What is a PET scan?
Positron emission tomography (PET or PET scan) 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 small amount of a radioactive substance, called a radionuclide (radiopharmaceutical or radiotracer), is used to perform the procedure. 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 scan) 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 (CAT scan) 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 radiotracers 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, in PET scans of the brain, a radioactive atom is applied to glucose (blood sugar) 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.
What are the reasons for a PET scan?
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 doctor to recommend a PET scan.
What are the risks of 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, or latex should notify their doctor.
If you are pregnant or suspect that you may be pregnant, you should notify your health care provider due to the risk of injury to the fetus from a PET scan. If you are lactating, or breastfeeding, you should notify your health care provider due to the risk of contaminating breast milk with the radionuclide.
There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor 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
Medications, such as insulin, tranquilizers, and sedatives
Notify your doctor if any of the above situations may apply to you.
How do I prepare for a PET scan or PET/CT scan?
PRECAUTIONS: If you are pregnant or think you may be pregnant, please check with your doctor before scheduling the exam. Other options will be discussed with you and your doctor.
BREASTFEEDING: If you are breastfeeding, we recommend that you do not breastfeed your child for 24 hours following the injection of the radiotracer.
CLOTHING: Please dress comfortably. You may be asked to change into a patient gown that is provided for you. You can lock up all personal belongings. Please remove all piercings and leave all jewelry at valuables at home.
ACTIVITY: Avoid strenous activity or exercise for 24 hours prior to the appointment.
AGENTS: You will be required to drink a liquid called barium, which is a contrast agent. The more contrast you are able to drink, the better the images are for the radiologist to visualize your digestive tract. The barium may cause some abdominal discomfort. If you have a colostomy bag, you are advised to bring an extra bag and possibly a chance of clothes.
Iodinated intravenous (IV) contrast is only used when the requesting physician asks for CT scan with IV contrast in addition to the PET/CT. If your doctor requested additional CT scan with IV contrast and you have a history of allergic reaction to iodinated contrast, then you must be pre-medicated before the IV contrast portion of the scan.
Premedication is ordered by your physician and is usually taken 24, 12 and two hours prior to the scan. If you have a contrast allergy but have not completed premedication, the PET/CT scan (without IV contrast) can still be performed. You can then arrange to have the IV contrast CT scan at a later date. This will not compromise your PET/CT scan.
ALLERGY: Please inform the access center representative when you schedule your scan if you have had an allergic reaction to any contrast dye in the past.
EAT/DRINK: Eat nothing for four hours before the appointment time. No gum. No coffee or other drinks. You are encouraged to drink plain water (no flavors) as much as you like until the time of the scan, unless specified by your other providers. There may be different food and drink recommendations from your physician if you also have upcoming surgery.
DIABETICS: If you are having your PET scan with Johns Hopkins radiology, you will be given specific instructions before your examination.
CARDIAC STUDIES: If you are having a PET/CT cardiac scan with Johns Hopkins radiology, you will receive detailed information about the preparation.
MEDICATION: You may take your medications as usual, with plain water only.
Based on your medical condition, your doctor may request other specific preparation.
What happens during a PET scan?
PET scans can be done on an outpatient basis. It is also possible that some hospital inpatients may undergo a PET examination for certain conditions.
Specific protocols may vary, but 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 radiotracer.
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 radiotracer, depending on the type of study being done. You will be positioned on a padded table inside the scanner.
The radiotracer will be injected into your IV. The examination will start 30 - 60 minutes after the injection. You will remain in the facility during this time. You will not be hazardous to other people, as the radiotracer emits less radiation than a standard X-ray.
After the radiotracer has been absorbed for the appropriate length of time, the scan will begin. The scanner table will move slowly so that the body part being studied is scanned.
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 technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.
What happens after a PET scan?
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.
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 doctor as this may indicate an infection or other type of reaction.
You may resume your usual diet and activities, unless your doctor advises you differently.
Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.