Positron emission tomography (PET) scan: Detecting conditions early

A positron emission tomography (PET) scan is a specific type of imaging test that can help your doctor see how your tissues and organs are functioning.

Unlike other scanning techniques, a PET scan isn't designed to show structural detail of organs. Instead, it shows images containing 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 useful in evaluating a variety of conditions — including neurological disease, heart disease, infections, certain inflammatory diseases and cancer.

What is a PET scan for?

A PET scan is an effective way for your doctor to examine the chemical activity in certain parts of your body, which may help detect abnormalities in those areas. In particular, a PET scan may be used to detect or monitor:

• Cancer. PET scans may be useful in determining the extent of spread of certain cancers, assessing how the cancer responds to treatment, and determining if the cancer has recurred. Cancers may use more energy than surrounding tissues and therefore appear brighter on the PET scan. PET scanning may also be valuable in assessing how certain cancers respond to chemotherapy.

  PET scans need to be interpreted carefully because noncancerous conditions can resemble cancer, and many types of cancer do not appear on PET scans. A PET scan can only suggest the presence of cancer; a biopsy is needed to 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.

• 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 frontal temporal dementia and Huntington's disease.

  Doctors and scientists are also combining PET scans with magnetic resonance imaging (MRI) to create 3-D 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. In the future, doctors may also use a PET scan to detect memory disorders and certain mental health disorders, such as schizophrenia and depression. Such uses are currently being investigated.

PET scan of the chest for cancer

PET scan of the chest for cancer

A PET scan, unlike a normal X-ray, can detect cancer before organ or gland enlargement occurs. Here a normal X-ray of the chest (left) is compared with a PET scan of the chest producing normal results (top right) and a PET scan revealing cancer that's spread to the lymph nodes (bottom right).

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PET scan of brain for depression

PET scan of brain for depression

A PET scan can compare brain activity during periods of depression (left) with normal brain activity (right). An increase of blue and green colors, along with decreased white and yellow areas, shows decreased brain activity due to depression.

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PET scan of the brain for Alzheimer's disease

PET scan of the brain for Alzheimer's disease

These PET scan images show normal brain activity (left) and reduced brain activity caused by Alzheimer's disease (right). The diminishing of the intense white and yellow areas in the image on the right indicates mild Alzheimer's disease, with the increase of blue and green colors showing decreased brain activity.

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How is a PET scan done?

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. These scans commonly use a form of blood sugar (glucose) that all cells in your body use for an energy source. The glucose is combined with a radioactive substance (radiotracer) that's generally injected into your bloodstream and allows the PET scanner to image which areas of your body are using more glucose.

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 positrons. These positrons are detected by a device called a PET scanner or gamma camera. 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. Areas that take up more glucose are more metabolically active and appear brighter on a PET scan. Areas that don't use much energy or that are damaged don't take up as much glucose and therefore aren't as bright on a PET scan.

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.

Risks 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.