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Single-photon emission tomography

 
, medical expert
Last reviewed: 31.05.2018
 
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One-photon emission tomography (OFET) gradually replaces the usual static scintigraphy, since it allows to achieve the best spatial resolution with the same amount of the same RFP. To detect much smaller areas of organ damage - hot and cold nodes. To perform the OFET, special gamma cameras are used. From ordinary they differ in that detectors (usually two) cameras rotate around the patient's body. During the rotation, the scintillation signals come to the computer from different camera angles, which makes it possible to build a layered image of the organ on the display screen (as with another layered imaging, X-ray computed tomography).

One-photon emission tomography is intended for the same purposes as static scintigraphy, i.e. To obtain an anatomical and functional image of the organ, but differs from the latter by a higher image quality. It allows to reveal smaller details and, consequently, to recognize the disease at earlier stages and with greater certainty. In the presence of a sufficient number of transverse "slices" obtained in a short period of time, a three-dimensional volumetric image of the organ can be constructed using a computer to get a more accurate idea of its structure and function.

There is another kind of layered radionuclide imaging - positron two-photon emission tomography (PET). Radiuclides emitting positrons, mainly ultrashort-living nuclides, whose half-life is several minutes, are used as RFPs: 11 C (20.4 min), 11 N (10 min), 15 O (2.03 min), 1 8 F (1O min). The positrons emitted by these radionuclides annihilate near the atoms with electrons, resulting in the emergence of two gamma quanta - photons (hence the name of the method) that fly from the annihilation point in strictly opposite directions. The flying quanta are detected by several gamma-camera detectors located around the subject.

The main advantage of PET is that its radionuclides can be used to label very important physiologically medicinal preparations, for example glucose, which, as is known, is actively involved in many metabolic processes. When a labeled glucose is introduced into a patient's body, it is actively involved in the tissue metabolism of the brain and the heart muscle. By registering with the help of PET the behavior of this drug in these organs, one can judge the nature of metabolic processes in tissues. In the brain, for example, early forms of circulatory disturbance or development of tumors are detected, and even a change in the physiological activity of the brain tissue is revealed in response to the action of physiological stimuli, light and sound. In the heart muscle determine the initial manifestations of metabolic disorders.

The spread of this important and very promising method in the clinic is constrained by the fact that ultrashort-lived radionuclides produce cyclotrons on nuclear particle accelerators. It is clear that working with them is possible 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, in the scanner detector there is a scintillation crystal of relatively small dimensions, several centimeters in diameter, so to view the entire organ under investigation it is necessary to move this crystal sequentially line by line (for example, as an electron beam in a cathode-ray tube). These movements are slow, so the duration of the study 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 in radionuclide diagnostics is rarely used, mainly where there are no gamma cameras.

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

However, the simplicity of the device of the radiograph and of the entire study as a whole is crossed out by a very significant shortcoming - low accuracy of the study. The thing is that in radiography, unlike scintigraphy, it is very difficult to observe the correct "geometry of the count", i.e. Place the detector exactly above the surface of the organ under examination. As a result of this inaccuracy, the radiograph detector often "sees" not what is needed, and the effectiveness of the investigation is low.

trusted-source[1], [2], [3], [4], [5], [6], [7], [8]

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