What is PET?
Positron emission tomography (PET) is a specialized radiology procedure used to examine various body tissues to identify certain conditions. PET may also be used to follow the progress of the treatment of certain conditions. While PET is most commonly used in the fields of neurology, oncology, and cardiology, applications in other fields are currently being studied.
PET is a type of nuclear medicine procedure. This means that a tiny amount of a radioactive substance, called a radionuclide (radiopharmaceutical or radioactive tracer), is used during the procedure to assist in the examination of the tissue under study. Specifically, PET studies evaluate the metabolism of a particular organ or tissue, so that information about the physiology (functionality) and anatomy (structure) of the organ or tissue is evaluated, as well as its biochemical properties. Thus, PET may detect biochemical changes in an organ or tissue that can identify the onset of a disease process before anatomical changes related to the disease can be seen with other imaging processes, such as computed tomography (CT) or magnetic resonance imaging (MRI).
PET is most often used by oncologists (doctors specializing in cancer treatment), neurologists and neurosurgeons (doctors specializing in treatment and surgery of the brain and nervous system), and cardiologists (doctors specializing in the treatment of the heart). However, as advances in PET technologies continue, this procedure is beginning to be used more widely in other areas.
PET is also being used in conjunction with other diagnostic tests such as computed tomography (CT) to provide more definitive information about malignant (cancerous) tumors and other lesions. The combination of PET and CT shows particular promise in the diagnosis and treatment of many types of cancer.
Until recently, PET procedures were performed in dedicated PET centers. The equipment used in these centers is quite expensive. However, a new technology called gamma camera systems (devices used to scan patients who have been injected with small amounts of radionuclides and currently in use with other nuclear medicine procedures) is now being adapted for use in PET scan procedures. The gamma camera system can complete a scan more quickly, and at less cost, than a traditional PET scan.
How does PET work?
PET works by using a scanning device (a machine with a large hole at its center) to detect positrons (subatomic particles) emitted by a radionuclide in the organ or tissue being examined.
The radionuclides used in PET scans are chemical substances such as glucose, carbon, or oxygen used naturally by the particular organ or tissue during its metabolic process. A radioactive substance is attached to the chemical required for the specific tests. For example, in PET scans of the brain, a radioactive substance is applied to glucose to create a radionuclide called fluorodeoxyglucose (FDG), because the brain uses glucose for its metabolism. FDG is widely used in PET scanning.
Other substances may be used for PET scanning, depending on the purpose of the scan. If blood flow and perfusion of an organ or tissue is of interest, the radionuclide may be a type of radioactive oxygen, carbon, nitrogen, or gallium.
The radionuclide is administered into a vein through an intravenous (IV) line. Next, the PET scanner slowly moves over the part of the body being examined. Positrons are emitted by the breakdown of the radionuclide. Gamma rays are created during the emission of positrons, and the scanner then detects the gamma rays. A computer analyzes the gamma rays and uses the information to create an image map of the organ or tissue being studied. The amount of the radionuclide collected in the tissue affects how brightly the tissue appears on the image, and indicates the level of organ or tissue function.
Other related procedures that may be performed include computed tomography (CT scan) and magnetic resonance imaging (MRI). Please see these procedures for additional information.