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Positron emission tomography
Medical expert of the article
Last reviewed: 03.07.2025
Positron emission tomography (PET) is a method for studying the metabolic and functional activity of body tissues in vivo. The method is based on the phenomenon of positron emission observed in a radiopharmaceutical introduced into the body during its distribution and accumulation in various organs. In neurology, the main application point of the method is the study of brain metabolism in a number of diseases. Changes in the accumulation of nuclides in any area of the brain suggest a violation of neuronal activity.
Indications for positron emission tomography
Indications for positron emission tomography include testing for myocardial hibernation in patients undergoing coronary artery bypass grafting or cardiac transplantation and distinguishing metastasis from necrosis and fibrosis in enlarged lymph nodes in patients with cancer. PET is also used to evaluate pulmonary nodules and determine whether they are metabolically active, and to diagnose lung cancer, neck cancer, lymphoma, and melanoma. CT can be combined with positron emission tomography to correlate morphologic and functional data.
Preparation for positron emission tomography
PET is performed on an empty stomach (the last meal is 4-6 hours before the examination). The duration of the examination is from 30 to 75 minutes, depending on the scope of the procedure. During the 30-40 minutes required for the introduction of the administered drug into the body's metabolic processes, patients should be in conditions that minimize the possibility of motor, speech and emotional activity in order to reduce the likelihood of false-positive results. For this, the patient is placed in a separate room with soundproof walls; the patient lies with his eyes closed.
Alternative methods
Other functional neuroimaging methods such as magnetic resonance spectroscopy, single-photon emission CT, perfusion and functional MRI may serve to some extent as an alternative to PET.
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Single photon emission tomography
A less expensive option for radioisotope examination of the intravital structure of the brain is single-photon emission computed tomography.
This method is based on the registration of quantum radiation emitted by radioactive isotopes. Unlike the PET method, single-photon emission computed tomography uses elements that do not participate in metabolism (Tc99, TI-01), and with the help of a y-camera rotating around the object, single quanta (photons) are registered rather than paired ones.
One of the modifications of the single-photon emission computed tomography method is visualization of local cerebral blood flow. The patient is given a gas mixture to inhale, containing xenon-133, which dissolves in the blood, and with the help of computer analysis, a three-dimensional picture of the distribution of photon emission sources in the brain is constructed with a spatial resolution of about 1.5 cm. This method is used, in particular, to study the characteristics of local cerebral blood flow in cerebrovascular diseases and in various types of dementia.
Evaluation of results
PET evaluation is performed using visual and semi-quantitative methods. Visual evaluation of PET data is performed using both black and white and various color scales, allowing one to determine the intensity of radiopharmaceutical accumulation in various parts of the brain, identify foci of pathological metabolism, and evaluate their localization, contours, and sizes.
In a semi-quantitative analysis, the ratio of radiopharmaceutical accumulation between two areas of equal size is calculated, one of which corresponds to the most active part of the pathological process, and the other to the unchanged contralateral area of the brain.
The use of PET in neurology allows us to solve the following problems:
- study the activity of certain areas of the brain when presented with various stimuli;
- conduct early diagnosis of diseases;
- carry out differential diagnostics of pathological processes with similar clinical manifestations;
- predict the course of the disease, evaluate the effectiveness of the therapy.
The main indications for the use of the technique in neurology are:
- cerebrovascular pathology;
- epilepsy;
- Alzheimer's disease and other forms of dementia;
- degenerative diseases of the brain (Parkinson's disease, Huntington's disease);
- demyelinating diseases;
- brain tumors.
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Epilepsy
PET with 18-fluorodeoxyglucose allows to detect epileptogenic foci, especially in focal forms of epilepsy, and to evaluate metabolic disorders in these foci. In the interictal period, the epileptogenic focus zone is characterized by glucose hypometabolism, and the area of reduced metabolism in some cases significantly exceeds the size of the focus established using structural neuroimaging methods. In addition, PET allows to detect epileptogenic foci even in the absence of electroencephalographic and structural changes, it can be used in the differential diagnosis of epileptic and non-epileptic seizures. The sensitivity and specificity of the method increase significantly with the combined use of PET with electroencephalography (EEG).
At the time of an epileptic seizure, an increase in regional glucose metabolism is observed in the area of the epileptogenic focus, often in combination with suppression in another area of the brain, and after the seizure, hypometabolism is again recorded, the severity of which begins to decrease reliably 24 hours after the seizure.
PET can also be successfully used in deciding on indications for surgical treatment of various forms of epilepsy. Preoperative assessment of the localization of epileptic foci makes it possible to choose the optimal treatment tactics and make a more objective prognosis of the outcome of the proposed intervention.
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Cerebrovascular pathology
In diagnostics of ischemic stroke, PET is considered as a method for determining viable, potentially recoverable brain tissue in the ischemic penumbra zone, which will allow for specifying indications for reperfusion therapy (thrombolysis). The use of central benzodiazepine receptor ligands, which serve as markers of neuronal integrity, allows for a fairly clear distinction between irreversibly damaged and viable brain tissue in the ischemic penumbra zone at an early stage of stroke. It is also possible to conduct differential diagnostics between fresh and old ischemic foci in patients with repeated ischemic episodes.
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Alzheimer's disease and other types of dementia
In diagnosing Alzheimer's disease, the sensitivity of PET ranges from 76 to 93% (average 86%), which is confirmed by autopsy study materials.
PET in Alzheimer's disease is characterized by a pronounced focal decrease in cerebral metabolism mainly in the neocortical association areas of the cortex (posterior cingulate, temporoparietal and frontal multimodal cortex), with changes being more pronounced in the dominant hemisphere. At the same time, the basal ganglia, thalamus, cerebellum and cortex responsible for primary sensory and motor functions remain relatively intact. The most typical for Alzheimer's disease is bilateral hypometabolism in the temporoparietal areas of the brain, which in advanced stages can be combined with a decrease in metabolism in the frontal cortex.
Dementia due to cerebrovascular disease is characterized by a predominant involvement of the frontal lobes, including the cingulate and superior frontal gyrus. Patients with vascular dementia also typically have patchy areas of decreased metabolism in the white matter and cortex, often involving the cerebellum and subcortices. Frontotemporal dementia shows decreased metabolism in the frontal, anterior, and medial temporal cortex. Patients with Lewy body dementia have bilateral temporoparietal metabolic deficits reminiscent of Alzheimer's disease, but often involve the occipital cortex and cerebellum, which are usually intact in Alzheimer's disease.
Pattern of metabolic changes in various conditions associated with dementia
Etiology of dementia |
Metabolic disturbance zones |
Alzheimer's disease |
Damage to the parietal, temporal, and posterior cingulate cortex occurs earliest with relative sparing of the primary sensorimotor and primary visual cortex and with sparing of the striatum, thalamus, and cerebellum. In the early stages, the deficit is often asymmetrical, but the degenerative process eventually manifests bilaterally. |
Vascular dementia |
Hypometabolism and hypoperfusion in affected cortical, subcortical areas and cerebellum |
Frontal type dementia |
The frontal cortex, anterior temporal cortex, and mediotemporal regions are affected first, with an initially higher degree of severity of damage than the parietal and lateral temporal cortex, with relative preservation of the primary sensorimotor and visual cortex. |
Huntington's chorea |
The caudate and lenticular nuclei are affected earlier with gradual diffuse involvement of the cortex |
Dementia in Parkinson's disease |
Alzheimer's disease-like features but with greater sparing of the mediotemporal region and lesser sparing of the visual cortex |
Dementia with Lewy bodies |
Disturbances typical of Alzheimer's disease, but with less preservation of the visual cortex and possibly the cerebellum |
The use of PET as a predictor of the development of Alzheimer's type dementia, especially in patients with mild and moderate cognitive impairment, is promising.
Currently, attempts are being made to study cerebral amyloidosis in vivo using PET, using special amyloid ligands, for the purpose of preclinical diagnosis of dementia in individuals with risk factors. Studying the severity and localization of cerebral amyloidosis also allows for reliable improvement of diagnostics at different stages of the disease. In addition, the use of PET, especially in dynamics, makes it possible to more accurately predict the course of the disease and objectively evaluate the effectiveness of the therapy.
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Parkinson's disease
PET with the use of the specific ligand B18-fluorodopa allows quantitative determination of the deficiency of dopamine synthesis and storage within presynaptic striatal terminals in Parkinson's disease. The presence of characteristic changes allows establishing a diagnosis and organizing preventive and therapeutic measures already in the early, sometimes preclinical stages of the disease.
The use of PET allows for differential diagnosis of Parkinson's disease with other diseases whose clinical picture includes extrapyramidal symptoms, such as multiple system atrophy.
The state of the dopamine receptors themselves can be assessed using PET with the H2-receptor ligand raclopride. In Parkinson's disease, the number of presynaptic dopaminergic terminals and the amount of dopamine transporter in the synaptic cleft are reduced, while in other neurodegenerative diseases (e.g., multiple system atrophy, progressive supranuclear palsy, and corticobasal degeneration), the number of dopamine receptors in the striatum is reduced.
In addition, the use of PET allows us to predict the course and rate of progression of the disease, evaluate the effectiveness of drug therapy, and help determine indications for surgical treatment.
Huntington's chorea and other hyperkinesias
PET results in Huntington's chorea are characterized by a decrease in glucose metabolism in the caudate nuclei, which makes possible preclinical diagnostics of the disease in individuals at high risk of developing the disease according to DNA testing results.
In torsion dystonia, PET with 18-fluorodeoxyglucose reveals a decrease in the regional level of glucose metabolism in the caudate and lentiform nuclei, as well as the frontal projection fields of the mediodorsal thalamic nucleus, with an intact overall metabolic level.
Multiple sclerosis
PET with 18-fluorodeoxyglucose in patients with multiple sclerosis demonstrates diffuse changes in brain metabolism, including in the gray matter. The identified quantitative metabolic disorders can serve as a marker of disease activity, as well as reflect the pathophysiological mechanisms of exacerbation development, help in predicting the course of the disease and assessing the effectiveness of therapy.
Brain tumors
CT or MRI allows obtaining reliable information about the localization and volume of tumor damage to brain tissue, but does not fully provide the ability to differentiate benign lesions from malignant ones with high accuracy. In addition, structural neuroimaging methods do not have sufficient specificity to differentiate tumor recurrence from radiation necrosis. In these cases, PET becomes the method of choice.
In addition to 18-fluorodeoxyglucose, other radiopharmaceuticals are used to diagnose brain tumors, such as 11 C-methionine and 11 C-tyrosine. In particular, PET with 11 C-methionine is a more sensitive method for detecting astrocytomas than PET with 18-fluorodeoxyglucose, and it can also be used to evaluate low-grade tumors. PET with 11 C-tyrosine allows differentiating malignant tumors from benign brain lesions. In addition, highly and poorly differentiated brain tumors show different absorption kinetics of this radiopharmaceutical.
Currently, PET is one of the most highly accurate and high-tech studies for diagnosing various diseases of the nervous system. In addition, this method can be used to study the functioning of the brain in healthy people for scientific research purposes.
The use of the method due to insufficient equipment and high cost remains extremely limited and is available only in large research centers, but the potential of PET is quite high. The introduction of a technique that provides for the simultaneous performance of MRI and PET with subsequent combination of the obtained images seems extremely promising, which will allow obtaining maximum information about both structural and functional changes in various parts of the brain tissue.
What is Positron Emission Tomography?
Unlike standard MRI or CT, which primarily provide an anatomical image of an organ, PET evaluates functional changes at the level of cellular metabolism, which can be recognized already in the early, preclinical stages of the disease, when structural neuroimaging methods do not reveal any pathological changes.
PET uses various radiopharmaceuticals labeled with oxygen, carbon, nitrogen, glucose, i.e. natural metabolites of the body, which are included in the metabolism together with its own endogenous metabolites. As a result, it becomes possible to evaluate processes occurring at the cellular level.
The most common radiopharmaceutical used in PET is fluorodeoxyglucose. Other radiopharmaceuticals commonly used in PET include 11C -methionine (MET) and 11C -tyrosine.
The radiation load at the maximum dose of the administered drug corresponds to the radiation load received by the patient during chest X-ray in two projections, so the examination is relatively safe. It is contraindicated for people suffering from diabetes mellitus, with a blood sugar level of more than 6.5 mmol/l. Contraindications also include pregnancy and lactation.