PET (Positron Emission Tomography)

Introduction

Positron emission tomography, also called PET imaging or a PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of positrons. Positrons are tiny particles emitted from a radioactive substance administered to the patient.  The subsequent views of the human body developed by this technique are used to evaluate a variety of diseases.

Common uses of the Procedure

PET scans are used most often to detect cancer and to examine the effects of cancer therapy by characterizing biochemical changes in the cancer.  These scans are performed on the whole body.  PET scans of the heart can be used to determine blood flow to the heart muscle and help evaluate signs of coronary artery disease.  PET scans of the heart can also be used to determine if areas of the heart that show decreased function are alive rather than scarred due to a prior heart attack, called a myocardial infraction.  Combined with a myocardial perfusion study, PET scans differentiate nonfunctioning heart muscle from heart muscle that would benefit from a procedure, such as angioplasty or coronary artery bypass surgery, which would reestablish adequate blood flow and improve heart function.  PET scans of the brain are used to evaluate patients who have memory disorders of an undetermined cause, who have suspected or proven brain tumors or who have seizure disorders that are not responsive to medical therapy and, therefore, are candidates for surgery.

Preparing for the Procedure

You should wear comfortable, loose-fitting clothes. You should not eat for four hours before the scan.  You will be encouraged to drink water.  Your doctor will instruct you regarding the use of medications before the test.   Diabetic patients should ask for any specific diet guidelines to control glucose levels during the day of the test.

About the Equipment

You will be taken to an examination room that houses the PET scanner, which has a hole in the middle and looks like a large doughnut.  Within this machine are multiple rings of detectors that record the emission of energy from the radioactive substance in your body and permit an image of your body to be obtained.  While lying on a cushioned examination table, you will be moved into the hole of the machine.  The images are displayed on the monitor of a nearby computer, which is similar in appearance to the personal computer you may have in your home.

The Procedure

Before the examination begins, a radioactive substance is produced in a machine called a cyclotron and attached, or tagged, to a natural body compound, most commonly glucose, but sometimes water or ammonia.  Once this substance is administered to the patient, the radioactivity localizes in the appropriate areas of the body and is detected by the PET scanner.

Different colors or degrees of brightness on a PET image represent different levels of tissue or organ function. For example, because healthy tissue uses glucose for energy, it accumulates some of the tagged glucose, which will show up on the PET images. However, cancerous tissue, which uses more glucose than normal tissue, will absorb more of the substance and appear brighter than normal tissue on the PET images.

A nurse or technologist will take you into a special PET examination room.  You will lie down on an examination table and be given the radioactive substance as an intravenous injection (although, in some cases, it will be given through an existing intravenous line or inhaled as a gas).  It will then take approximately 30 to 60 minutes for the substance to travel through your body and be absorbed by the tissue under study.  During this time, you will be asked to rest quietly in a partially darkened room and to avoid significant movement or talking, which may alter the localization of the administered substance.  After that time, scanning begins.  This takes an additional 30 to 45 minutes.

Some patients, specifically those with heart disease, may undergo a stress test in which PET scans are obtained while they are at rest, and again after undergoing the administration of a pharmaceutical to alter the blood flow to the heart.

Usually, there are no restrictions on daily routine after the test, although you should drink plenty of fluids to flush the radioactive substance from your body.

What You Will Experience During the Procedure

The administration of the radioactive substance will feel like a slight pinprick if given by intravenous injection.  You will then be made as comfortable as possible on the examination table before you are positioned in the PET scanner for the test.  You will be asked to remain still for the duration of the examination.  Patients who are claustrophobic may feel some anxiety while positioned in the scanner.  Also, some patients find it uncomfortable to hold one position for more than a few minutes.  You will not feel anything related to the radioactivity of the substance in your body.

Benefits and Risks of PET Scans
Because PET allows study of body function, it can help physicians detect alterations in biochemical processes that suggest disease before changes in anatomy are apparent on other imaging tests such as CT or MRI scans.

The radioactive substance may expose radiation to the fetus of patients who are pregnant or the infants of women who are breast-feeding.  The risk to the fetus or infant should be considered related to the potential information gain from the result of the PET examination.  If you are pregnant you should inform the PET imaging staff before the examination is performed.

Limitations of Positron Emission Tomography

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

Also, because the radioactive substance decays quickly and is effective for a short period of time, it must be produced in a laboratory near the PET scanner.  The value of a PET scan is enhanced when it is part of a larger diagnostic work-up. This often entails comparison of the PET scan with other imaging studies such as CT or MRI.