PET-CT scan of the body is a radionuclide tomographic diagnostic method that is used for a simultaneous and detailed examination of the entire body. Positron emission tomography makes it possible to study the distribution and accumulation of pre-administered radiopharmaceutical (RP) in internal organs and tissues. With the development of malignant tumors, metastases, inflammatory or dystrophic pathologies, the activity of chemical processes changes, which is manifested by a change in the color scheme on the obtained images. At the moment, radiopreparations are administered exclusively intravenously for PET, and their most common representatives are – 18F-fluorodeoxyglucose, 11C-methionine and 11C-tyrosine.
PET CT scan of the body is used in clinical situations when it is impossible to determine a specific area of study and identify pathological changes using other techniques. These cases include:
- Symptoms of a malignant tumor of unclear localization. PET is performed in the presence of persistent subfebrility, increased fatigue and sweating, anemia, rapid weight loss and other signs of a cancerous tumor that cannot be detected by radiography, ultrasound, CT and MRI.
- Search for distant metastases. When detecting malignant neoplasms in the late stages, there is a high probability of the presence of metastatic lesions of the lymph nodes or organs of other anatomical areas.
- Search for a primary cancer tumor. PET-CT scan of the body is used to identify cancer in situations where the lymph node affected by metastasis is first diagnosed.
- Monitoring the effectiveness of treatment. Positron emission tomography of the whole body is used to search for cancer relapses or metastases in patients after surgical treatment, courses of chemotherapy and/or radiation therapy.
- Screening. Full-body scanning is used as monitoring in healthy people and people with a pathological fear of cancer – carcinophobia.
Limitations in the use of whole-body PET are similar to those for computed tomography with X-ray contrast enhancement. Most contraindications to positron emission scanning are associated with the use of radiopharmaceuticals and radiation exposure to the patient.
- Pregnancy. Ionizing radiation emitted by a radioactive label has a detrimental effect on fetal maturation. Depending on the trimester of gestation, this can lead to miscarriage or severe congenital malformations.
- Breastfeeding. RP injected intravenously can be excreted from a woman’s body along with breast milk, so a woman is recommended to stop breastfeeding for the next 12-24 hours.
- Individual intolerance to RP. The administration of pharmacological agents to patients with hypersensitivity to them is prohibited. This can provoke the development of anaphylactic shock and other severe allergic reactions.
- Diabetes mellitus and hyperglycemia (blood glucose level over 8 mmol/l). The composition of some RP may include glucose, which can increase the overall level of sugar in the blood, thereby causing severe hyperglycemia.
- Severe renal failure. Violation of the excretory function of the kidneys leads to a delay of the radiopharmaceutical in the patient’s body.
- Psychomotor agitation. Acute mental disorders, claustrophobia, severe pain require the use of anesthetic support, because they do not allow the patient to maintain the necessary stationary position during the scan.
Preparation for PET-CT
To obtain the most accurate PET results and eliminate threats to the patient’s health, special preparatory measures are provided before scanning the entire body. They include the following items:
- Providing the radiologist with a referral for PET and all medical documentation that relates to the current disease (epicrisis, outpatient records, conclusions of doctors of related specialties, the results of previously conducted studies).
- Exclusion from the diet of foods with a high content of vegetable fiber and carbohydrates, alcoholic and carbonated beverages the day before PET.
- A light dinner on the eve of the study, consisting of cottage cheese or other fermented milk products.
- Consumption of 0.5-0.7 liters of pure still water no later than 1 hour before the introduction of the radiopharmaceutical. In total, you need to have 1-1.5 liters of water with you.
- Compliance with hunger 6 hours before the start of the scan.
- Conducting an allergy test with an administered RP to determine the patient’s sensitivity to a pharmacological agent.
- Evaluation of renal excretory function using blood test, the results of a biochemical blood test and a Rehberg sample.
Patients suffering from diabetes mellitus require a preliminary consultation of the treating endocrinologist with correction of insulin therapy, taking into account the dose and type of administered RP.
Methodology of conducting
- The radiologist briefly explains the essence of PET, the sequence of actions of the patient, the progress of the study.
- Empty the bladder before starting the scan. This is due to a very intensive accumulation of the drug in the genitourinary system.
- The patient is taken to the locker room, where he changes into special underwear or a bathrobe, removes all metal objects – jewelry, hair clips, etc. You can stay in your clothes provided that they are warm and do not contain metal elements.
- Next, the patient is taken to a separate room in which a radiopharmaceutical is administered by intravenous injection. The time of distribution in the body is from 1 to 2 hours and depends on the type and dose of the drug. For this period of time, the patient sits in a separate chair. Physical and speech activity is strictly limited, as they increase the consumption of RP.
- After proper distribution of the drug throughout the patient’s body, the patient is taken to a room with a positron emission tomograph scanning module, laid on a couch.
- The patient’s position inside the scanner is lying on his back, arms along the trunk, legs straightened. Rollers are placed under the neck, knees and elbow joints. There are options for couches with stands for knees and elbows. In such cases, the legs are slightly bent at the knees, and the arms are bent at the elbows and thrown over the head.
- Throughout the study, you need to maintain a stationary position. If this is not possible, a medical sedation is performed by an anesthesiologist.
- The duration of PET-CT scan of the body is from 20 minutes to 1 hour. At this time, the patient can hear the noise and clicks that accompany the operation of the tomograph. As a rule, this procedure does not cause physical sensations, complications and side reactions.
- After the end of the scan, you need to drink the remaining clean water. This will speed up the process of removing RP from the body.
Interpretation of results
PET-CT scan allows to study the activity of metabolic processes of organs and tissues by assessing the amount of the injected radiopharmaceutical. At the same time, the higher the metabolic rate in the organ, the more RP accumulates in it. All anatomical structures in a normal state are characterized by a certain rate of chemical reactions, which is displayed in the appropriate color on the tomogram. In order to simplify the analysis of images and standardize the data obtained with PET, a comparative reference scale of the intensity of accumulation of RP is used – the SUV scale. According to her, the degree of normal accumulation of radiopharmaceuticals in the tissues of the whole body can be divided into 4 levels:
- I – corresponds to soft tissues and muscles in a relaxed state, which almost do not accumulate RP. The tomogram shows the least bright colors – gray, dark blue.
- II – corresponds to the liver and other internal organs. They have a relatively low metabolic rate and are visualized in shades of green.
- III – intermediate level of activity of chemical processes “between the liver and the brain.” Most often it is characterized by a range of colors from yellow to red.
- IV – characterizes the brain. The highest level of metabolic activity among all normal tissues of the body. The PET is displayed in bright red or white.
Depending on the features of the tomograph and its pre-setting, these colors may differ. An abnormal increase in metabolic activity in a particular organ is a sign of pathological disorders. Based on the reference indicators of the SUV scale, it is possible not only to diagnose pathology before the development of structural changes, but also to assess its nature.
- The II level of activity of RP accumulation on the SUV scale corresponds to inflammatory changes in this area.
- Level III is a sign of a tumor or metastasis, rarely of chronic inflammation.
- The IV level of RP accumulation outside the brain tissue indicates a primary tumor or metastatic lesion.
Accordingly, low biochemical activity in uncharacteristic organs is a sign of ischemic and degenerative changes. A decrease in the level of RP accumulation in various anatomical structures by 1-2 levels, according to the SUV scale, may indicate the presence of coronary heart disease, myocardial infarction, multiple sclerosis, Alzheimer’s disease and other diseases.