Norm of Positron Emission Tomography (PET)
Requires interpretation according to the type of study being performed.
Usage of Positron Emission Tomography (PET)
Enables noninvasive regional tissue physiology study of metabolic changes in body tissues. Comparison of cerebral blood flow and energy metabolism; evaluation for leakage of the blood-brain barrier; study of brain pharmacology; evaluation of brain hemodynamics in cerebrovascular disease; localization of seizure foci in clients with focal seizures; evaluation of regional myocardial blood flow, metabolism, and thus viability; study of the distribution of pulmonary edema; study of solid tumor proliferation, blood flow, glucose, and oxygen utilization alone and in response to therapy; diagnosing, staging, and restaging of cancer (lung, colorectal, lymphoma, melanoma, head and neck, and esophageal).
Description of Positron Emission Tomography (PET)
Positron emission tomography (PET) is a noninvasive radiographic method for studying blood flow and metabolic changes occurring in specific organs or regions of the body tissues. It involves the injection or inhalation of gamma ray–emitting, biologically compatible radioisotopes and the creation of images of radioisotope distribution in the body. As the radioisotopes disintegrate, they emit positrons, which are positively charged particles similar to electrons. As the positrons are captured by electrons, both are destroyed, resulting in the emission of two photons, which travel outward in opposite directions. The photons are detected simultaneously by the PET camera, an event known as a “coincidence.” The summation of these coincidences allows for the creation of a continuous map of the metabolic activity of the body. A computer then creates pictures of cross-sections of the body area studied, which show brighter areas according to the amount of radioisotope present.
Some examples of radioisotopes include oxygen-15, nitrogen-13, carbon-11, and fluorine-18, which are labeled onto substances such as water, carbon dioxide, or glucose. Because the radioisotopes are biologically compatible, they take the place of the body's chemical elements (such as oxygen, nitrogen, or fluorine), and the resulting scan gives a true representation of the physiologic function of the body processes. The choice of radioisotope and material to be labeled is based on the body function to be studied. For example, blood flow is studied using 15O-labeled HO-, glucose metabolism is studied using 18F-labeled glucose, tissue perfusion is studied using 13N-labeled NH2, and anaerobic metabolism is studied using 11C-labeled acetate. Some conditions in which the use of PET has been studied include Alzheimer's disease, asthma, brain tumors, cerebral atrophy, cerebrovascular disorders, chronic obstructive pulmonary disease, coronary artery disease, epilepsy, head trauma, Huntington's disease, myocardial infarction, obsessive compulsive disorder, pulmonary edema, schizophrenia, and unstable angina.
The newest equipment, called “Dual Mode Imaging,” combines PET with structural imaging modalities such as Ultrafast CT or MRI for improved imaging results. See Dual modality imaging.
Professional Considerations of Positron Emission Tomography (PET)
Consent form MAY be required.
During pregnancy, risks of cumulative radiation exposure to the fetus from this and other previous or future imaging studies must be weighed against the benefits of the procedure. Although formal limits for client exposure are relative to this risk:benefit comparison, the United States Nuclear Regulatory Commission requires that the cumulative dose equivalent to an embryo/ fetus from occupational exposure not exceed 0.5 rem (5 mSv). Radiation dosage to the fetus is proportional to the distance of the anatomy studied from the abdomen and decreases as pregnancy progresses. For pregnant clients, consult the radiologist/ radiology department to obtain estimated fetal radiation exposure from this procedure.
- A premedication may be prescribed to be given before transport to the nuclear medicine department.
- Diuretics should be withheld before the study unless an indwelling urinary catheter is present or will be inserted.
- If pelvic imaging is to be performed, an indwelling urinary catheter should be inserted.
- The client should have a meal before the procedure. Diabetic clients should be given their morning insulin before the procedure.
- If abdominal imaging is indicated, a bowel preparation may be prescribed.
- See Client and Family Teaching.
- The client is placed in a supine position on the scanning table, with an arm supported in extension.
- Intravenous access is established.
- A heparin flush solution is slowly infused.
- An arterial line may be inserted for some procedures.
- For brain scans, a polymer clay (Polyform)-molded face mask is placed over the temporal level of the face and secured to the headrest to immobilize the client's head. The mask is marked with a reference point to ensure exact repeat positioning for any necessary future PET studies.
- The scanning table is moved into position within the lumen of the positron emission scanner.
- Once the client is positioned, he or she must remain motionless throughout the study.
- Some studies are conducted by having the client inhale the radioisotope. Others use intravenous injection.
- An example of the steps involved in one type of scan follows.
- Cardiac PET:
- a. 15O-labeled HO- is injected intravenously, and a 15-minute test scan is conducted to verify proper positioning.
- b. A 30-minute transmission scan is then performed to correct for the attenuation of the chest and lungs.
- c. 13N-labeled NH2 is injected intravenously and allowed to equilibrate for 3 minutes. Then a PET study is performed for approximately 30 minutes to study cardiac tissue perfusion.
- d. Finally, glucose metabolism of the heart is studied. If the client has diabetes, with a glucose level >150 mg/dL, insulin may be given before this step. If the client has a low blood glucose level, either orally administered glucose or intravenous 50% dextrose in water will be given. Fluorodeoxyglucose (FDG) is injected intravenously to study glucose metabolism of the heart. After waiting 30 minutes for the FDG to circulate, one performs a 30-minute PET study.
- The arterial line, if inserted for the PET study, is discontinued, and the site should be monitored for the development of hematoma.
Client and Family Teaching
- You must remain motionless in an enclosed space for 1–3 hours.
- Wear comfortable clothing to the test.
- You may bring a cassette tape to listen to during the study.
- Do not drink large quantities of fluid or caffeine-containing beverages within 2 hours before the study unless you have been informed that an indwelling catheter will be inserted.
- Have a meal before the procedure.
- You may need a bowel preparation if abdominal imaging is indicated.
- Lactating women should not breast-feed for at least 20 hours after the scan.
Factors That Affect Results
- Hypoglycemia may alter the results of PET glucose metabolism.
- Movement more than about 1 cm may blur the resulting pictures. The ability of the client to remain motionless in an enclosed space affects whether an accurate study can be obtained.
- Clients with insulin-dependent diabetes must have insulin administered the day of the study if glucose metabolism will be a focus of PET.
- Anxiety in the client that causes tension in the neck area can cause increased uptake of the fluorine type of isotope, which can be misinterpreted as metastases.
- PET takes 1–3 hours. The half-life of the specific radioisotope used affects the length of the study.
- Claustrophobia may occur during the procedure.