The mechanism of occurrence of X-rays. What are x-rays - properties and applications of radiation. X-ray radiation - bremsstrahlung and characteristic
They are emitted with the participation of electrons, in contrast to gamma radiation, which is nuclear. Artificial X-rays are created by strongly accelerating charged particles and by moving electrons from one energy level to another, releasing a large amount of energy. Devices that can be obtained are X-ray tubes and particle accelerators. Its natural sources are radioactively unstable atoms and space objects.
Discovery history
It was made in November 1895 by Roentgen, a German scientist who discovered the fluorescence effect of barium platinum cyanide during the operation of a cathode ray tube. He described the characteristics of these rays in some detail, including the ability to penetrate living tissue. They were called X-rays by the scientist, the name "X-ray" took root in Russia later.
What characterizes this type of radiation
It is logical that the features of this radiation are due to its nature. An electromagnetic wave is what X-rays are. Its properties are the following:
X-ray radiation - harm
Of course, at the time of opening and long years after that, no one realized how dangerous it was.
Where are x-rays used?
- Medicine. X-ray diagnostics - "transmission" of living organisms. X-ray therapy - the effect on tumor cells.
- The science. Crystallography, chemistry and biochemistry use them to reveal the structure of matter.
- Industry. Detection of defects in metal parts.
- Safety. X-ray equipment is used to detect dangerous items in luggage at airports and other places.
a brief description of x-ray radiation
X-rays are electromagnetic waves (flux of quanta, photons), the energy of which is located on the energy scale between ultraviolet radiation and gamma radiation (Fig. 2-1). X-ray photons have energies from 100 eV to 250 keV, which corresponds to radiation with a frequency of 3×10 16 Hz to 6×10 19 Hz and a wavelength of 0.005–10 nm. Electromagnetic spectra X-rays and gamma rays overlap to a large extent.
Rice. 2-1. Electromagnetic radiation scale
The main difference between these two types of radiation is the way they occur. X-rays are obtained with the participation of electrons (for example, during the deceleration of their flow), and gamma rays - with the radioactive decay of the nuclei of some elements.
X-rays can be generated during deceleration of an accelerated stream of charged particles (the so-called bremsstrahlung) or when high-energy transitions occur in the electron shells of atoms (characteristic radiation). Medical devices use X-ray tubes to generate X-rays (Figure 2-2). Their main components are a cathode and a massive anode. The electrons emitted due to the difference in electrical potential between the anode and the cathode are accelerated, reach the anode, upon collision with the material of which they are decelerated. As a result, bremsstrahlung X-rays are produced. During the collision of electrons with the anode, the second process also occurs - electrons are knocked out of the electron shells of the anode atoms. Their places are occupied by electrons from other shells of the atom. During this process, a second type of X-ray radiation is generated - the so-called characteristic X-ray radiation, the spectrum of which largely depends on the anode material. Anodes are most often made of molybdenum or tungsten. There are special devices for focusing and filtering X-rays in order to improve the resulting images.
Rice. 2-2. Scheme of the X-ray tube device:
The properties of X-rays that determine their use in medicine are penetrating power, fluorescent and photochemical effects. The penetrating power of X-rays and their absorption by the tissues of the human body and artificial materials are the most important properties that determine their use in radiation diagnostics. The shorter the wavelength, the greater the penetrating power of X-rays.
There are "soft" X-rays with low energy and radiation frequency (respectively, with the largest wavelength) and "hard" X-rays with high photon energy and radiation frequency, which have a short wavelength. The wavelength of X-ray radiation (respectively, its "hardness" and penetrating power) depends on the magnitude of the voltage applied to the X-ray tube. The higher the voltage on the tube, the greater the speed and energy of the electron flow and the shorter the wavelength of the x-rays.
During the interaction of X-ray radiation penetrating through the substance, qualitative and quantitative changes occur in it. The degree of absorption of X-rays by tissues is different and is determined by the density and atomic weight of the elements that make up the object. The higher the density and atomic weight of the substance of which the object (organ) under study consists, the more X-rays are absorbed. The human body contains tissues and organs of different densities (lungs, bones, soft tissues, etc.), which explains the different absorption of X-rays. The visualization is based on the artificial or natural difference in the absorption of X-rays by various organs and tissues. internal organs and structures.
To register the radiation that has passed through the body, its ability to cause fluorescence of certain compounds and to have a photochemical effect on the film is used. For this purpose, special screens for fluoroscopy and photographic films for radiography are used. In modern X-ray machines, special systems of digital electronic detectors - digital electronic panels - are used to register attenuated radiation. In this case, X-ray methods are called digital.
Due to the biological effect of X-rays, it is necessary to protect patients during the examination. This is achieved
maximum short time irradiation, the replacement of fluoroscopy with radiography, the strictly justified use of ionizing methods, protection by shielding the patient and staff from exposure to radiation.
Modern medical diagnostics and treatment of certain diseases cannot be imagined without devices that use the properties of X-rays. The discovery of X-rays occurred more than 100 years ago, but even now work continues on the creation of new methods and apparatus to minimize the negative effect of radiation on the human body.
Who and how discovered X-rays
Under natural conditions, the flux of X-rays is rare and is emitted only by certain radioactive isotopes. X-rays or X-rays were only discovered in 1895 by the German scientist Wilhelm Röntgen. This discovery happened by chance, during an experiment to study the behavior of light rays under conditions approaching vacuum. The experiment involved a cathode gas discharge tube with reduced pressure and a fluorescent screen, which each time began to glow at the moment when the tube began to act.
Interested in a strange effect, Roentgen conducted a series of studies showing that the resulting visible to the eye radiation is able to penetrate various barriers: paper, wood, glass, some metals, and even through the human body. Despite the lack of understanding of the very nature of what is happening, whether such a phenomenon is caused by the generation of a stream of unknown particles or waves, the following pattern was noted - radiation easily passes through the soft tissues of the body, and much harder through solid living tissues and inanimate substances.
Roentgen was not the first to study this phenomenon. In the middle 19th century, Frenchman Antoine Mason and Englishman William Crookes studied similar possibilities. However, it was Roentgen who first invented the cathode tube and an indicator that could be used in medicine. He was the first to publish a scientific work, which brought him the title of the first Nobel laureate among physicists.
In 1901, a fruitful collaboration began between the three scientists, who became the founding fathers of radiology and radiology.
X-ray properties
X-rays are part of the total spectrum electromagnetic radiation. The wavelength is between gamma and ultraviolet rays. X-rays have all the usual wave properties:
- diffraction;
- refraction;
- interference;
- propagation speed (it is equal to light).
To artificially generate an X-ray flux, special devices are used - X-ray tubes. X-ray radiation arises from the contact of fast tungsten electrons with substances evaporating from a hot anode. Against the background of interaction, short-length electromagnetic waves arise, which are in the spectrum from 100 to 0.01 nm and in the energy range of 100-0.1 MeV. If the wavelength of the rays is less than 0.2 nm - this is hard radiation, if the wavelength is greater than the specified value, they are called soft x-rays.
It is significant that the kinetic energy arising from the contact of electrons and the anode substance is 99% converted into heat energy and only 1% is X-rays.
X-ray radiation - bremsstrahlung and characteristic
X-radiation is a superposition of two types of rays - bremsstrahlung and characteristic. They are generated in the handset simultaneously. Therefore, X-ray irradiation and the characteristic of each specific X-ray tube - the spectrum of its radiation, depends on these indicators, and represents their superposition.
Bremsstrahlung or continuous X-rays are the result of deceleration of electrons evaporating from a tungsten filament.
Characteristic or line X-rays are formed at the moment of rearrangement of the atoms of the substance of the anode of the X-ray tube. The wavelength of the characteristic rays directly depends on the atomic number chemical element used to make the tube anode.
The listed properties of X-rays allow them to be used in practice:
- invisible to the ordinary eye;
- high penetrating ability through living tissues and inanimate materials that do not transmit visible light;
- ionization effect on molecular structures.
Principles of X-ray Imaging
The property of x-rays on which imaging is based is the ability to either decompose or cause some substances to glow.
X-ray irradiation causes a fluorescent glow in cadmium and zinc sulfides - green, and in calcium tungstate - blue color. This property is used in the technique of medical X-ray transillumination, and also increases the functionality of X-ray screens.
The photochemical effect of X-rays on light-sensitive silver halide materials (illumination) makes it possible to carry out diagnostics - to take X-ray images. This property is also used in measuring the amount of the total dose that laboratory assistants receive in X-ray rooms. Wearable dosimeters have special sensitive tapes and indicators. The ionizing effect of X-ray radiation makes it possible to determine the qualitative characteristics of the obtained X-rays.
A single exposure to conventional X-rays increases the risk of cancer by only 0.001%.
Areas where X-rays are used
The use of X-rays is acceptable in the following industries:
- Safety. Fixed and portable devices for detecting dangerous and prohibited items at airports, customs or in crowded places.
- Chemical industry, metallurgy, archaeology, architecture, construction, restoration work - to detect defects and carry out chemical analysis substances.
- Astronomy. It helps to observe cosmic bodies and phenomena with the help of X-ray telescopes.
- military industry. For the development of laser weapons.
The main application of X-rays is in the medical field. Today, the section of medical radiology includes: radiodiagnostics, radiotherapy (X-ray therapy), radiosurgery. Medical schools produce narrow-profile specialists - radiologists.
X-Radiation - harm and benefit, effects on the body
The high penetrating power and ionizing effect of X-rays can cause a change in the structure of the DNA of the cell, therefore it is dangerous for humans. The harm from X-ray radiation is directly proportional to the received radiation dose. Different organs respond to irradiation to varying degrees. The most susceptible include:
- bone marrow and bone tissue;
- lens of the eye;
- thyroid;
- mammary and sex glands;
- lung tissue.
Uncontrolled use of X-ray radiation can cause reversible and irreversible pathologies.
Consequences of X-ray exposure:
- damage to the bone marrow and the occurrence of pathologies of the hematopoietic system - erythrocytopenia, thrombocytopenia, leukemia;
- damage to the lens, with the subsequent development of cataracts;
- cellular mutations that are inherited;
- development of oncological diseases;
- getting radiation burns;
- development of radiation sickness.
Important! Unlike radioactive substances, X-rays do not accumulate in the tissues of the body, which means that there is no need to remove X-rays from the body. The harmful effect of X-rays ends when the medical device is turned off.
The use of X-rays in medicine is permissible not only for diagnostic (traumatology, dentistry), but also for therapeutic purposes:
- from x-rays in small doses, the metabolism in living cells and tissues is stimulated;
- certain limiting doses are used for the treatment of oncological and benign neoplasms.
Methods for diagnosing pathologies using X-rays
Radiodiagnostics includes the following methods:
- Fluoroscopy is a study in which an image is obtained on a fluorescent screen in real time. Along with the classical real-time imaging of a body part, today there are X-ray television transillumination technologies - the image is transferred from a fluorescent screen to a television monitor located in another room. Several digital methods have been developed for processing the resulting image, followed by transferring it from the screen to paper.
- Fluorography is the cheapest method for examining the chest organs, which consists in making a small picture of 7x7 cm. Despite the possibility of error, it is the only way to conduct a mass annual examination of the population. The method is not dangerous and does not require the withdrawal of the received radiation dose from the body.
- Radiography - obtaining a summary image on film or paper to clarify the shape of an organ, its position or tone. Can be used to assess peristalsis and the condition of the mucous membranes. If there is a choice, then among modern X-ray devices, preference should be given neither to digital devices, where the x-ray flux can be higher than that of old devices, but to low-dose X-ray devices with direct flat semiconductor detectors. They allow you to reduce the load on the body by 4 times.
- Computed X-ray tomography is a technique that uses x-rays to obtain the required number of images of sections of a selected organ. Among the many varieties of modern CT devices, low-dose high-resolution CT scanners are used for a series of repeated studies.
Radiotherapy
X-ray therapy is one of the methods local treatment. Most often, the method is used to destroy cancer cells. Since the effect of exposure is comparable to surgical removal, this treatment method is often called radiosurgery.
Today, x-ray treatment is carried out in the following ways:
- External (proton therapy) - the radiation beam enters the patient's body from the outside.
- Internal (brachytherapy) - the use of radioactive capsules by implanting them into the body, with the placement closer to the cancerous tumor. The disadvantage of this method of treatment is that until the capsule is removed from the body, the patient needs to be isolated.
These methods are gentle, and their use is preferable to chemotherapy in some cases. Such popularity is due to the fact that the rays do not accumulate and do not require removal from the body, they have a selective effect, without affecting other cells and tissues.
Safe X-ray exposure rate
This indicator of the norm of permissible annual exposure has its own name - a genetically significant equivalent dose (GED). There are no clear quantitative values for this indicator.
- This indicator depends on the age and desire of the patient to have children in the future.
- It depends on which organs were examined or treated.
- The GZD is affected by the level of natural radioactive background of the region where a person lives.
Today, the following average GZD standards are in force:
- the level of exposure from all sources, with the exception of medical ones, and without taking into account the natural radiation background - 167 mRem per year;
- the norm for an annual medical examination is not more than 100 mRem per year;
- the total safe value is 392 mRem per year.
X-ray radiation does not require excretion from the body, and is dangerous only in case of intense and prolonged exposure. Modern medical equipment uses low-energy radiation of short duration, so its use is considered relatively harmless.
In 1895, Roentgen discovered that if through a glass tube with two soldered electrodes, from which air is pumped out to a pressure of 103 mm Hg. Art., skip an electric current, then the anode emits special, hitherto unknown, invisible to the eye rays. He called them X-rays. In Russia and in many other countries they began to be called x-rays. Roentgen, examining their properties, found the following:
1. They have a strong penetrating power, which depends on the nature of the substance and its thickness. Due to this property, they are widely used in medicine and industry.
2. They cause the glow (luminescence) of some bodies. With the help of screens of such substances, they can be observed.
3. Have an effect on the film (photochemical effect).
4. Able to actively ionize air and other substances.
5. They have a biological effect on body tissues, which has been used in the treatment of malignant tumors.
However, Roentgen himself did not reveal the nature of X-rays. Many researchers found similarities between X-rays and light - they propagated in a straight line and did not deviate in either an electric or magnetic field. But, if we assume the same nature of light and X-rays, then X-rays would have to have wave and quantum properties. However, X-ray diffraction for a long time could not get. In 1910, P.N. Lebedev suggested using natural crystals as a diffraction grating for X-rays, and in 1912 the German physicist Laue performed this experiment. The flow of X-ray light was directed through the diaphragm onto the crystal, while on the screen or photographic film around the central bright spot (non-diffracted rays) a series of bright points appeared, arranged in a certain order.
The distance between the atoms of the crystal lattice, on the order of 1A°, is commensurate with the wavelength, and these gaps are the centers of secondary waves, which, when diffracted, give maxima in the form of white spots. But since Since the atoms are not strictly located one next to the other like the slits of a diffraction grating, then the maxima are arranged in a complex order than in a diffraction grating. Such a picture is called a Lauegram. This experiment showed that X-rays are of a wave nature.
Laue's experience allowed the use of X-ray diffraction:
1. To determine the wavelength, knowing the distance between atoms.
2. To determine the structure of substances according to the Lauegram, knowing the wavelength of X-rays.
A method for studying molecular structures, i.e. determination of the position of atoms in a molecule and their nature using x-rays, called x-ray diffraction analysis. To study biological structures, various phenomena of the interaction of X-rays with matter can be used: absorption, scattering and diffraction, inactivation (changes in the structure of molecules and the functions of their constituent parts under the action of X-rays). The method of scattering and diffraction of X-rays uses their wave properties. X-rays scattered by the atoms that make up the molecules interfere and give a picture - a Lauegram, in which the position and intensity of the maxima depend on the position of the atoms in the molecule and on the relative position of the molecules. If the molecules are arranged randomly, for example, in solutions, then the scattering does not depend on the internal structure of the molecules, but mainly on their size and shape.
Later, other properties of X-rays were studied:
1. Interference.
2. Refraction.
3. Total internal reflection.
4. Polarization.
5. Spectral composition.
6. Interaction with matter.
Get x-rays with an x-ray tube.
It consists of a glass container with as high a vacuum as possible (10 -6 - 10 -7 mm Hg), in which there are two electrodes.
Cathode - is a source of electrons and is made in the form of a spiral. The anode consists of a massive copper rod, on the end section of which there is a tungsten plate (anode mirror). The electrons are accelerated in the electric field and interact with the anode mirror. As a result of the interaction, an X-ray flux is formed. The whole tube is surrounded by a lead casing, there is only a small window for the radiation to exit. Because the anode during operation is very hot, it is cooled with water or oil. In some tubes, the anode is made to rotate. The wavelength of X-rays is from 0.001 to 2 nm. X-ray radiation is characterized by intensity and rigidity.
Intensity is the amount of energy carried by x-rays through an area of 1 cm 2 in 1 s.
The hardness of X-ray radiation is determined by its ability to pass through a substance, and the penetrating power depends on the wavelength. X-ray radiation arises as a result of the interaction of the electron flow with the atoms of the anode mirror.
An electron moving in a direction can be represented electric shock. Getting into the electric field of the atom, the movement of the electron slows down, which corresponds to a decrease in current. Current reduction
will cause a changing magnetic field around the electron, and a changing magnetic field will induce a changing electric field at adjacent points, etc., thus. When an electron is decelerated by an atom, an electromagnetic wave is produced. There is also quantum theory explaining the origin of bremsstrahlung X-rays. In addition to circular or elliptical stationary orbits, called periodic, there are also non-closed orbits of electrons (parabolic, hyperbolic), along which an electron can move without emitting or absorbing energy. Approaching the atom with a speed υ 1, the electron moves along a stationary non-closed orbit with energy E 1, slowing down, it moves to another stationary orbit with energy E 2, while a quantum of energy is emitted. The initial kinetic energy of an electron depends only on the accelerating voltage mυ 1 2 /2=eU and is a constant value. The final energy, depending on the braking conditions, can take any values from mυ 1 2 /2 to 0. Therefore, the energy of the emitted quantum can be any in the range from 0 to mυ 1 2 /2 . The emission spectrum is continuous, limited from the side
short wavelengths.
hv \u003d (mυ 1 2) / 2 - (mυ 2 2) / 2
The minimum quantum energy is determined from this equation,
If (mυ 2 2)/2= 0 , then or hv min \u003d (mυ 1 2) / 2
hc/λ max =eU, where λmax = (hc)/(eU)
An electron, interacting with an anode atom, can remove an orbital electron from the K, L, M orbit closest to the nucleus to a more distant one or even beyond the atom. An electron from a more distant orbit will move to the vacated place. In this case, an X-ray quantum is emitted, the wavelength of which is determined by the difference between the allowed energy states of the atom (hv = E 2 - E 1). Therefore, radiation can only be of certain wavelengths, the spectrum of such radiation will be line, and radiation is called characteristic.
When the anode material is bombarded with electrons, both types of radiation exist. Consider the scheme of the x-ray machine.
The X-ray apparatus includes the following components:
1. X-ray tube (RT)
2. Step-up transformer (TP2).
3. Step-down transformer (TR,).
4. Autotransformer (ATR).
5. High voltage rectifier (B).
The primary winding of the step-up transformer is fed from the AC mains through an autotransformer. The autotransformer serves to regulate the voltage between the anode and cathode. Changing the voltage changes the wavelength λ min \u003d l,24 / U , and the wavelength characterizes the radiation hardness, i.e. The autotransformer is used to adjust the X-ray hardness. The voltage between the anode and cathode of an x-ray tube in medical x-ray machines is up to 60 kV, in industrial ones - 200 - 250 kV. The tube is powered by direct current. As a rectifier, high-voltage diodes or kenotrons are used, one-half-wave and two-half-wave circuits are used. To power the glow of the tube, a step-down transformer TR 1 is used. A rheostat R is placed in the primary circuit of this transformer. By changing the resistance, we change the cathode filament current, and, consequently, its temperature and the number of emitted electrons. The number of electrons characterizes the intensity of X-ray radiation, thus. Rheostat R serves to change the radiation intensity, which is determined by the following formula:
Ф = kJU 2 Z",
where J is the anode current, U is the voltage between the cathode and the anode of the tube, Z is the ordinal number of the substance of the anode mirror. Protection against exposure to x-ray radiation given by medical and diagnostic devices is as follows:
1. Screening of the radiation source. The X-ray tube is self-protective. The chamber is covered with lead sheets.
2.Individual protection of service personnel (apron, gloves, screen glass is made of leaded material).
3. Protected by law (shorter working hours, additional leave, special meals, etc.)
When X-rays interact with a substance, some of them are reflected from the surface, some pass through the substance without interaction, and some pass inside the substance, interacting with atoms.
In this case, three cases of interaction can arise.
1. If the photon does not have sufficient energy to transfer the orbital electron to a higher energy level, then the interaction occurs by elastic collision, the direction of the photon changes, and the energy and wavelength remain the same hv 1 = hv 2 This interaction is called coherent or classical scattering.
2. If the quantum energy is equal to or slightly exceeds the work function of the electron from the metal, then the interaction occurs photoelectric effect, the energy of a photon is expended on the work of getting an electron out of an atom and imparting kinetic energy to it.
hv 1 \u003d A out + (mυ 2) / 2
If the energy is less than the work function, but sufficient to transfer an electron from one orbit to another (with a higher energy level), then radiation in the visible part of the spectrum can occur, x-ray luminescence or activation of molecules. Both types of interaction are combined common name - true absorption.
3. If the energy of a photon significantly exceeds the work of an electron, which is more typical for hard short-wave radiation and external electrons of an atom, then during the interaction the photon gives up part of the energy. A photon with a lower energy and a recoil photoelectron appear. This phenomenon is called incoherent scattering or Compton effect.
The resulting new photon and electron are called secondary radiation. Secondary radiation can cause new reactions (coherent scattering, true absorption, Compton effect) with the formation of tertiary electrons, quanta, etc. As a result of all these processes, ionization of the substance and radiation with a longer wavelength occurs, which is scattered in all directions.
The parallel flow of X-rays is weakened when passing through a substance. The weakening obeys Bouguer's law: Ф \u003d Ф 0 e - μd
Fo is the flow incident on the substance, F is the flow passing through the substance, μ is the linear attenuation coefficient, d is the thickness of the substance layer.
For X-ray radiation used in medicine with a photon energy of 150-200 keV for deep therapy; 60-100 keV for diagnostics; attenuation coefficient is determined by the formula:
μ = kpZ 3 λ 3 ,
k is the coefficient of proportionality, depending on the choice of units of measurement, p is the density of the substance, Z is the ordinal number of the element, λ is the radiation wavelength.
If an inhomogeneous substance is placed in the path of X-ray radiation, then on a fluorescent screen we will get shadows of individual details
substances. Such a heterogeneous substance is the human body. Translucent with X-rays, according to the shape and size, as well as the intensity of the shadow image, they judge the normal or pathological state of the organs. This method of diagnosing diseases is called X-ray diagnostics. There are two main methods of X-ray diagnostics: fluoroscopy and radiography. During fluoroscopy, the shadow image of organs is observed on a fluorescent screen. On the screen, denser tissues (heart, blood vessels) are seen as dark, little absorbing tissues (lung fields) as light. During radiography, the shadow image is photographed on film. The image is obtained negative (reverse) in relation to the image on the screen.
In addition to the basic methods, special methods of X-ray diagnostics are used.
1. Contrast radiography. To obtain a more contrasting image, special substances are used that are injected into tissues - negative contrast agents (air, oxygen) are used in dense tissues (brain), positive contrast agents (barium salts, iodine-based colloids) for low-absorbing tissues.
2. Fluorography. Photographing an x-ray image from a screen onto a small format film. The screen, optics and camera film are combined into a large light-tight system, which allows you to shoot in a dark room. This method is used for mass survey of the population.
3. Electroradiography differs from conventional radiography in the manner in which the image is acquired; with this method, a beam of x-rays that have passed through the patient's body is directed to a pre-infected selenium plate. X-rays that have passed through the body change the potential of the plate in its different sections, respectively, the intensity of the radiation falling on these areas - a “latent electrical image” appears on the plate. To "develop" the image, the selenium plate is sprayed with graphite powder, which is attracted to those places where the charge has been preserved and does not linger in those places that have lost their charge under the action of X-rays. This image is easily transferred to plain paper. After erasing the powder, the plate can be used again. More than 1000 shots can be taken on one plate. The main advantages of electroroentgenography are that it allows you to quickly obtain images without the cost of film, without a wet photo process, without darkening, and has a higher resolution.
4. X-ray computed tomography. This method consists in moving the X-ray tube along a certain trajectory in order to photograph the object from various positions. At the same time, the image on the film also moves. However, the shooting is done in such a way that the x-ray beam always passes the same point O. If you move this point, then you can get a layered shadow image in the image (tomography - layered recording). Reading such images is quite difficult. Computer technology helps the doctor in this matter, so the word computed tomography is added. X-ray computed tomography makes it possible to obtain an image with details of about 1 mm, two formations differ in contrast with a difference in absorption of about 0.1%.
5. X-ray television. With the help of special X-ray image photoamplifiers (URI), a weak image on the screen is recorded and enhanced, and using television transmission equipment, an image is obtained on the TV screen. The image on the TV screen of considerable brightness, provides the identification of relatively small details of the object, allows you to take photos and films.
X-rays are used to "treat" malignant neoplasms - X-ray therapy. When living tissues are irradiated with X-rays, the functional state of cells changes. The primary effect of X-rays on matter is ionization. It was found that at lethal doses, about 1 million ions are formed in the cell (there are 10 14 atoms in the cell). During the primary exchange of energy, no visible structural changes occur in atoms and molecules. Modern physiology considers the primary effects of the interaction of ionizing radiation with matter (including X-rays) in two aspects: interaction with water molecules in aqueous solutions and the effect on organic compounds. In aqueous solutions, radicals (OH -, H +), hydroperoxide and peroxide compounds (H 2 O 2) are formed, which have high chemical activity. When exposed to organic compounds, excited molecules, radicals, ions, peroxides are formed, which are also very active chemically. That. The primary interaction is physical laws excitation and ionization of molecules. The ionization of atoms and molecules causes secondary processes that develop according to biological laws. Active peroxide compounds oxidize and change cellular enzymes, which causes a disruption in the normal course of biochemical processes - cells lose the ability to synthesize certain types of proteins, without which cell division is impossible. Mutations occur, the course of protein, carbohydrate, peptide and cholesterol metabolism changes. In such reactions, protein molecules can break down and break down into amino acids, up to the formation of very toxic histamine-like compounds, under the influence of which dystrophic and necrotic changes develop. X-rays have a particularly strong effect on fast-growing, poorly differentiated cells - hematopoietic organs, skin, gonads, which makes it possible to use X-rays to irradiate cancerous tumors of these formations. It should be remembered that radiation acts not only on a biological object subjected to radiation, but also on subsequent generations, through the hereditary apparatus of cells.
LECTURE
X-RAY RADIATION
2. Bremsstrahlung X-ray, its spectral properties.
3. Characteristic x-ray radiation (for review).
4. Interaction of X-ray radiation with matter.
5.Physical foundations use of x-rays in medicine.
X-rays (X - rays) were discovered by K. Roentgen, who in 1895 became the first Nobel Laureate in physics.
1. The nature of X-rays
x-ray radiation - electromagnetic waves with a length of 80 to 10 -5 nm. Long-wave X-ray radiation is blocked by short-wave UV radiation, short-wave - by long-wave g-radiation.
X-rays are produced in x-ray tubes. fig.1.
K - cathode
1 - electron beam
2 - X-ray radiation
Rice. 1. X-ray tube device.
The tube is a glass flask (with a possible high vacuum: the pressure in it is about 10–6 mm Hg) with two electrodes: anode A and cathode K, to which a high voltage is applied U (several thousand volts). The cathode is a source of electrons (due to the phenomenon of thermionic emission). Anode is a metal rod inclined surface in order to direct the emerging x-rays at an angle to the axis of the tube. It is made of a highly heat-conducting material to remove the heat generated during electron bombardment. On the beveled end there is a plate made of refractory metal (for example, tungsten).
The strong heating of the anode is due to the fact that the main number of electrons in the cathode beam, having hit the anode, experience numerous collisions with the atoms of the substance and transfer a large amount of energy to them.
Under the action of high voltage, the electrons emitted by the hot cathode filament are accelerated to high energies. The kinetic energy of an electron is mv 2 /2. It is equal to the energy that it acquires by moving in the electrostatic field of the tube:
mv 2 /2 = eU(1)
where m , e are the mass and charge of the electron, U is the accelerating voltage.
The processes leading to the appearance of bremsstrahlung X-rays are due to the intense deceleration of electrons in the anode material by the electrostatic field of the atomic nucleus and atomic electrons.
The origin mechanism can be represented as follows. Moving electrons are some kind of current that forms its own magnetic field. Electron deceleration - a decrease in current strength and, accordingly, a change in induction magnetic field, which will cause the occurrence of a variable electric field, i.e. appearance of an electromagnetic wave.
Thus, when a charged particle flies into matter, it slows down, loses its energy and speed, and emits electromagnetic waves.
2. Spectral properties of X-ray bremsstrahlung .
So, in the case of electron deceleration in the anode material, bremsstrahlung radiation.
The bremsstrahlung spectrum is continuous . The reason for this is as follows.
When the electrons slow down, each of them has a part of the energy used to heat the anode (E 1 = Q ), the other part to create an X-ray photon (E 2 = hv ), otherwise, eU = hv + Q . The relationship between these parts is random.
Thus, the continuous spectrum of bremsstrahlung X-rays is formed due to the deceleration of many electrons, each of which emits one X-ray quantum. hv(h ) of a strictly defined value. The value of this quantum different for different electrons. Dependence of the X-ray energy flux on the wavelength l , i.e. the X-ray spectrum is shown in Fig.2.
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Fig.2. Bremsstrahlung spectrum: a) at different voltages U in the tube; b) at different temperatures T of the cathode.
Short-wave (hard) radiation has a greater penetrating power than long-wave (soft) radiation. Soft radiation is more strongly absorbed by matter.
From the side of short wavelengths, the spectrum ends abruptly at a certain wavelength l m i n . Such short-wavelength bremsstrahlung occurs when the energy acquired by an electron in an accelerating field is completely converted into photon energy ( Q = 0):
eU = hv max = hc/ l min , l min = hc/(eU), (2)
l min (nm) = 1.23 / U kV
The spectral composition of the radiation depends on the voltage on the X-ray tube, with increasing voltage, the value l m i n shifts towards short wavelengths (Fig. 2 a).
When the temperature T of the cathode incandescence changes, the electron emission increases. Therefore, the current increases I in the tube, but the spectral composition of the radiation does not change (Fig. 2b).
Energy flow Ф * bremsstrahlung is directly proportional to the square of the voltage U between anode and cathode, current strength I in tube and atomic number Z anode materials:
F \u003d kZU 2 I. (3)
where k \u003d 10 -9 W / (V 2 A).
3. Characteristic X-rays (for familiarization).
Increasing the voltage on the X-ray tube leads to the fact that against the background of a continuous spectrum, a line appears, which corresponds to the characteristic X-ray radiation. This radiation is specific to the anode material.
The mechanism of its occurrence is as follows. At a high voltage, accelerated electrons (with high energy) penetrate deep into the atom and knock electrons out of its inner layers. On free places electrons pass from the upper levels, as a result of which photons of characteristic radiation are emitted.
The spectra of characteristic X-ray radiation differ from optical spectra.
- Uniformity.
The uniformity of the characteristic spectra is due to the fact that the inner electron layers of different atoms are identical and differ only energetically due to the force action from the nuclei, which increases with the increase in the ordinal number of the element. Therefore, the characteristic spectra shift towards higher frequencies with increasing nuclear charge. This was experimentally confirmed by an employee of Roentgen - Moseley, who measured X-ray transition frequencies for 33 elements. They made the law.
MOSELY'S LAW – the square root of the characteristic radiation frequency is linear function element's serial number:
A × (Z – B ), (4)
where v is the spectral line frequency, Z is the atomic number of the emitting element. A, B are constants.
The importance of Moseley's law lies in the fact that this dependence can be used to accurately determine the atomic number of the element under study from the measured frequency of the X-ray line. This played a big role in the placement of the elements in the periodic table.
– Independence from a chemical compound.
The characteristic X-ray spectra of an atom do not depend on the chemical compound in which the atom of the element enters. For example, the X-ray spectrum of an oxygen atom is the same for O 2, H 2 O, while the optical spectra of these compounds differ. This feature of the x-ray spectrum of the atom was the basis for the name " characteristic radiation".
4. Interaction of X-ray radiation with matter
The impact of X-ray radiation on objects is determined primary processes x-ray interaction photon with electrons atoms and molecules of matter.
X-ray radiation in matter absorbed or dissipates. In this case, various processes can occur, which are determined by the ratio of the X-ray photon energy hv and ionization energy A and (ionization energy A and - the energy required to remove internal electrons from the atom or molecule).
A) Coherent scattering(scattering of long-wave radiation) occurs when the relation
hv< А и.
For photons, due to interaction with electrons, only the direction of movement changes (Fig. 3a), but the energy hv and the wavelength do not change (hence this scattering is called coherent). Since the energy of the photon and the atom do not change, coherent scattering does not affect biological objects, but when creating protection against X-ray radiation, one should take into account the possibility of changing the primary direction of the beam.
b) photoelectric effect happens when
hv ³ A and .
In this case, two cases can be realized.
1. The photon is absorbed, the electron is detached from the atom (Fig. 3b). Ionization occurs. The detached electron acquires kinetic energy: E k \u003d hv - A and . If the kinetic energy is large, then the electron can ionize neighboring atoms by collision, forming new ones. secondary electrons.
2. The photon is absorbed, but its energy is not enough to detach the electron, and excitation of an atom or molecule(Fig. 3c). This often leads to the subsequent emission of a photon in the visible radiation region (X-ray luminescence), and in tissues - to the activation of molecules and photochemical reactions. The photoelectric effect occurs mainly on the electrons of the inner shells of atoms with high Z.
V) Incoherent scattering(Compton effect, 1922) occurs when the photon energy is much greater than the ionization energy
hv » A and.
In this case, the electron is detached from the atom (such electrons are called recoil electrons), acquires some kinetic energy E to , the energy of the photon itself decreases (Fig. 4d):
hv=hv" + A and + E k. (5)
The resulting radiation with a changed frequency (length) is called secondary, it scatters in all directions.
Recoil electrons, if they have sufficient kinetic energy, can ionize neighboring atoms by collision. Thus, as a result of incoherent scattering, secondary scattered X-ray radiation is formed and the atoms of the substance are ionized.
These (a, b, c) processes can cause a number of subsequent ones. For example (Fig. 3d), if, during the photoelectric effect, electrons are detached from the atom on the inner shells, then electrons with more high levels, which is accompanied by the secondary characteristic X-ray emission of the given substance. Photons of secondary radiation, interacting with electrons of neighboring atoms, can, in turn, cause secondary phenomena.
coherent scattering
hv< А И
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photoelectric effect
photon is absorbed, e - detached from the atom - ionization
hv \u003d A and + E to
atom A excited by the absorption of a photon, R – X-ray luminescence
incoherent scattering
hv » A and
hv \u003d hv "+ A and + E to
secondary processes in the photoelectric effect
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Rice. 3 Mechanisms of interaction of X-rays with matter
Physical basis for the use of X-rays in medicine
When X-rays fall on a body, it is slightly reflected from its surface, but mainly passes deep into, while it is partially absorbed and scattered, and partially passes through.
The law of weakening.
The X-ray flux is attenuated in matter according to the law:
F \u003d F 0 e - m × x (6)
where m – linear attenuation factor, which essentially depends on the density of the substance. It is equal to the sum of three terms corresponding to coherent scattering m 1, incoherent m 2 and photoelectric effect m 3:
m \u003d m 1 + m 2 + m 3. (7)
The contribution of each term is determined by the photon energy. Below are the ratios of these processes for soft tissues (water).
Energy, keV | photoelectric effect | Compton - effect |
| 100 % |
enjoy mass attenuation coefficient, which does not depend on the density of the substance r :
m m = m / r . (8)
The mass attenuation coefficient depends on the energy of the photon and on the atomic number of the absorbing substance:
m m = k l 3 Z 3 . (9)
Mass attenuation coefficients of bone and soft tissue (water) differ: m m bones / m m water = 68.
If an inhomogeneous body is placed in the path of X-rays and a fluorescent screen is placed in front of it, then this body, absorbing and attenuating the radiation, forms a shadow on the screen. By the nature of this shadow, one can judge the shape, density, structure, and in many cases the nature of bodies. Those. a significant difference in the absorption of x-ray radiation by different tissues allows you to see the image of the internal organs in the shadow projection.
If the organ under study and the surrounding tissues equally attenuate x-rays, then contrast agents are used. So, for example, filling the stomach and intestines with a mushy mass of barium sulfate ( BaS 0 4), you can see their shadow image (the ratio of attenuation coefficients is 354).
Use in medicine.
In medicine, X-ray radiation with photon energy from 60 to 100-120 keV is used for diagnostics and 150-200 keV for therapy.
X-ray diagnostics – Recognition of diseases by transilluminating the body with X-rays.
Radiodiagnosis is used in various options which are listed below.
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1. With fluoroscopy the x-ray tube is located behind the patient. In front of it is a fluorescent screen. There is a shadow (positive) image on the screen. In each individual case, the appropriate hardness of the radiation is selected so that it passes through soft tissues, but is sufficiently absorbed by dense ones. Otherwise, a uniform shadow is obtained. On the screen, the heart, the ribs are visible dark, the lungs are light.
2. When radiography the object is placed on a cassette, which contains a film with a special photographic emulsion. x-ray tube located above the object. The resulting radiograph gives a negative image, i.e. the opposite in contrast to the picture observed during transillumination. In this method, there is a greater clarity of the image than in (1), therefore, details are observed that are difficult to see when transilluminated.
A promising variant of this method is X-ray tomography and "machine version" - computer tomography.
3. With fluoroscopy, on a sensitive small format film, an image is captured with big screen. When viewed, the pictures are examined on a special magnifier.
X-ray therapy - the use of X-rays to destroy malignant tumors.
The biological effect of radiation is to disrupt vital activity, especially rapidly multiplying cells.
COMPUTED TOMOGRAPHY (CT)
The method of X-ray computed tomography is based on image reconstructionof a certain section of the patient's body by registering a large number of X-ray projections of this section, made at different angles. Information from the sensors that register these projections enters the computer, which special program calculates distribution tightly sample sizein the investigated section and displays it on the display screen. The resulting imagesection of the patient's body is characterized by excellent clarity and high information content. The program allows you toincrease image contrast V dozens and even hundreds of times. This expands the diagnostic capabilities of the method.
Videographers (devices with digital X-ray image processing) in modern dentistry.
In dentistry it x-ray examination is the main diagnostic method. However, a number of traditional organizational and technical features of X-ray diagnostics make it not quite comfortable for both the patient and dental clinics. First of all, this is the need for the patient to come into contact with ionizing radiation, which often creates a significant radiation load on the body, this is also the need for a photoprocess, and, consequently, the need for photoreagents, including toxic ones. This is, finally, a bulky archive, heavy folders and envelopes with x-ray films.
In addition, the current level of development of dentistry makes the subjective assessment of radiographs by the human eye insufficient. As it turned out, of the variety of shades of gray contained in the x-ray image, the eye perceives only 64.
Obviously, to obtain a clear and detailed image of the hard tissues of the dentoalveolar system with minimal radiation exposure, other solutions are needed. The search led to the creation of so-called radiographic systems, videographers - digital radiography systems.
Without technical details, the principle of operation of such systems is as follows. X-ray radiation enters through the object not on a photosensitive film, but on a special intraoral sensor (special electronic matrix). The corresponding signal from the matrix is transmitted to a digitizing device (analog-to-digital converter, ADC) that converts it into digital form and is connected to the computer. Special software builds an x-ray image on the computer screen and allows you to process it, save it on a hard or flexible storage medium (hard drive, floppy disks), print it as a picture as a file.
In a digital system, an x-ray image is a collection of dots having different digital grayscale values. The information display optimization provided by the program makes it possible to obtain an optimal frame in terms of brightness and contrast at a relatively low radiation dose.
In modern systems created, for example, by firms Trophy (France) or Schick (USA) when forming a frame, 4096 shades of gray are used, the exposure time depends on the object of study and, on average, is hundredths - tenths of a second, reduction of radiation exposure in relation to the film - up to 90% for intraoral systems, up to 70% for panoramic videographers.
When processing images, videographers allow:
1. Get positive and negative images, false color images, embossed images.
2. Increase contrast and magnify the area of interest in the image.
3. Assess the change in the density of dental tissues and bone structures, to control the uniformity of the filling of the channels.
4. In endodontics to determine the length of the channel of any curvature, and in surgery to select the size of the implant with an accuracy of 0.1 mm.
5. Unique system caries detector with elements of artificial intelligence in the analysis of the picture allows you to detect caries in the stain stage, root caries and hidden caries.
* « Ф" in formula (3) refers to the entire range of emitted wavelengths and is often referred to as "Integral Energy Flux".
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