Single-photon emission tomography

Single-photon emission tomography (SPET) is gradually replacing conventional static scintigraphy, as it allows for better spatial resolution with the same amount of the same radiopharmaceutical, i.e. to detect much smaller areas of organ damage - hot and cold nodes. Special gamma cameras are used to perform SPET. They differ from conventional cameras in that the detectors (usually two) of the camera rotate around the patient's body. During the rotation, scintillation signals are sent to the computer from different shooting angles, which makes it possible to construct a layered image of the organ on the display screen (as with another layered visualization - X-ray computed tomography).

Single-photon emission tomography is intended for the same purposes as static scintigraphy, i.e. to obtain an anatomical and functional image of an organ, but differs from the latter in its higher image quality. It allows for the detection of finer details and, therefore, for recognizing the disease at earlier stages and with greater reliability. With a sufficient number of transverse "sections" obtained in a short period of time, a computer can be used to construct a three-dimensional volumetric image of the organ on the display screen, allowing for a more accurate representation of its structure and function.

There is another type of layered radionuclide visualization - positron two-photon emission tomography (PET). Radionuclides that emit positrons are used as RFP, mainly ultra-short-lived nuclides with a half-life of several minutes - ¹¹ C (20.4 min), ¹¹ N (10 min), ¹⁵ O (2.03 min), ¹⁸ F (10 min). The positrons emitted by these radionuclides annihilate near atoms with electrons, which results in the emergence of two gamma quanta - photons (hence the name of the method), flying away from the annihilation point in strictly opposite directions. The flying away quanta are recorded by several detectors of the gamma camera, located around the person being examined.

The main advantage of PET is that the radionuclides used can be used to label very important physiological drugs, such as glucose, which is known to be actively involved in many metabolic processes. When labeled glucose is introduced into a patient's body, it is actively included in the tissue metabolism of the brain and heart muscle. By recording the behavior of this drug in the above-mentioned organs using PET, one can judge the nature of metabolic processes in the tissues. In the brain, for example, early forms of circulatory disorders or tumor development are detected in this way, and even changes in the physiological activity of brain tissue in response to physiological stimuli - light and sound - are detected. In the heart muscle, initial manifestations of metabolic disorders are determined.

The spread of this important and very promising method in the clinic is restrained by the fact that ultra-short-lived radionuclides are produced in nuclear particle accelerators - cyclotrons. It is clear that it is possible to work with them only if the cyclotron is located directly in the medical institution, which, for obvious reasons, is available only to a limited number of medical centers, mainly large research institutes.

Scanning is intended for the same purposes as scintigraphy, i.e. to obtain a radionuclide image. However, the scanner detector contains a scintillation crystal of relatively small size, several centimeters in diameter, so to view the entire organ being examined, this crystal must be moved sequentially line by line (for example, like an electron beam in a cathode-ray tube). These movements are slow, as a result of which the duration of the examination is tens of minutes, sometimes 1 hour or more. The quality of the image obtained in this case is low, and the evaluation of the function is only approximate. For these reasons, scanning is rarely used in radionuclide diagnostics, mainly where there are no gamma cameras.

To register functional processes in organs - accumulation, excretion or passage of radiopharmaceuticals - some laboratories use radiography. The radiograph has one or more scintillation sensors that are installed above the patient's body surface. When radiopharmaceuticals are introduced into the patient's body, these sensors detect the gamma radiation of the radionuclide and convert it into an electrical signal, which is then recorded on chart paper in the form of curves.

However, the simplicity of the radiograph device and the entire study as a whole is crossed out by a very significant drawback - low accuracy of the study. The fact is that with radiography, unlike scintigraphy, it is very difficult to maintain the correct "counting geometry", i.e. to place the detector exactly above the surface of the organ being examined. As a result of such inaccuracy, the radiograph detector often "sees" something other than what is needed, and the effectiveness of the study is low.

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