Radionuclide study

History of the discovery of radionuclide diagnostics

The distance between physics laboratories, where scientists recorded the tracks of nuclear particles, and everyday clinical practice seemed depressingly long. The very idea of using nuclear physics phenomena to examine patients might seem, if not crazy, then fabulous. However, this was the idea that was born in the experiments of the Hungarian scientist D. Hevesi, who later won the Nobel Prize. One autumn day in 1912, E. Rutherford showed him a pile of lead chloride lying in the basement of the laboratory and said: "Here, take care of this pile. Try to isolate radium D from the lead salt."

After numerous experiments conducted by D. Hevesi together with the Austrian chemist A. Paneth, it became clear that it was impossible to separate lead and radium D chemically, since they were not separate elements, but isotopes of one element - lead. They differ only in that one of them is radioactive. When decaying, it emits ionizing radiation. This means that a radioactive isotope - a radionuclide - can be used as a marker when studying the behavior of its non-radioactive twin.

Fascinating prospects opened up for doctors: introducing radionuclides into the patient's body and monitoring their location using radiometric devices. In a relatively short time, radionuclide diagnostics became an independent medical discipline. Abroad, radionuclide diagnostics in combination with the therapeutic use of radionuclides is called nuclear medicine.

The radionuclide method is a method of studying the functional and morphological state of organs and systems using radionuclides and indicators labeled with them. These indicators - they are called radiopharmaceuticals (RP) - are introduced into the patient's body, and then, using various devices, the speed and nature of their movement, fixation and removal from organs and tissues are determined.

In addition, tissue samples, blood and patient secretions can be used for radiometry. Despite the introduction of negligible amounts of the indicator (hundredths and thousandths of a microgram) that do not affect the normal course of life processes, the method has an exceptionally high sensitivity.

A radiopharmaceutical is a chemical compound that is approved for administration to humans for diagnostic purposes and that contains a radionuclide in its molecule. The radionuclide must have a radiation spectrum of a certain energy, cause minimal radiation exposure, and reflect the condition of the organ being examined.

In this regard, a radiopharmaceutical is selected taking into account its pharmacodynamic (behavior in the body) and nuclear-physical properties. The pharmacodynamics of a radiopharmaceutical is determined by the chemical compound on the basis of which it is synthesized. The possibilities of registering an RFP depend on the type of decay of the radionuclide with which it is labeled.

When choosing a radiopharmaceutical for examination, the physician must first of all take into account its physiological orientation and pharmacodynamics. Let us consider this using the example of the introduction of an RFP into the blood. After injection into a vein, the radiopharmaceutical is initially evenly distributed in the blood and transported to all organs and tissues. If the physician is interested in the hemodynamics and blood filling of organs, he will choose an indicator that circulates in the bloodstream for a long time, without going beyond the walls of blood vessels into the surrounding tissues (for example, human serum albumin). When examining the liver, the physician will prefer a chemical compound that is selectively captured by this organ. Some substances are captured from the blood by the kidneys and excreted in the urine, so they are used to examine the kidneys and urinary tract. Some radiopharmaceuticals are tropic to bone tissue, which makes them indispensable in examining the musculoskeletal system. By studying the transportation times and the nature of the distribution and elimination of the radiopharmaceutical from the body, the doctor judges the functional state and structural and topographic features of these organs.

However, it is not enough to consider only the pharmacodynamics of a radiopharmaceutical. It is necessary to take into account the nuclear-physical properties of the radionuclide included in its composition. First of all, it must have a certain radiation spectrum. To obtain an image of organs, only radionuclides emitting γ-rays or characteristic X-ray radiation are used, since these radiations can be registered with external detection. The more γ-quanta or X-ray quanta are formed during radioactive decay, the more effective this radiopharmaceutical is in diagnostic terms. At the same time, the radionuclide should emit as little corpuscular radiation as possible - electrons that are absorbed in the patient's body and do not participate in obtaining an image of organs. From this standpoint, radionuclides with a nuclear transformation of the isomeric transition type are preferable.

Radionuclides with a half-life of several tens of days are considered long-lived, several days - medium-lived, several hours - short-lived, several minutes - ultra-short-lived. For obvious reasons, they tend to use short-lived radionuclides. The use of medium-lived and especially long-lived radionuclides is associated with increased radiation exposure, the use of ultra-short-lived radionuclides is difficult for technical reasons.

There are several ways to obtain radionuclides. Some of them are formed in reactors, some in accelerators. However, the most common way to obtain radionuclides is the generator method, i.e. the production of radionuclides directly in the radionuclide diagnostics laboratory using generators.

A very important parameter of a radionuclide is the energy of electromagnetic radiation quanta. Very low energy quanta are retained in tissues and, therefore, do not reach the detector of a radiometric device. Very high energy quanta partially pass through the detector, so their registration efficiency is also low. The optimal range of quantum energy in radionuclide diagnostics is considered to be 70-200 keV.

An important requirement for a radiopharmaceutical is the minimum radiation exposure during its administration. It is known that the activity of the applied radionuclide decreases due to two factors: the decay of its atoms, i.e. a physical process, and its elimination from the body - a biological process. The decay time of half of the atoms of the radionuclide is called the physical half-life T 1/2. The time during which the activity of the drug introduced into the body decreases by half due to its elimination is called the biological half-life. The time during which the activity of the radiopharmaceutical introduced into the body decreases by half due to physical decay and elimination is called the effective half-life (Ef).

For radionuclide diagnostic studies, they try to choose a radiopharmaceutical drug with the shortest T 1/2. This is understandable, because the radiation load on the patient depends on this parameter. However, a very short physical half-life is also inconvenient: you need to have time to deliver the radiopharmaceutical to the laboratory and conduct the study. The general rule is: the Tdar of the drug should be close to the duration of the diagnostic procedure.

As already noted, currently laboratories most often use the generator method of obtaining radionuclides, and in 90-95% of cases this is the radionuclide ^99m Tc, which is used to label the vast majority of radiopharmaceuticals. In addition to radioactive technetium, ¹³³ Xe, ⁶⁷ Ga, and very rarely other radionuclides are used.

Radiopharmaceuticals most frequently used in clinical practice.

RFP	Scope of application
99m Tc-albumin	Blood flow study
99m 'Tc-labeled erythrocytes	Blood flow study
99m Tc-colloid (technifit)	Liver examination
99m Tc-butyl-IDA (bromeside)	Examination of the biliary system
99m Tc-pyrophosphate (technifor)	Skeletal examination
99m Ts-MAA	Lung examination
133 He	Lung examination
67 Ga-citrate	Tumorotropic drug, heart examination
99m Ts-sestamibi	Tumorotropic drug
99m Tc-monoclonal antibodies	Tumorotropic drug
201 T1-chloride	Heart, brain research, tumorotropic drug
99m Tc-DMSA (technemek)	Kidney examination
131 T-hippuran	Kidney examination
99 Tc-DTPA (pentatech)	Examination of the kidneys and blood vessels
99m Tc-MAG-3 (technemag)	Kidney examination
99m Tc-pertechnetate	Examination of the thyroid gland and salivary glands
18 F-DG	Brain and Heart Research
123 I-MIBG	Adrenal gland examination

Various diagnostic devices have been developed to perform radionuclide studies. Regardless of their specific purpose, all of these devices are designed according to a single principle: they have a detector that converts ionizing radiation into electrical impulses, an electronic processing unit, and a data presentation unit. Many radiodiagnostic devices are equipped with computers and microprocessors.

Scintillators or, less commonly, gas counters are usually used as detectors. A scintillator is a substance in which flashes of light, or scintillations, occur under the action of rapidly charged particles or photons. These scintillations are captured by photomultiplier tubes (PMTs), which convert the flashes of light into electrical signals. The scintillation crystal and PMT are placed in a protective metal casing, a collimator, which limits the "field of vision" of the crystal to the size of the organ or body part being studied.

Usually, a radiodiagnostic device has several replaceable collimators, which are selected by the doctor depending on the objectives of the study. The collimator has one large or several small holes through which radioactive radiation penetrates the detector. In principle, the larger the hole in the collimator, the higher the sensitivity of the detector, i.e. its ability to register ionizing radiation, but at the same time its resolution is lower, i.e. the ability to separately distinguish small radiation sources. Modern collimators have several dozen small holes, the position of which is selected taking into account the optimal "vision" of the object of study! In devices designed to determine the radioactivity of biological samples, scintillation detectors are used in the form of so-called well counters. Inside the crystal there is a cylindrical channel into which a test tube with the material under study is placed. Such a detector design significantly increases its ability to capture weak radiation from biological samples. Liquid scintillators are used to measure the radioactivity of biological fluids containing radionuclides with soft β-radiation.

All radionuclide diagnostic studies are divided into two large groups: studies in which the radiopharmaceutical is introduced into the patient’s body – in vivo studies, and studies of the patient’s blood, tissue pieces and secretions – in vitro studies.

Any in vivo study requires psychological preparation of the patient. The purpose of the procedure, its importance for diagnostics, and the procedure should be explained to him. It is especially important to emphasize the safety of the study. As a rule, there is no need for special preparation. The patient should only be warned about his behavior during the study. In vivo studies use various methods of administering the radiopharmaceutical depending on the objectives of the procedure. Most methods involve injecting the radiopharmaceutical mainly into a vein, much less often into an artery, organ parenchyma, or other tissues. The radiopharmaceutical is also used orally and by inhalation (inhalation).

Indications for radionuclide examination are determined by the attending physician after consultation with a radiologist. As a rule, it is carried out after other clinical, laboratory and non-invasive radiation procedures, when the need for radionuclide data on the function and morphology of a particular organ becomes clear.

There are no contraindications to radionuclide diagnostics, there are only restrictions provided for by the instructions of the Ministry of Health.

Among the radionuclide methods, the following are distinguished: radionuclide visualization methods, radiography, clinical and laboratory radiometry.

The term "visualization" is derived from the English word "vision". It denotes obtaining an image, in this case using radioactive nuclides. Radionuclide visualization is the creation of a picture of the spatial distribution of the radiopharmaceutical in organs and tissues when it is introduced into the patient's body. The main method of radionuclide visualization is gamma scintigraphy (or simply scintigraphy), which is performed on a device called a gamma camera. A variant of scintigraphy performed on a special gamma camera (with a movable detector) is layer-by-layer radionuclide visualization - single-photon emission tomography. Rarely, mainly due to the technical complexity of obtaining ultra-short-lived positron-emitting radionuclides, two-photon emission tomography is also performed on a special gamma camera. Sometimes an outdated method of radionuclide visualization is used - scanning; it is performed on a device called a scanner.

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