Radionuclide research

Opening history radionuclide diagnostics

Depressingly long seemed the distance between physical laboratories, where scientists registered tracks of nuclear particles, and everyday clinical practice. The very idea of the possibility of using nuclear-physical phenomena for the examination of patients could seem, if not insane, then fantastic. However, exactly such an idea was born in the experiments of the Hungarian scientist D.Heveshi, later the Nobel Prize winner. In one of the autumn days of 1912 E.Reserford showed him a pile of lead chloride, lying in the basement of the laboratory, and said: "Here, take this pile. Try to distinguish Radium from the salt of lead. "

After many experiments conducted by D.Heveshi together with the Austrian chemist A.Panet, it became clear that it is impossible to separate lead and radium D chemically, since these are not individual elements, but the isotopes of one element - lead. They differ only in that one of them is radioactive. Disintegrating, it emits ionizing radiation. Hence, a radioactive isotope, a radionuclide, can be used as a mark when studying the behavior of its non-radioactive twin.

Before the doctors opened up a tempting prospect: introducing into the patient's body radionuclides, to monitor their location with the help of radiometric instruments. Within a relatively short period, radionuclide diagnostics has become an independent medical discipline. Abroad, radionuclide diagnostics in combination with the therapeutic use of radionuclides are called nuclear medicine.

The radionuclide method is a method for studying the functional and morphological state of organs and systems with the help of radionuclides and labeled indicators. These indicators - they are called radiopharmaceuticals (RFPs) - are injected into the patient's body, and then using the various instruments determine the speed and nature of movement, fixation and removal from organs and tissues.

In addition, pieces of tissue, blood and discharge of the patient can be used for radiometry. Despite the introduction of negligibly small 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 the chemical compound permitted for administration to a person with a diagnostic purpose, in the molecule of which a radionuclide is contained. Radionut should have a spectrum of radiation of a certain energy, determine the minimum radiation load and reflect the condition of the organ under investigation.

In this regard, the radiopharmaceutical is chosen 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 possibility of registering RFPs depends on the type of decay of the radionuclide with which it is labeled.

Choosing a radiopharmaceutical for research, a physician should first of all take into account his physiological focus and pharmacodynamics. Consider this for example the introduction of RFP in the blood. After the injection into the vein, the radiopharmaceutical is initially evenly distributed in the blood and transported to all organs and tissues. If the doctor is interested in hemodynamics and blood filling of the organs, he will choose an indicator that circulates for a long time in the bloodstream without leaving the walls of the vessels in 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 serve to study the kidneys and urinary tract. Individual radiopharmaceuticals are tropic to bone tissue, and therefore they are indispensable in the study of the osteoarticular apparatus. Studying the terms of transportation and the nature of the distribution and removal of the radiopharmaceutical from the body, the doctor judges the functional state and structural-topographic features of these organs.

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

Radionuclides, whose half-life is several dozen days, are considered to be long-lived, several days are medium-lived, several hours are short-lived, and a few minutes are ultrashort-lived. For understandable reasons they tend to use short-lived radionuclides. The use of medium-lived and, especially, long-lived radionuclides is associated with increased radiation load, the use of ultrashort-lived radionuclides is hampered 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 of obtaining radionuclides is generator, i.e. Production of radionuclides directly in the laboratory of radionuclide diagnostics with the help of generators.

A very important parameter of the radionuclide is the energy of quanta of electromagnetic radiation. Quanta of very low energies are retained in the tissues and, therefore, do not reach the detector of the radiometric device. Quanta of very high energies partly fly through the detector, so the effectiveness of their registration is also low. The optimum range of quantum energy in radionuclide diagnostics is 70-200 keV.

An important requirement for a radiopharmaceutical is the minimum radiation load when it is administered. It is known that the activity of the applied radionuclide decreases due to the action of two factors: the decay of its atoms, i.e. Physical process, and removing it from the body - the biological process. The decay time of half the radionuclide atoms is called the physical half-life of T 1/2. The time for which the activity of the drug, introduced into the body, is reduced by half due to its excretion, is called the period of biological half-elimination. The time during which the activity of the RFP introduced into the body is reduced by half due to physical decay and elimination is called the effective half-life (TEF)

For radionuclide diagnostic studies, they seek to select a radiopharmaceutical with the least prolonged T 1/2. This is understandable because the radial load on the patient depends on this parameter. However, a very short physical half-life is also inconvenient: it is necessary to have time to deliver RFP to the laboratory and conduct a study. The general rule is this: The drug must approach the duration of the diagnostic procedure.

As already noted, now in the laboratories the generator method of obtaining radionuclides is used more often, and in 90-95% of cases it is the radionuclide ^99m Tc, which is labeled with the overwhelming majority of radiopharmaceutical preparations. In addition to radioactive technetium, ¹³³ Xe, ⁶⁷ Ga , sometimes very rarely other radionuclides are used.

RFP, the most commonly used in clinical practice.

RFP	Application area
99m Tc Albumin	Blood flow examination
99m 'Tc-labeled erythrocytes	Blood flow examination
99m Tc-colloid (technicalite)	Liver examination
99m Tc-butyl-IDA (bromeside)	Examination of the bile excretory system
99m Tc-pyrophosphate (technophore)	Study of the skeleton
99m Tc-MAA	Lung examination
133 Xe	Lung examination
67 Ga-citrate	Tumorotropic drug, heart examination
99m Tc-sesthabit	Tumorotropic drug
99m Tc-monoclonal antibodies	Tumorotropic drug
201 T1-chloride	Study of the heart, brain, tumorotropic drug
99m Tc-DMSA (technical reference)	Kidney examination
131 T-Hippuran	Kidney examination
99 Tc-DTPA (pentatech)	Study of the kidneys and blood vessels
99m Tc-MAG-3 (technomag)	Kidney examination
99m Tc-pertechnetate	Thyroid and salivary gland research
18 F-DG	Study of the brain and heart
123 I-MIBG	Study of the adrenal glands

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

Scintillators or, more rarely, gas counters are usually used as a detector. The scintillator is a substance in which light flashes-scintillations-are produced by the action of rapidly charged particles or photons. These scintillations are captured by photoelectric multipliers (PMTs), which convert light flashes into electrical signals. The scintillation crystal and the photomultiplier are placed in a protective metal casing, a collimator that limits the "field of vision" of the crystal to the size of the organ or the studied part of the patient's body.

Usually the radiodiagnostic device has several removable collimators, which the doctor chooses, depending on the research tasks. In the collimator there is one large or several small holes through which the radioactive radiation penetrates into the detector. In principle, the larger the hole in the collimator, the higher the sensitivity of the detector, i. E. Its ability to detect ionizing radiation, but at the same time its resolving power is lower, i.e. Distinguish between small sources of radiation. In modern collimators there are several tens of small holes, the position of which is chosen taking into account the optimal "vision" of the object of investigation! 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 tube with the material to be examined is placed. Such a detector device significantly increases its ability to capture weak radiation from biological samples. To measure the radioactivity of biological fluids containing radionuclides with soft β-radiation, liquid scintillators are used.

All radionuclide diagnostic studies are divided into two large groups: studies in which RFPs are introduced into the patient's body, in vivo studies, and studies of blood, tissue fragments and patient discharge-in vitro studies.

When performing any in vivo study, the patient's psychological preparation is required. He needs to clarify the purpose of the procedure, its importance for diagnosis, and the procedure. It is especially important to emphasize the safety of the study. In special training, as a rule, there is no need. It is only necessary to warn the patient about his behavior during the study. In vivo studies, various methods of administering RFP depending on the objectives of the procedure are used. In most methods, RFP is injected primarily into the vein, much less often into the artery, organ parenchyma, and other tissues. RFP is also used orally and by inhalation (inhalation).

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

Contraindications to radionuclide diagnostics are not present, there are only restrictions provided by instructions of the Ministry of Health.

Radionuclide methods distinguish between radionuclide imaging methods, radiography, clinical and laboratory radiometry.

The term "visualization" is derived from the English word "vision". They designate the acquisition of an image, in this case by radioactive nuclides. Radionuclide imaging is the creation of a picture of the spatial distribution of RFP in organs and tissues when it is introduced into the patient's body. The main method of radionuclide imaging is gamma scintigraphy (or simply scintigraphy), which is carried out on an apparatus called a gamma camera. A variant of scintigraphy performed on a special gamma camera (with a movable detector) is layered radionuclide imaging - single-photon emission tomography. Rarely, mainly because of the technical complexity of obtaining ultrashort-living positronizing radionuclides, two-photon emission tomography is also performed on a special gamma camera. Sometimes an already outdated method of radionuclide imaging is used - scanning; it is performed on an apparatus called a scanner.

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