioned in the PET scanner. You're then asked to remain still for the duration of the examination, which takes about 30 to 45 minutes. Moving around during the test can reduce the quality of the images. If you're claustrophobic, you may feel some anxiety while positioned in the scanner. Be sure to tell the nurse or technologist about any discomfort.

Depending on which organ or tissue is being examined, additional tests involving other dyes or chemicals may be used, possibly increasing your total appointment time to three to four hours. For example, if you're being examined for heart disease, you may undergo a stress test in which a PET scan is obtained while you're at rest followed by the administration of other drugs to examine the blood flow to your heart while you exercise.

In general, there are no restrictions on your daily routine after the test. However, drink plenty of fluids to flush the radioactive substance from your body.

Different colors or degrees of brightness on the images from a PET scan represent different levels of tissue and organ function. A radiologist with special training in reading PET scans interprets the images from your PET scan. It usually takes one to three days to interpret, report and deliver the test results to your doctor. To enhance the radiologist's study of your PET scan, films from other tests, such as those from recent computerized tomography (CT) or magnetic resonance imaging (MRI) scans, may be compared with those of a PET scan.

Although a radioactive substance is used during a PET scan, the amount of radiation that you're exposed to is low. The amount of radiation in a radiotracer dose isn't enough to affect the normal processes of your body.

However, the radiotracer may expose radiation to the fetus of a pregnant woman or to the infant of a woman who is breast-feeding. You and your doctor can discuss risk to the fetus or infant versus the reasoning and benefit of having a PET scan performed.

PET scans offer significant advantages over other forms of imaging, such as CT or MRI scans, in diagnosing disease. However, PET scanning often is used along with a CT or MRI scan to produce a three-dimensional view of an organ or tissue. Today PET scanners are being made with PET and CT scanning devices all in one.

Until recently, a medical center with a PET scanner required a special laboratory and machine (cyclotron) to produce radioactive substances needed to conduct a PET scan. Because of this, few medical centers could afford in-house PET scanning. However, a number of facilities now make and distribute the radioactive substance, making it feasible for more medical facilities to offer PET scanning.