Positron emission tomography (PET ) allows computer visualization of biochemical processes in the tissues of the body and is used in diagnostics and biomedical research. PET is most effectively used in the study of biochemical processes that occur with the participation of glucose and oxygen in the brain and heart. In PET, a chemical is “labeled” with short-lived, positrons emitting isotopes of carbon, oxygen, nitrogen or fluorine that are introduced into the body. Due to the fact that the isotopes used in PET have a short half-life, PET plants are usually located in close proximity to cyclotrons. The activity of such radioactive substances is quantitatively measured in the target organ using scintillator photomultipliers. When radionuclides decay, positrons annihilate with electrons, exciting gamma rays, i.e. photon flux. External detection and localization of a positron source inside the brain is based on the fact that during the annihilation of a positron and an electron two photons arise simultaneously and move in opposite
directions. Thus, conditions are created for recording two identical signals from the target organ. In this case, only those signals that are created by coinciding in time and oppositely directed photons are detected and participate in the construction of the image. This greatly increases the accuracy of the visualization of the object. Building a picture of the distribution of a radioactive isotope in the brain using PET is a multi-stage computerized procedure, as a result of which the concentration of radioactivity in sections of the brain is represented by different colors or different shades of gray. The spatial resolution of PET is approximately 8 mm, in some cases reaches 3 mm.
The rate of local brain metabolism using PET can be estimated by glucose or oxygen. When measured by oxygen, the subject inhales the oxygen isotope 15 O 2 until the level of radioactivity in the brain tissue reaches a stable level (10-15 minutes using the steady state method). In this case, additional measurements of local cerebral blood flow are necessary. When glucose is measured, carbon -substituted ( 11 C) and fluoro-substituted ( 18 F) glucose derivatives are used. Measurements with glucose derivatives last about 40 minutes, which is too long compared with the time required to perform most psychological tests.
PET is also used to study local cerebral blood flow. H 2 15 O is used as a blood flow tracer . Resolution of about 5 mm. Indicators of cerebral blood flow obtained using this method are characterized by great variability, since H 2 O does not freely diffuse in the brain. To improve the accuracy of determined differences of local cerebral blood flow characteristics when performing two consecutive planks comrade .
PET is a very expensive method, and currently in Russia studies are carried out only in some medical centers.
Single photon emission computed tomography (SPECT), in English Single photon emission tomography (SPECT) is aimed at determining the regional concentration of radionuclides inside organs. Unlike PET, in the SPECT technique, substances are introduced into the brain that do not participate in the metabolism, for example, the thallium-201 isotope or technetium-99, emitting gamma rays. The Tc-99 radioisotope with a half-life of about 6 hours emits a single photon with an energy of 140 KeV, which is easily detected by gamma cameras. Technetium is embedded in radiopharmaceuticals, which are proteins or other organic molecules. The choice of organic molecules for SPECT is determined by their ability to be absorbed by target organs, and depending on the intensity of gamma radiation of certain organs or their parts, we can talk about the metabolic rate in various target organs. SPECT does not provide an accurate anatomical picture of the organ under study.
SPECT requires collimation (creating a parallel orientation) of gamma rays emitted by radiopharmaceuticals. The collimators, together with photomultipliers and a NaI crystal, necessary to create a glow when interacting with gamma rays, form an Anger chamber. The size of the cameras depends on what objects are being investigated.
SPECT is still used to build a three-dimensional display of local cerebral blood flow, but there is the potential to apply it also for other purposes. The main advantage of SPECT over PET is that the long half-life of the radioactive substances used in SPECT does not require the cyclotron to be placed in close proximity to the facility. The disadvantages of the method include the fact that, due to the long half-life of radio drugs, repeated studies cannot be carried out at short intervals on the same person.