Positron Emission Tomography (PET Scan)
What is positron emission tomography (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 (physicians specializing in cancer treatment), neurologists and neurosurgeons (physicians specializing in treatment and surgery of the brain and nervous system), and cardiologists (physicians 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 lung 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.
In general, PET scans may be used to evaluate organs and/or tissues for the presence of disease or other conditions. PET may also be used to evaluate the function of organs such as the heart or brain. Another use of PET scans is in the evaluation of the treatment of cancer.
More specific reasons for PET scans include, but are not limited to, the following:
- to diagnose dementias such as Alzheimer's disease, as well as other neurological conditions such as Parkinson's disease (a progressive disease of the nervous system in which a fine tremor, muscle weakness, and a peculiar type of gait are seen), Huntington’s disease (a hereditary disease of the nervous system which causes increasing dementia, bizarre involuntary movements, and abnormal posture), epilepsy (a brain disorder involving recurrent seizures), and cerebrovascular accident (stroke)
- to locate the specific surgical site prior to surgical procedures of the brain
- to evaluate the brain after trauma to detect hematoma (blood clot), bleeding, and/or perfusion (blood and oxygen flow) of the brain tissue
- to detect the spread of cancer to other parts of the body from the original cancer site
- to evaluate the effectiveness of cancer treatment
- to evaluate the perfusion to the myocardium (heart muscle) as an aid in determining the usefulness of a therapeutic procedure to improve blood flow to the myocardium
- to further identify lung lesions or masses detected on chest x-ray and/or chest CT
- to assist in the management and treatment of lung cancer by staging lesions and following the progress of lesions after treatment
- to detect recurrence of tumors earlier than with other diagnostic modalities
There may be other reasons for your physician to recommend a PET scan.
The amount of the radionuclide injected into your vein for the procedure is small enough that there is no need for precautions against radioactive exposure. The injection of the radionuclide may cause some slight discomfort. Allergic reactions to the radionuclide are rare, but may occur.
For some patients, having to lie still on the scanning table for the length of the procedure may cause some discomfort or pain.
Patients who are allergic to or sensitive to medications, contrast dyes, iodine, shellfish, or latex should notify their physician.
If you are pregnant or suspect that you may be pregnant, you should notify your physician due to the risk of injury to the fetus from a PET scan. If you are lactating, or breastfeeding, you should notify your physician due to the risk of contaminating breast milk with the radionuclide.
There may be other risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.
Certain factors or conditions may interfere with the accuracy of a PET scan. These factors include, but are not limited to, the following:
- high blood glucose levels in diabetics
- caffeine, alcohol, or tobacco consumed within 24 hours of the procedure
- excessive anxiety (may affect brain function)
- medications, such as insulin, tranquilizers, and sedatives
Notify your physician if any of the above situations may apply to you.
- Your physician will explain the procedure and offer you the opportunity to ask any questions that you might have about the procedure.
- You may be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if something is not clear.
- Notify the radiologist or technologist if you are allergic to latex or shellfish and/or sensitive to medications, contrast dyes, or iodine.
- Fasting for a certain period of time prior to the procedure is required, usually for at least four hours. Your physician will give you special instructions ahead of time as to the number of hours you are to withhold food and drink. Your physician will also inform you as to the use of medications prior to the PET scan.
- Notify your physician if you are pregnant or suspect you may be pregnant.
- Notify your physician of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.
- You should not consume any caffeine or alcohol, or use tobacco, for at least 24 hours prior to the procedure.
- If you are a diabetic who uses insulin, you may be instructed to take your pre-procedure insulin dose with a meal three to four hours prior to the procedure. Your physician will give you specific instructions based on your individual situation. Also, a fasting blood sugar test may be obtained prior to the procedure. If your blood sugar is elevated, you may be given insulin to lower the blood sugar.
- Based upon your medical condition, your physician may request other specific preparation.
PET scans may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.
Generally, a PET scan follows this process:
- You will be asked to remove any clothing, jewelry, or other objects that may interfere with the scan.
- If you are asked to remove clothing, you will be given a gown to wear.
- You will be asked to empty your bladder prior to the start of the procedure.
- One or two intravenous (IV) lines will be started in the hand or arm for injection of the radionuclide.
- Certain types of scans of the abdomen or pelvis may require that a urinary catheter be inserted into the bladder to drain urine during the procedure.
- In some cases, an initial scan may be performed prior to the injection of the radionuclide, depending on the type of study being done. You will be positioned on a padded table inside the scanner.
- The radionuclide will be injected into your vein. The radionuclide will be allowed to concentrate in the organ or tissue for about 30 to 60 minutes. You will remain in the facility during this time. You 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.
- When the scan has been completed, the IV line will be removed. If a urinary catheter has been inserted, it will be removed.
While the PET scan itself causes no pain, having to lie still for the length of the procedure might cause some discomfort or pain, particularly in the case of a recent injury or invasive procedure such as surgery. The radiologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.
You should move slowly when getting up from the scanner table to avoid any dizziness or lightheadedness from lying flat for the length of the procedure.
You will be instructed to drink plenty of fluids and empty your bladder frequently for 24 to 48 hours after the test to help flush the remaining radionuclide from your body.
The IV site will be checked for any signs of redness or swelling. If you notice any pain, redness, and/or swelling at the IV site after you return home following your procedure, you should notify your physician as this may indicate an infection or other type of reaction.
Your physician may give you additional or alternate instructions after the procedure, depending on your particular situation.