Clinical radiometry

Clinical radiometry is the measurement of the radioactivity of the whole body or part of it after the administration of the RFP. Usually in clinical practice gamma-emitting radionuclides are used. After introduction into the body of RFP containing such a radionuclide, its radiation is captured by a scintillation detector located above the corresponding part of the patient's body. The results of the investigation are usually presented on the light board in the form of the number of pulses recorded for a certain period of time, or in the form of counting speed (in pulses per minute). In clinical practice this method is not of great importance. Usually it is used in those cases when it is necessary to identify and evaluate the incorporation of radionuclides in case of accidental ingestion into the human body - by negligence, in case of catastrophes.

A more interesting method is the radiometry of the whole body. When it is carried, the person is placed in a special low-background camera containing several specially oriented scintillation detectors. This makes it possible to record the radioactive radiation of the entire body, and under conditions of minimal influence of the natural radioactive background, which, as is known, can be very high in some regions of the Earth's surface. If any part of the body (organ) is closed with a lead plate during radiometry, it is possible to estimate the contribution of this part of the body (or located under the organ plate) to the total radioactivity of the organism. In this way it is possible to study the metabolism of proteins, vitamins, iron, determine the volume of extracellular water. This method is also used when examining people with random incorporation of radionuclides (instead of the usual clinical radiometry).

Automated radiometers are used for laboratory radiometry. In them on the conveyor are placed test tubes with radioactive material. Under the control of the microprocessor, the tubes are automatically fed to the well meter window; After the radiometry is performed, the tubes are automatically changed. The results of the measurement are counted in the computer, and after appropriate processing they are fed to the printer. In modern radiometers, automatic calculations are performed in complex calculations, and the doctor receives ready information, for example, the concentration of hormones and enzymes in the blood, indicating the accuracy of the measurements. If the amount of work on laboratory radiometry is small, then simpler radiometers are used with manual displacement of the tubes and performing radiometry manually, in the non-automatic mode.

Radionuclide diagnostics in vitro (from Latin vitrum - glass, since all studies are performed in test tubes) refers to microanalysis and occupies a boundary position between radiology and clinical biochemistry. It makes it possible to detect the presence of various substances of endogenous and exogenous origin in biological fluids (blood, urine), located there in negligible concentrations or, as the chemists say, disappearing concentrations. These substances include hormones, enzymes, drugs, injected into the body with a therapeutic purpose, and others.

At various diseases, for example at a cancer or a myocardial infarction, in an organism there are substances, specific for these diseases. They are called markers (from English mark - label). The concentration of markers is as insignificant as the hormones: literally, single molecules in 1 ml of blood.

All these unique in their accuracy studies can be performed with the use of radioimmunoassay, developed in 1960 by American researchers S. Berson and R. Yalou, who subsequently was awarded the Nobel Prize for this work. Widespread introduction of it into clinical practice marked a revolutionary leap in microanalysis and radionuclide diagnostics. For the first time, physicians were given the opportunity, and very real, to decipher the mechanisms of the development of many diseases and diagnose them at the very top nnih stages. The endocrinologists, therapists, obstetricians, and pediatricians have most visibly felt the value of the new method.

The principle of the radioimmunological method consists in the competitive binding of the desired stable and similar labeled substances with a specific sensing system.

To perform this analysis, standard reagent kits are issued, each of which is designed to determine the concentration of any one particular substance.

As can be seen in the figure, the binding system (most often it is specific antibodies or antisera) interacts simultaneously with two antigens, one of which is sought, the other is its labeled analogue. Apply solutions in which the labeled antigen is always more than antibodies. In this case, a real fight of labeled and unlabeled antigens is played out for being associated with antibodies. The latter belong to the class G immunoglobulins.

They must be narrowly specific; react only with the antigen to be tested. Antibodies accept on their open binding sites (sites) only specific antigens, and in quantities proportional to the amount of antigens. This mechanism is figuratively described as a "lock and key" phenomenon: the larger the initial content of the desired antigen in the reacting solutions, the less the radioactive analogue of the antigen will be captured by the binding system and the greater part of it will remain unbound.

Simultaneously with the determination of the concentration of the substance sought in the patient's blood, under the same conditions and with the same reagents, a standard serum with exactly the concentration of the desired antigen is tested. By the ratio of the radioactivities of the reacted components, a calibration curve is constructed which reflects the dependence of the radioactivity of the sample on the concentration of the test substance. Then, comparing the radioactivity of the samples of the material obtained from the patient, with the calibration curve, the concentration of the substance sought in the sample is determined.

Radionuclide analysis in vitro became known as radioimmunoassay because it is based on the use of immunological antigen-antibody responses. However, in the future, other types of research were created that were similar in purpose and methodology, but differed in details in vitro. So, if an antibody is used as a labeled substance, and not an antigen, the analysis is called immunoradiometric; If the tissue receptors are taken as the binding system, they talk about radio-receptor analysis.

Radionuclide test in vitro consists of 4 stages.

• The first stage is the mixing of the analyzed biological sample with the reagents from the kit containing the antiserum (antibody) and the binding system. All manipulations with solutions are carried out by special semiautomatic micropipettes, in some laboratories they are carried out with the help of automatic devices.
• The second stage is the incubation of the mixture. It lasts until the dynamic equilibrium is reached: depending on the specificity of the antigen, its duration varies from a few minutes to several hours and even a day.
• The third stage is the separation of free and bound radioactive matter. For this purpose, the sorbents available in the kit (ion exchange resins, coal, etc.), which precipitate heavier antigen-antibody complexes, are used.
• The fourth stage is the radiometry of the samples, the construction of calibration curves, the determination of the concentration of the substance sought. All these works are performed automatically using a radiometer equipped with a microprocessor and a printing device.

As can be seen from the above, the radioimmunoassay is based on the use of the radioactive label of antigens. However, in principle, other substances, in particular enzymes, luminescent substances or high fluorescent molecules, can be used as an antigen or antibody label. On this new methods of microanalysis are based: immunoenzyme, immunoluminescent, immunofluorescent. Some of them are very promising and competing with radioimmunoassay.

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