Positron Emission Tomography (PET/CT) Scan
What is positron emission tomography (PET/CT)?
Positron emission tomography (PET) is a type of nuclear medicine procedure that measures metabolic activity of the cells of body tissues. Used mostly in patients with brain or heart conditions and cancer, PET helps to visualize the biochemical changes taking place in the body and may identify cancer in the body and other conditions of the brain and heart.
PET differs from other nuclear medicine examinations in that PET detects metabolism within body tissues, whereas other types of nuclear medicine examinations detect the amount of a radioactive substance collected in body tissue in a certain location to examine the tissue’s function.
Since PET is a type of nuclear medicine procedure, this means that a tiny amount of a radioactive substance called a radiopharmaceutical 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. 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 (PET/CT) or magnetic resonance imaging (PET/MRI).
The radionuclides used in PET scans are made by attaching a radioactive atom to chemical substances that are used naturally by the particular organ or tissue during its metabolic process. For example, a radioactive atom is applied to glucose (blood sugar) to create a radionuclide called fluorodeoxyglucose (FDG), because most tissues use glucose for 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.
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 may also be used in conjunction with other diagnostic tests, such as computed tomography (CT) or magnetic resonance imaging (MRI) to provide more definitive information about malignant (cancerous) tumors and other lesions. Newer technology combines PET and CT into one scanner, known as PET/CT.
How is a PET/CT scan performed?
PET works by using a scanning device (a machine with a large hole at its center) to detect photons (subatomic particles) emitted by a radionuclide in the organ or tissue being examined. The radionuclides used in PET scans are made by attaching a radioactive atom to chemical substances that are used naturally by the particular organ or tissue during its metabolic process.
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.
Although each facility may have specific protocols in place, a PET/CT scan procedure follows this process:
- The patient will be asked to remove any clothing, jewelry, or other objects that may interfere with the scan.
- The patient will be asked to empty his or her bladder prior to the start of the procedure.
- An IV line will be started in the hand or arm for injection of the radionuclide.
- The radionuclide will be injected into the IV. The radionuclide will be allowed to concentrate in the organ or tissue for about 30 to 60 minutes. The patient will remain in the facility in a quiet, relaxed state during this time. The patient will not be hazardous to other people, as the radionuclide emits less radiation than a standard X-ray.
- After the radionuclide has been absorbed for the appropriate length of time, the scan will begin. The scanner will move slowly over the body part being studied.