Positron emission tomography (PET-CT) is a method of tomographic diagnostics using radionuclide preparations. Thanks to the use of various radiopharmaceuticals (fluorodeoxyglucose, C-tyrosine, C-methionine, etc.), it allows us to study various processes, including the functional and metabolic activity of tissues. PET-CT is a promising new research method that can replace several diagnostic procedures. The high information content and reasonable price of PET-CT contribute to the expansion of the scope of application of this technique in oncology, cardiology, neurology and other fields of medicine.
History of origin
PET-CT is a combination of two inventions: computed tomography technology and the possibility of using short-lived radionuclide indicators to track the characteristics of regional metabolism. Despite the fact that the principles of using radionuclide drugs were developed back in the 50s, and tests of computed tomographs were carried out in the early 70s of the last century, PET-CT began to be used in clinical practice less than 30 years ago.
Initially, PET-CT was prescribed to detect pathological changes in the heart and central nervous system. A little later, the researchers discovered that, thanks to the use of the isotope 18-FDG, which has a tropicity to cells with high activity of metabolic processes, PET-CT can be successfully used in diagnosing and determining the prevalence of malignant neoplasms. The end of the 90s of the last century was marked by a rapid increase in the popularity of PET-CT and the recognition of the technique among American oncologists and radiologists.
Before starting PET-CT, a rapidly decaying radionuclide drug is injected into the patient’s body. During the decay of the drug, positrons arise, which interact with electrons, forming gamma quanta in the process of mutual annihilation. Gamma quanta are recorded by special equipment (a PET-CT scanner). Then the information is processed by a computer, the received data is converted into a color image. The more radiopharmaceutical there is in a certain area– the brighter the image of this area turns out to be.
Tracking the dynamics of the distribution and excretion of radionuclide drug during PET-CT makes it possible to obtain information about the rate and features of metabolism in healthy and pathologically altered tissues. The result of PET-CT is a series of images with “hot” (bright) and “cold” (pale) zones. The use of various drugs makes it possible to study the state of certain tissues at the level of cellular metabolism, even before the appearance of clinical signs of the disease.
Differences of PET-CT
Along with other modern diagnostic techniques (CT, MRI), PET-CT is a non-invasive procedure that provides reliable information about the condition of various organs and tissues. PET-CT does not cause unpleasant sensations, is easily tolerated by patients and does not pose a danger to the health of patients. During the procedure, minimal amounts of radionuclide substances are used. The radiation dose for PET-CT corresponds to the radiation dose during traditional CT or conventional radiography.
The main difference between PET-CT and other research methods is the nature of the data obtained. Most diagnostic techniques, including CT, MRI and conventional radiography, primarily provide information about the features of the anatomical structure of organs. With the help of PET-CT, images are obtained that reflect the functional states of various parts of the organ at the level of cellular metabolism. This makes it possible to detect diseases at an early stage, when they are not yet reflected at the level of the structure, structure and location of the organ.
Unlike radiography, CT and MRI, which can be performed both with and without the use of certain medications, PET-CT is always performed after the introduction of a pharmaceutical. At the same time, iodine–containing contrast agents are used in CT and radiography, contrast agents based on paramagnetic ions of rare earth elements are used in MRI, and various radionuclide preparations with a very short half-life are used in PET-CT.
MRI, CT and radiography are used in the diagnosis of various diseases of all organs and systems – from lesions of the brain and ENT organs to diseases of the digestive tract and genitals; from injuries and malformations to oncological and inflammatory processes. PET-CT is not yet as widespread as the methods listed above. This diagnostic procedure is still gaining a worthy place in the list of modern non-invasive studies. Currently, PET-CT is mainly used in the detection of oncological diseases, heart diseases and central nervous system.
PET-CT is prescribed at the final stage of the examination, after radiography, ultrasound, CT, MRI, scintigraphy and other studies. Indications for PET-CT are diagnostic difficulties in identifying pathological foci, the need to assess the functional activity of these foci to determine the prevalence of pathological processes, early detection of disorders in the structurally unchanged areas of the affected organ or in other organs. In addition, PET-CT is used to draw up a treatment plan, evaluate the effectiveness of therapy and detect relapses in the long term.
PET-CT in oncology is used to detect areas with an increased level of metabolism, characteristic of malignant neoplasms. The procedure is prescribed for early detection of primary tumors and secondary metastatic lesions. PET-CT is used to accurately differentiate neoplasias and chronic inflammatory changes, determine the prevalence of already diagnosed neoplasms, detect the primary node with already confirmed metastases and accurately stage oncological processes.
In addition, PET-CT is prescribed when choosing a site for a biopsy, when planning surgical interventions, assessing changes in the state of the tumor against the background of radiation therapy or chemotherapy, differentiating recurrent neoplasms and post-therapeutic changes. The reliability of PET-CT results in oncological diseases is 80-95%, which is significantly higher than during CT, the accuracy of which ranges from 50 to 68%.
PET-CT of the heart allows you to assess the level of perfusion, detect areas of myocardial ischemia and determine their viability. PET-CT is prescribed in the process of early diagnosis of coronary heart disease and in case of suspected past heart attacks. In addition, PET-CT is used at the stage of determining indications for surgical intervention and when choosing the type of operation (stenting, balloon angioplasty, coronary artery bypass grafting, etc.).
PET-CT of the brain makes it possible to assess the level of brain tissue metabolism, identify areas of hemodynamic disorders, detect violations of the vital activity of brain cells and determine the cause of such disorders. PET-CT is used in cases of suspicion of the presence of bulky formations, differential diagnosis of malignant and benign tumors, as well as in assessing the prevalence of oncological processes. PET-CT is also used during the diagnosis and differential diagnosis of epilepsy, Parkinson’s disease, multiple sclerosis and dementias of various origins (vascular dementia, Alzheimer’s disease, etc.).
Due to the radiation load that the patient is exposed to during the study, pregnancy and age up to 14 years are considered as the main contraindications to PET-CT. The exception is situations when the procedure is necessary for vital reasons. Contraindications to PET-CT are also severe renal failure, subcompensated and decompensated diabetes mellitus. In the presence of these pathological conditions, replacement of PET-CT with other studies or preliminary therapy is required to achieve a compensation state.
In addition, there are certain limitations due to previous treatment. PET-CT is prescribed no earlier than a month after the completion of chemotherapy or surgery. The time interval between the end of radiation therapy and positron emission tomography should be at least 3 months. Lactation is not a contraindication to PET-CT if the patient refrains from breastfeeding for 8 hours after the completion of the study.
Preparation for PET-CT
Within 6 hours before the start of all types of procedures, you should refrain from eating and not drink anything but water. Caffeine-containing beverages should be excluded from the diet within 24 hours before PET-CT of the heart. When taking medications, you must strictly follow the doctor’s instructions (cancellation or appointment of some medications on the eve or on the day of the study is possible). You should choose loose clothing without metal parts. It is recommended to come to the medical institution 10-15 minutes before the start of the procedure.
Before starting PET-CT, the patient informs the specialist about the presence of chronic diseases, taking certain medications, as well as the type and time of the end of previously performed treatment. The patient presents to the doctor extracts from medical records and the results of previous studies (radiography, CT, MRI, ultrasound).
Methodology of conducting
The patient is asked to change clothes (in some medical institutions, during PET-CT, they are allowed to remain in their clothes if there are no metal elements on it that prevent the procedure) and a radiopharmaceutical is administered. Sometimes, simultaneously with a radionuclide substance, a light-acting muscle relaxant is administered. Then the patient is laid on a couch and asked to wait 30-45 minutes for the drug to be distributed throughout the body. During this period, you should remain motionless in order to avoid false positive results of the study.
After the specified time, the patient is transferred to the office for PET-CT and placed on a special table. The PET scanner is an open ring with sensors mounted on it. The absence of an enclosed space precludes the development of an attack of claustrophobia. The duration of PET-CT varies from 30 to 75 minutes. During the study, the patient should not experience unpleasant sensations. If unpleasant symptoms appear, you should inform a specialist about the deterioration of the condition.
After the completion of PET-CT, it is recommended to drink copious amounts, which contributes to the elimination of the radiopharmaceutical from the body. After the introduction of a muscle relaxant, the patient needs an accompanying person. It is not recommended to involve children and pregnant women as an escort, since the radionuclide drug in the patient’s body is a source of ionizing radiation. It usually takes 1-2 days to prepare a conclusion based on the results of PET-CT. Less often, documents are provided after a few hours.