What is x-ray radiation, its properties and application. What is X-rays and how is it used in medicine How are X-rays formed?
X-rays are a type of high-energy electromagnetic radiation. It is actively used in various branches of medicine.
X-rays are electromagnetic waves whose photon energy on the scale of electromagnetic waves is between ultraviolet radiation and gamma radiation (from ~10 eV to ~1 MeV), which corresponds to wavelengths from ~10^3 to ~10^−2 angstroms ( from ~10^−7 to ~10^−12 m). That is, it is incomparably harder radiation than visible light, which is on this scale between ultraviolet and infrared (“thermal”) rays.
The boundary between X-rays and gamma radiation is distinguished conditionally: their ranges intersect, gamma rays can have an energy of 1 keV. They differ in origin: gamma rays are emitted during processes occurring in atomic nuclei, while X-rays are emitted during processes involving electrons (both free and those in the electron shells of atoms). At the same time, it is impossible to determine from the photon itself during which process it arose, that is, the division into the X-ray and gamma ranges is largely arbitrary.
The X-ray range is divided into “soft X-ray” and “hard”. The boundary between them lies at the wavelength level of 2 angstroms and 6 keV of energy.
The X-ray generator is a tube in which a vacuum is created. There are electrodes - a cathode, to which a negative charge is applied, and a positively charged anode. The voltage between them is tens to hundreds of kilovolts. The generation of X-ray photons occurs when electrons “break off” from the cathode and crash into the anode surface at high speed. The resulting X-ray radiation is called “bremsstrahlung”, its photons have different wavelengths.
At the same time, photons of the characteristic spectrum are generated. Part of the electrons in the atoms of the anode substance is excited, that is, it goes to higher orbits, and then returns to its normal state, emitting photons of a certain wavelength. Both types of X-rays are produced in a standard generator.
Discovery history
On November 8, 1895, the German scientist Wilhelm Conrad Roentgen discovered that some substances under the influence of "cathode rays", that is, the flow of electrons generated by a cathode ray tube, begin to glow. He explained this phenomenon by the influence of certain X-rays - so (“X-rays”) this radiation is now called in many languages. Later V.K. Roentgen studied the phenomenon he had discovered. On December 22, 1895, he gave a lecture on this topic at the University of Würzburg.
Later it turned out that X-ray radiation had been observed before, but then the phenomena associated with it were not given much importance. The cathode ray tube was invented a long time ago, but before V.K. X-ray, no one paid much attention to the blackening of photographic plates near it, etc. phenomena. The danger posed by penetrating radiation was also unknown.
Types and their effect on the body
“X-ray” is the mildest type of penetrating radiation. Overexposure to soft x-rays is similar to ultraviolet exposure, but in a more severe form. A burn forms on the skin, but the lesion is deeper, and it heals much more slowly.
Hard X-ray is a full-fledged ionizing radiation that can lead to radiation sickness. X-ray quanta can break the protein molecules that make up the tissues of the human body, as well as the DNA molecules of the genome. But even if an X-ray quantum breaks a water molecule, it doesn't matter: in this case, chemically active free radicals H and OH are formed, which themselves are able to act on proteins and DNA. Radiation sickness proceeds in a more severe form, the more the hematopoietic organs are affected.
X-rays have mutagenic and carcinogenic activity. This means that the probability of spontaneous mutations in cells during irradiation increases, and sometimes healthy cells can degenerate into cancerous ones. Increasing the likelihood of malignant tumors is a standard consequence of any exposure, including x-rays. X-rays are the least dangerous type of penetrating radiation, but they can still be dangerous.
X-ray radiation: application and how it works
X-ray radiation is used in medicine, as well as in other areas of human activity.
Fluoroscopy and computed tomography
The most common application of X-rays is fluoroscopy. "Transillumination" of the human body allows you to get a detailed image of both the bones (they are most clearly visible) and images of the internal organs.
Different transparency of body tissues in x-rays is associated with their chemical composition. Features of the structure of bones is that they contain a lot of calcium and phosphorus. Other tissues are composed mainly of carbon, hydrogen, oxygen and nitrogen. The phosphorus atom exceeds the weight of the oxygen atom almost twice, and the calcium atom - 2.5 times (carbon, nitrogen and hydrogen are even lighter than oxygen). In this regard, the absorption of X-ray photons in the bones is much higher.
In addition to two-dimensional “pictures”, radiography makes it possible to create a three-dimensional image of an organ: this type of radiography is called computed tomography. For these purposes, soft x-rays are used. The amount of exposure received in a single image is small: it is approximately equal to the exposure received during a 2-hour flight in an airplane at an altitude of 10 km.
X-ray flaw detection allows you to detect small internal defects in products. Hard x-rays are used for it, since many materials (metal, for example) are poorly “translucent” due to the high atomic mass of their constituent substance.
X-ray diffraction and X-ray fluorescence analysis
X-rays have properties that allow them to examine individual atoms in detail. X-ray diffraction analysis is actively used in chemistry (including biochemistry) and crystallography. The principle of its operation is the diffraction scattering of X-rays by atoms of crystals or complex molecules. Using X-ray diffraction analysis, the structure of the DNA molecule was determined.
X-ray fluorescence analysis allows you to quickly determine the chemical composition of a substance.
There are many forms of radiotherapy, but they all involve the use of ionizing radiation. Radiotherapy is divided into 2 types: corpuscular and wave. Corpuscular uses flows of alpha particles (nuclei of helium atoms), beta particles (electrons), neutrons, protons, heavy ions. Wave uses rays of the electromagnetic spectrum - x-rays and gamma.
Radiotherapy methods are used primarily for the treatment of oncological diseases. The fact is that radiation primarily affects actively dividing cells, which is why the hematopoietic organs suffer this way (their cells are constantly dividing, producing more and more new red blood cells). Cancer cells are also constantly dividing and are more vulnerable to radiation than healthy tissue.
A level of radiation is used that suppresses the activity of cancer cells, while moderately affecting healthy ones. Under the influence of radiation, it is not the destruction of cells as such, but the damage to their genome - DNA molecules. A cell with a destroyed genome may exist for some time, but can no longer divide, that is, tumor growth stops.
Radiation therapy is the mildest form of radiotherapy. Wave radiation is softer than corpuscular radiation, and X-rays are softer than gamma radiation.
During pregnancy
It is dangerous to use ionizing radiation during pregnancy. X-rays are mutagenic and can cause abnormalities in the fetus. X-ray therapy is incompatible with pregnancy: it can only be used if it has already been decided to have an abortion. Restrictions on fluoroscopy are softer, but in the first months it is also strictly prohibited.
In case of emergency, X-ray examination is replaced by magnetic resonance imaging. But in the first trimester they try to avoid it too (this method has appeared recently, and with absolute certainty to speak about the absence of harmful consequences).
An unequivocal danger arises when exposed to a total dose of at least 1 mSv (in old units - 100 mR). With a simple x-ray (for example, when undergoing fluorography), the patient receives about 50 times less. In order to receive such a dose at a time, you need to undergo a detailed computed tomography.
That is, the mere fact of a 1-2-fold “X-ray” at an early stage of pregnancy does not threaten with serious consequences (but it’s better not to risk it).
Treatment with it
X-rays are used primarily in the fight against malignant tumors. This method is good because it is highly effective: it kills the tumor. It is bad because healthy tissues are not much better, there are numerous side effects. The organs of hematopoiesis are at particular risk.
In practice, various methods are used to reduce the effect of x-rays on healthy tissues. The beams are directed at an angle in such a way that a tumor appears in the zone of their intersection (due to this, the main absorption of energy occurs just there). Sometimes the procedure is performed in motion: the patient's body rotates relative to the radiation source around an axis passing through the tumor. At the same time, healthy tissues are in the irradiation zone only sometimes, and the sick - all the time.
X-rays are used in the treatment of certain arthrosis and similar diseases, as well as skin diseases. In this case, the pain syndrome is reduced by 50-90%. Since the radiation is used in this case is softer, side effects similar to those that occur in the treatment of tumors are not observed.
Modern medicine uses many physicians for diagnosis and therapy. Some of them have been used relatively recently, while others have been practiced for more than a dozen or even hundreds of years. Also, a hundred and ten years ago, William Conrad Roentgen discovered the amazing X-rays, which caused a significant resonance in the scientific and medical world. And now doctors all over the planet use them in their practice. The topic of our today's conversation will be X-rays in medicine, we will discuss their application in a little more detail.
X-rays are one of the varieties of electromagnetic radiation. They are characterized by significant penetrating qualities, which depend on the wavelength of radiation, as well as on the density and thickness of the irradiated materials. In addition, X-rays can cause the glow of a number of substances, affect living organisms, ionize atoms, and also catalyze some photochemical reactions.
The use of X-rays in medicine
To date, the properties of x-rays allow them to be widely used in x-ray diagnostics and x-ray therapy.
X-ray diagnostics
X-ray diagnostics is used when carrying out:
X-ray (transmission);
- radiography (picture);
- fluorography;
- X-ray and computed tomography.
Fluoroscopy
To conduct such a study, the patient needs to position himself between the X-ray tube and a special fluorescent screen. A specialist radiologist selects the required hardness of the X-rays, receiving on the screen a picture of the internal organs, as well as the ribs.
Radiography
For this study, the patient is placed on a cassette containing a special film. The X-ray machine is placed directly above the object. As a result, a negative image of the internal organs appears on the film, which contains a number of fine details, more detailed than during a fluoroscopic examination.
Fluorography
This study is carried out during mass medical examinations of the population, including for the detection of tuberculosis. At the same time, a picture from a large screen is projected onto a special film.
Tomography
When conducting tomography, computer beams help to obtain images of organs in several places at once: in specially selected transverse sections of tissue. This series of x-rays is called a tomogram.
Computed tomogram
Such a study allows you to register sections of the human body by using an X-ray scanner. After the data is entered into the computer, getting one picture in cross section.
Each of the listed diagnostic methods is based on the properties of the X-ray beam to illuminate the film, as well as on the fact that human tissues and bone skeleton differ in different permeability to their effects.
X-ray therapy
The ability of X-rays to influence tissues in a special way is used to treat tumor formations. At the same time, the ionizing qualities of this radiation are especially actively noticeable when exposed to cells that are capable of rapid division. It is these qualities that distinguish the cells of malignant oncological formations.
However, it is worth noting that X-ray therapy can cause a lot of serious side effects. Such an impact aggressively affects the state of the hematopoietic, endocrine and immune systems, the cells of which also divide very quickly. Aggressive influence on them can cause signs of radiation sickness.
The effect of X-ray radiation on humans
During the study of x-rays, doctors found that they can lead to changes in the skin that resemble a sunburn, but are accompanied by deeper damage to the skin. Such ulcers heal for a very long time. Scientists have found that such lesions can be avoided by reducing the time and dose of radiation, as well as using special shielding and remote control methods.
The aggressive influence of X-rays can also manifest itself in the long term: temporary or permanent changes in the composition of the blood, susceptibility to leukemia and early aging.
The effect of x-rays on a person depends on many factors: on which organ is irradiated, and for how long. Irradiation of the hematopoietic organs can lead to blood ailments, and exposure to the genital organs can lead to infertility.
Carrying out systematic irradiation is fraught with the development of genetic changes in the body.
The real harm of x-rays in x-ray diagnostics
During the examination, doctors use the minimum possible amount of x-rays. All radiation doses meet certain acceptable standards and cannot harm a person. X-ray diagnostics poses a significant danger only for the doctors who carry it out. And then modern methods of protection help to reduce the aggression of the rays to a minimum.
The safest methods of radiodiagnosis include radiography of the extremities, as well as dental x-rays. In the next place of this rating is mammography, followed by computed tomography, and after it is radiography.
In order for the use of X-rays in medicine to bring only benefit to a person, it is necessary to conduct research with their help only according to indications.
In 1895, the German physicist W. Roentgen discovered a new, previously unknown type of electromagnetic radiation, which was named X-ray in honor of its discoverer. W. Roentgen became the author of his discovery at the age of 50, holding the post of rector of the University of Würzburg and having a reputation as one of the best experimenters of his time. One of the first to find a technical application for Roentgen's discovery was the American Edison. He created a handy demonstration apparatus and already in May 1896 organized an X-ray exhibition in New York, where visitors could look at their own hand on a luminous screen. After Edison's assistant died from the severe burns he received from constant demonstrations, the inventor stopped further experiments with X-rays.
X-ray radiation began to be used in medicine due to its high penetrating power. Initially, X-rays were used to examine bone fractures and locate foreign bodies in the human body. Currently, there are several methods based on X-rays. But these methods have their drawbacks: radiation can cause deep damage to the skin. Appearing ulcers often turned into cancer. In many cases, fingers or hands had to be amputated. Fluoroscopy(synonymous with translucence) is one of the main methods of X-ray examination, which consists in obtaining a planar positive image of the object under study on a translucent (fluorescent) screen. During fluoroscopy, the subject is between a translucent screen and an x-ray tube. On modern X-ray translucent screens, the image appears at the moment the X-ray tube is turned on and disappears immediately after it is turned off. Fluoroscopy makes it possible to study the function of the organ - heart pulsation, respiratory movements of the ribs, lungs, diaphragm, peristalsis of the digestive tract, etc. Fluoroscopy is used in the treatment of diseases of the stomach, gastrointestinal tract, duodenum, diseases of the liver, gallbladder and biliary tract. At the same time, the medical probe and manipulators are inserted without tissue damage, and the actions during the operation are controlled by fluoroscopy and are visible on the monitor.
Radiography - method of X-ray diagnostics with the registration of a fixed image on a photosensitive material - special. photographic film (X-ray film) or photographic paper with subsequent photo processing; With digital radiography, the image is fixed in the computer's memory. It is performed on X-ray diagnostic devices - stationary, installed in specially equipped X-ray rooms, or mobile and portable - at the patient's bedside or in the operating room. On radiographs, the elements of the structures of various organs are displayed much more clearly than on a fluorescent screen. Radiography is performed in order to detect and prevent various diseases, its main goal is to help doctors of various specialties correctly and quickly make a diagnosis. An x-ray image captures the state of an organ or tissue only at the time of exposure. However, a single radiograph captures only anatomical changes at a certain moment, it gives the statics of the process; through a series of radiographs taken at certain intervals, it is possible to study the dynamics of the process, that is, functional changes. Tomography. The word tomography can be translated from Greek as slice image. This means that the purpose of tomography is to obtain a layered image of the internal structure of the object of study. Computed tomography is characterized by high resolution, which makes it possible to distinguish subtle changes in soft tissues. CT allows to detect such pathological processes that cannot be detected by other methods. In addition, the use of CT makes it possible to reduce the dose of X-ray radiation received by patients during the diagnostic process.
Fluorography- a diagnostic method that allows you to get an image of organs and tissues, was developed at the end of the 20th century, a year after X-rays were discovered. In the pictures you can see sclerosis, fibrosis, foreign objects, neoplasms, inflammations that have a developed degree, the presence of gases and infiltrate in the cavities, abscesses, cysts, and so on. Most often, a chest x-ray is performed, which allows to detect tuberculosis, a malignant tumor in the lungs or chest, and other pathologies.
X-ray therapy- This is a modern method with which the treatment of certain pathologies of the joints is performed. The main directions of treatment of orthopedic diseases by this method are: Chronic. Inflammatory processes of the joints (arthritis, polyarthritis); Degenerative (osteoarthritis, osteochondrosis, deforming spondylosis). The purpose of radiotherapy is the inhibition of the vital activity of cells of pathologically altered tissues or their complete destruction. In non-tumor diseases, X-ray therapy is aimed at suppressing the inflammatory reaction, inhibiting proliferative processes, reducing pain sensitivity and secretory activity of the glands. It should be borne in mind that the sex glands, hematopoietic organs, leukocytes, and malignant tumor cells are most sensitive to X-rays. The radiation dose in each case is determined individually.
For the discovery of X-rays, Roentgen was awarded the first Nobel Prize in Physics in 1901, and the Nobel Committee emphasized the practical importance of his discovery.
Thus, X-rays are invisible electromagnetic radiation with a wavelength of 105 - 102 nm. X-rays can penetrate some materials that are opaque to visible light. They are emitted during the deceleration of fast electrons in matter (continuous spectrum) and during transitions of electrons from the outer electron shells of the atom to the inner ones (linear spectrum). Sources of X-ray radiation are: X-ray tube, some radioactive isotopes, accelerators and accumulators of electrons (synchrotron radiation). Receivers - film, luminescent screens, nuclear radiation detectors. X-rays are used in X-ray diffraction analysis, medicine, flaw detection, X-ray spectral analysis, etc.
X-ray radiation (synonymous with X-rays) is with a wide range of wavelengths (from 8·10 -6 to 10 -12 cm). X-ray radiation occurs when charged particles, most often electrons, decelerate in the electric field of the atoms of a substance. The resulting quanta have different energies and form a continuous spectrum. The maximum photon energy in such a spectrum is equal to the energy of incident electrons. In (see) the maximum energy of X-ray quanta, expressed in kiloelectron-volts, is numerically equal to the magnitude of the voltage applied to the tube, expressed in kilovolts. When passing through a substance, X-rays interact with the electrons of its atoms. For X-ray quanta with energies up to 100 keV, the most characteristic type of interaction is the photoelectric effect. As a result of such an interaction, the quantum energy is completely spent on pulling out an electron from the atomic shell and imparting kinetic energy to it. With an increase in the energy of an X-ray quantum, the probability of the photoelectric effect decreases and the process of scattering of quanta on free electrons, the so-called Compton effect, becomes predominant. As a result of such an interaction, a secondary electron is also formed and, in addition, a quantum with an energy less than the energy of the primary quantum flies out. If the energy of an X-ray quantum exceeds one megaelectron-volt, a so-called pairing effect can occur, in which an electron and a positron are formed (see). Consequently, when passing through a substance, a decrease in the energy of X-ray radiation occurs, i.e., a decrease in its intensity. Since low-energy quanta are more likely to be absorbed in this case, X-ray radiation is enriched with higher-energy quanta. This property of X-rays is used to increase the average energy of quanta, i.e., to increase its rigidity. An increase in the hardness of X-ray radiation is achieved using special filters (see). X-ray radiation is used for X-ray diagnostics (see) and (see). See also Ionizing radiation.
X-ray radiation (synonym: x-rays, x-rays) - quantum electromagnetic radiation with a wavelength of 250 to 0.025 A (or energy quanta from 5 10 -2 to 5 10 2 keV). In 1895, it was discovered by V.K. Roentgen. The spectral region of electromagnetic radiation adjacent to x-rays, whose energy quanta exceed 500 keV, is called gamma radiation (see); radiation, whose energy quanta are below 0.05 keV, is ultraviolet radiation (see).
Thus, representing a relatively small part of the vast spectrum of electromagnetic radiation, which includes both radio waves and visible light, X-ray radiation, like any electromagnetic radiation, propagates at the speed of light (about 300 thousand km / s in a vacuum) and is characterized by a wavelength λ ( the distance over which the radiation propagates in one period of oscillation). X-ray radiation also has a number of other wave properties (refraction, interference, diffraction), but it is much more difficult to observe them than for longer-wavelength radiation: visible light, radio waves.
X-ray spectra: a1 - continuous bremsstrahlung spectrum at 310 kV; a - continuous bremsstrahlung spectrum at 250 kV, a1 - spectrum filtered with 1 mm Cu, a2 - spectrum filtered with 2 mm Cu, b - K-series of the tungsten line.
To generate x-rays, x-ray tubes are used (see), in which radiation occurs when fast electrons interact with atoms of the anode substance. There are two types of x-rays: bremsstrahlung and characteristic. Bremsstrahlung X-ray radiation, which has a continuous spectrum, is similar to ordinary white light. The distribution of intensity depending on the wavelength (Fig.) is represented by a curve with a maximum; in the direction of long waves, the curve falls gently, and in the direction of short waves, it steeply and breaks off at a certain wavelength (λ0), called the short-wavelength boundary of the continuous spectrum. The value of λ0 is inversely proportional to the voltage on the tube. Bremsstrahlung arises from the interaction of fast electrons with atomic nuclei. The bremsstrahlung intensity is directly proportional to the strength of the anode current, the square of the tube voltage, and the atomic number (Z) of the anode material.
If the energy of electrons accelerated in the X-ray tube exceeds the critical value for the anode substance (this energy is determined by the tube voltage Vcr, which is critical for this substance), then characteristic radiation occurs. The characteristic spectrum is line, its spectral lines form a series, denoted by the letters K, L, M, N.
The K series is the shortest wavelength, the L series is longer wavelength, the M and N series are observed only in heavy elements (Vcr of tungsten for the K-series is 69.3 kv, for the L-series - 12.1 kv). Characteristic radiation arises as follows. Fast electrons knock atomic electrons out of the inner shells. The atom is excited and then returns to the ground state. In this case, electrons from the outer, less bound shells fill the spaces vacated in the inner shells, and photons of characteristic radiation with an energy equal to the difference between the energies of the atom in the excited and ground states are emitted. This difference (and hence the energy of the photon) has a certain value, characteristic of each element. This phenomenon underlies the X-ray spectral analysis of elements. The figure shows the line spectrum of tungsten against the background of a continuous spectrum of bremsstrahlung.
The energy of electrons accelerated in the X-ray tube is converted almost entirely into thermal energy (the anode is strongly heated in this case), only an insignificant part (about 1% at a voltage close to 100 kV) is converted into bremsstrahlung energy.
The use of x-rays in medicine is based on the laws of absorption of x-rays by matter. The absorption of x-rays is completely independent of the optical properties of the absorber material. The colorless and transparent lead glass used to protect personnel in X-ray rooms absorbs X-rays almost completely. In contrast, a sheet of paper that is not transparent to light does not attenuate X-rays.
The intensity of a homogeneous (i.e., a certain wavelength) X-ray beam, when passing through an absorber layer, decreases according to an exponential law (e-x), where e is the base of natural logarithms (2.718), and the exponent x is equal to the product of the mass attenuation coefficient (μ / p) cm 2 /g per absorber thickness in g / cm 2 (here p is the density of the substance in g / cm 3). X-rays are attenuated by both scattering and absorption. Accordingly, the mass attenuation coefficient is the sum of the mass absorption and scattering coefficients. The mass absorption coefficient increases sharply with increasing atomic number (Z) of the absorber (proportional to Z3 or Z5) and with increasing wavelength (proportional to λ3). This dependence on the wavelength is observed within the absorption bands, at the boundaries of which the coefficient exhibits jumps.
The mass scattering coefficient increases with increasing atomic number of the substance. For λ≥0,3Å the scattering coefficient does not depend on the wavelength, for λ<0,ЗÅ он уменьшается с уменьшением λ.
A decrease in the absorption and scattering coefficients with decreasing wavelength causes an increase in the penetrating power of X-rays. The mass absorption coefficient for bones [absorption is mainly due to Ca 3 (PO 4) 2 ] is almost 70 times greater than for soft tissues, where absorption is mainly due to water. This explains why the shadow of the bones stands out so sharply on the radiographs against the background of soft tissues.
The propagation of an inhomogeneous X-ray beam through any medium, along with a decrease in intensity, is accompanied by a change in the spectral composition, a change in the quality of the radiation: the long-wave part of the spectrum is absorbed to a greater extent than the short-wave part, the radiation becomes more uniform. Filtering out the long-wavelength part of the spectrum makes it possible to improve the ratio between deep and surface doses during X-ray therapy of foci located deep in the human body (see X-ray filters). To characterize the quality of an inhomogeneous X-ray beam, the concept of "half attenuation layer (L)" is used - a layer of a substance that attenuates the radiation by half. The thickness of this layer depends on the voltage on the tube, the thickness and material of the filter. Cellophane (up to an energy of 12 keV), aluminum (20–100 keV), copper (60–300 keV), lead, and copper (>300 keV) are used to measure half attenuation layers. For X-rays generated at voltages of 80-120 kV, 1 mm of copper is equivalent in filtering capacity to 26 mm of aluminum, 1 mm of lead is equivalent to 50.9 mm of aluminum.
Absorption and scattering of X-rays is due to its corpuscular properties; X-rays interact with atoms as a stream of corpuscles (particles) - photons, each of which has a certain energy (inversely proportional to the X-ray wavelength). The energy range of X-ray photons is 0.05-500 keV.
The absorption of X-ray radiation is due to the photoelectric effect: the absorption of a photon by the electron shell is accompanied by the ejection of an electron. The atom is excited and, returning to the ground state, emits characteristic radiation. The emitted photoelectron carries away all the energy of the photon (minus the binding energy of the electron in the atom).
Scattering of X-ray radiation is due to the electrons of the scattering medium. There are classical scattering (the wavelength of the radiation does not change, but the direction of propagation changes) and scattering with a change in wavelength - the Compton effect (the wavelength of the scattered radiation is greater than the incident one). In the latter case, the photon behaves like a moving ball, and the scattering of photons occurs, according to the figurative expression of Comnton, like a game of billiards with photons and electrons: colliding with an electron, the photon transfers part of its energy to it and scatters, having already less energy (respectively, the wavelength of the scattered radiation increases), the electron flies out of the atom with a recoil energy (these electrons are called Compton electrons, or recoil electrons). The absorption of X-ray energy occurs during the formation of secondary electrons (Compton and photoelectrons) and the transfer of energy to them. The energy of X-rays transferred to a unit mass of a substance determines the absorbed dose of X-rays. The unit of this dose 1 rad corresponds to 100 erg/g. Due to the absorbed energy in the substance of the absorber, a number of secondary processes occur that are important for X-ray dosimetry, since it is on them that X-ray measurement methods are based. (see Dosimetry).
All gases and many liquids, semiconductors and dielectrics, under the action of X-rays, increase electrical conductivity. Conductivity is found by the best insulating materials: paraffin, mica, rubber, amber. The change in conductivity is due to the ionization of the medium, i.e., the separation of neutral molecules into positive and negative ions (ionization is produced by secondary electrons). Ionization in air is used to determine the exposure dose of X-ray radiation (dose in air), which is measured in roentgens (see Ionizing Radiation Doses). At a dose of 1 r, the absorbed dose in air is 0.88 rad.
Under the action of X-rays, as a result of the excitation of the molecules of a substance (and during the recombination of ions), in many cases a visible glow of the substance is excited. At high intensities of X-ray radiation, a visible glow of air, paper, paraffin, etc. is observed (metals are an exception). The highest yield of visible light is given by such crystalline phosphors as Zn·CdS·Ag-phosphorus and others used for screens in fluoroscopy.
Under the action of X-rays, various chemical processes can also take place in a substance: the decomposition of silver halides (a photographic effect used in X-rays), the decomposition of water and aqueous solutions of hydrogen peroxide, a change in the properties of celluloid (clouding and release of camphor), paraffin (clouding and bleaching) .
As a result of complete conversion, all the X-ray energy absorbed by the chemically inert substance is converted into heat. The measurement of very small amounts of heat requires highly sensitive methods, but is the main method for absolute measurements of X-rays.
Secondary biological effects from exposure to x-rays are the basis of medical radiotherapy (see). X-rays, the quanta of which are 6-16 keV (effective wavelengths from 2 to 5 Å), are almost completely absorbed by the skin integument of the tissue of the human body; they are called boundary rays, or sometimes Bucca rays (see Bucca rays). For deep X-ray therapy, hard filtered radiation with effective energy quanta from 100 to 300 keV is used.
The biological effect of x-ray radiation should be taken into account not only in x-ray therapy, but also in x-ray diagnostics, as well as in all other cases of contact with x-rays that require the use of radiation protection (see).
High penetrating ability - able to penetrate certain media. X-rays penetrate best through gaseous media (lung tissue), poorly penetrate through substances with high electron density and large atomic mass (in humans - bones).
Fluorescence - glow. In this case, the energy of X-rays is converted into the energy of visible light. Currently, the principle of fluorescence underlies the device of intensifying screens designed for additional illumination of X-ray film. This allows you to reduce the radiation load on the body of the patient under study.
Photochemical - the ability to induce various chemical reactions.
Ionizing ability - under the influence of X-rays, ionization of atoms occurs (decomposition of neutral molecules into positive and negative ions that make up an ion pair.
Biological - damage to cells. For the most part, it is due to the ionization of biologically significant structures (DNA, RNA, protein molecules, amino acids, water). Positive biological effects - antitumor, anti-inflammatory.
Beam tube device
X-rays are produced in an X-ray tube. An X-ray tube is a glass container with a vacuum inside. There are 2 electrodes - cathode and anode. The cathode is a thin tungsten spiral. The anode in the old tubes was a heavy copper rod, with a bevelled surface facing the cathode. On the beveled surface of the anode, a plate of refractory metal was soldered - the mirror of the anode (the anode is very hot during operation). In the center of the mirror is focus of x-ray tube This is where X-rays are produced. The smaller the focus value, the clearer the contours of the subject being shot are. Small focus is considered 1x1 mm, and even less.
In modern X-ray machines, electrodes are made from refractory metals. Typically, tubes with a rotating anode are used. During operation, the anode is rotated by a special device, and the electrons flying from the cathode fall into the optical focus. Due to the rotation of the anode, the position of the optical focus changes all the time, so such tubes are more durable and do not wear out for a long time.
How are x-rays obtained? First, the cathode filament is heated. To do this, using a step-down transformer, the voltage on the tube is reduced from 220 to 12-15V. The cathode filament heats up, the electrons in it begin to move faster, some of the electrons go beyond the filament and a cloud of free electrons forms around it. After that, a high voltage current is turned on, which is obtained using a step-up transformer. In diagnostic X-ray machines, high voltage current is used from 40 to 125 KV (1KV=1000V). The higher the voltage on the tube, the shorter the wavelength. When a high voltage is turned on, a large potential difference is obtained at the poles of the tube, the electrons “break off” from the cathode and rush to the anode at high speed (the tube is the simplest charged particle accelerator). Thanks to special devices, the electrons do not scatter to the sides, but fall into almost one point of the anode - the focus (focal spot) and are decelerated in the electric field of the anode atoms. When the electrons decelerate, electromagnetic waves arise, i.e. X-rays. Thanks to a special device (in old tubes - the bevel of the anode), x-rays are directed to the patient in the form of a divergent beam of rays, a "cone".
X-ray imaging
X-ray film is a layered structure, the main layer is a polyester composition up to 175 microns thick, coated with a photographic emulsion (silver iodide and bromide, gelatin).
Film development - silver is restored (where the rays passed through - blackening of the film area, where they lingered - lighter areas)
Fixer - washing out silver bromide from areas where the rays passed through and did not linger.
The device of a modern radiological room
1. The X-ray room itself, where the apparatus is located and the patients are examined. The area of the X-ray room must be at least 50 m2
2. Control room, where the control panel is located, with the help of which the X-ray laboratory assistant controls the entire operation of the apparatus.
3. A photographic laboratory where cassettes are loaded with film, images are developed and fixed, they are washed and dried. A modern method of photo processing of medical X-ray films is the use of roller-type processors. In addition to undoubted convenience in work, processors provide high stability of the photo processing process. The time of a complete cycle from the moment the film enters the processing machine to the receipt of a dry X-ray pattern ("from dry to dry") does not exceed several minutes.
4. Doctor's office, where the radiologist analyzes and describes the radiographs taken.
Methods of protection for medical personnel and for patients from x-ray radiation
3 main protection methods: protection by shielding, distance and time.
1 .Shield protection:
X-rays are placed in the path of special devices made of materials that absorb x-rays well. It can be lead, concrete, barite concrete, etc. The walls, floor, ceiling in X-ray rooms are protected, made of materials that do not transmit rays into neighboring rooms. The doors are protected with lead material. The observation windows between the X-ray room and the control room are made of leaded glass. The x-ray tube is placed in a special protective casing that does not let x-rays through, and the rays are directed to the patient through a special "window". A tube is attached to the window, which limits the size of the x-ray beam. In addition, the X-ray machine diaphragm is installed at the exit of the rays from the tube. It consists of 2 pairs of plates perpendicular to each other. These plates can be moved and moved apart like curtains. In this way, the irradiation field can be increased or decreased. The larger the irradiation field, the greater the harm, therefore aperture is an important part of protection, especially in children. In addition, the doctor himself is irradiated less. And the quality of the pictures will be better. Another example of shielding is sewn up - those parts of the body of the subject that are not currently subject to shooting should be covered with sheets of lead rubber. There are also aprons, skirts, gloves made of special protective material.
2 .Protection by time:
The patient should be irradiated during x-ray examination for as little time as possible (hurry, but not to the detriment of diagnosis). In this sense, images give a lower radiation load than transillumination, because. very slow shutter speeds (time) are used in the pictures. Time protection is the main way to protect both the patient and the radiologist himself. When examining patients, the doctor, ceteris paribus, tries to choose a research method that takes less time, but not to the detriment of diagnosis. In this sense, fluoroscopy is more harmful, but, unfortunately, it is often impossible to do without fluoroscopy. So in the study of the esophagus, stomach, intestines, both methods are used. When choosing a research method, we are guided by the rule that the benefits of research should be greater than the harm. Sometimes, due to the fear of taking an extra picture, errors in diagnosis occur, treatment is incorrectly prescribed, which sometimes costs the patient's life. It is necessary to remember about the dangers of radiation, but do not be afraid of it, it is worse for the patient.
3 .Protection distance:
According to the quadratic law of light, the illumination of a given surface is inversely proportional to the square of the distance from the light source to the illuminated surface. In relation to X-ray examination, this means that the radiation dose is inversely proportional to the square of the distance from the focus of the X-ray tube to the patient (focal length). With an increase in the focal length by 2 times, the radiation dose decreases by 4 times, with an increase in the focal length by 3 times, the radiation dose decreases by 9 times.
A focal length of less than 35 cm is not allowed for fluoroscopy. The distance from the walls to the X-ray apparatus must be at least 2 m, otherwise secondary rays are formed that occur when the primary beam of rays hits the surrounding objects (walls, etc.). For the same reason, extra furniture is not allowed in X-ray rooms. Sometimes, when examining seriously ill patients, the personnel of the surgical and therapeutic departments help the patient stand behind the screen for transillumination and stand next to the patient during the examination, supporting him. As an exception, this is allowed. But the radiologist must make sure that the nurses and nurses helping the sick put on a protective apron and gloves and, if possible, do not stand close to the patient (protection by distance). If several patients came to the X-ray room, they are called to the procedural room by 1 person, i.e. There should only be 1 person at a time in the study.
Physical bases of radiography and fluorography. Their shortcomings and advantages. Advantages of digital over film.
Principles of radiography
For diagnostic radiography, it is advisable to take pictures in at least two projections. This is due to the fact that the radiograph is a flat image of a three-dimensional object. And as a result, the localization of the detected pathological focus can be established only with the help of 2 projections.
Imaging technique
The quality of the resulting X-ray image is determined by 3 main parameters. The voltage applied to the X-ray tube, the current strength and the operating time of the tube. Depending on the studied anatomical formations, and the weight and size data of the patient, these parameters can vary significantly. There are average values for different organs and tissues, but it should be borne in mind that the actual values will differ depending on the device where the examination is performed and the patient who is being X-rayed. An individual table of values is compiled for each device. These values are not absolute and are adjusted as the study progresses. The quality of the images performed is largely dependent on the ability of the radiographer to adequately adapt the table of average values to a particular patient.
Image recording
The most common way to record an X-ray image is to fix it on an X-ray sensitive film and then develop it. Currently, there are also systems that provide digital data recording. Due to the high cost and complexity of manufacturing, this type of equipment is somewhat inferior to analog equipment in terms of prevalence.
X-ray film is placed in special devices - cassettes (they say - the cassette is loaded). The cassette protects the film from visible light; the latter, like x-rays, has the ability to reduce metallic silver from AgBr. Cassettes are made of a material that does not transmit light, but transmits x-rays. Inside the cassettes are intensifying screens, the film is laid between them; when taking a picture, not only the x-rays themselves fall on the film, but also the light from the screens (the screens are covered with fluorescent salt, so they glow and enhance the action of the x-rays). This allows you to reduce the radiation load on the patient by 10 times.
When taking a picture, x-rays are directed to the center of the object being photographed (centration). After shooting in a photo lab, the film is developed in special chemicals and fixed (fixed). The fact is that on those parts of the film that were not hit by x-rays during the shooting or there were few of them, silver was not restored, and if the film is not placed in a fixer (fixer) solution, then when examining the film, silver is restored under the influence of visible light. Sveta. The entire film will turn black and no image will be visible. When fixing (fixing), unreduced AgBr from the film goes into the fixer solution, so there is a lot of silver in the fixer, and these solutions are not poured out, but surrendered to X-ray centers.
A modern method of photo processing of medical X-ray films is the use of roller-type processors. In addition to undoubted convenience in work, processors provide high stability of the photo processing process. The time of a complete cycle from the moment the film enters the processing machine to the receipt of a dry X-ray pattern ("from dry to dry") does not exceed several minutes.
X-rays are an image made in black and white - a negative. Black - areas with low density (lungs, gas bubble of the stomach. White - with high density (bones).
Fluorography- The essence of FOG is that with it, an image of the chest is first obtained on a fluorescent screen, and then a picture is taken not of the patient himself, but of his image on the screen.
Fluorography gives a reduced image of the object. There are small frame (eg 24×24 mm or 35×35 mm) and large frame (eg 70×70 mm or 100×100 mm) techniques. The latter, in terms of diagnostic capabilities, approaches radiography. FOG is used for preventive examination of the population(hidden diseases such as cancer and tuberculosis are detected).
Both stationary and mobile fluorographic devices have been developed.
Currently, film fluorography is gradually being replaced by digital. Digital methods make it possible to simplify work with an image (an image can be displayed on a monitor screen, printed, transmitted over a network, stored in a medical database, etc.), reduce radiation exposure to the patient and reduce the cost of additional materials (film, developer for films).
There are two common methods of digital fluorography. The first technique, like conventional fluorography, uses photographing an image on a fluorescent screen, only a CCD matrix is used instead of an X-ray film. The second technique uses layer-by-layer transverse scanning of the chest with a fan-shaped X-ray beam with detection of the transmitted radiation by a linear detector (similar to a conventional paper document scanner, where the linear detector moves along a sheet of paper). The second method allows the use of much lower doses of radiation. Some drawback of the second method is the longer time for obtaining the image.
Comparative characteristics of the dose load in various studies.
A conventional film chest fluorogram provides the patient with an average individual radiation dose of 0.5 millisievert (mSv) per procedure (digital fluorogram - 0.05 mSv), while a film radiograph - 0.3 mSv per procedure (digital radiograph - 0 .03 mSv), and computed tomography of the chest - 11 mSv per procedure. Magnetic resonance imaging does not carry radiation exposure
Benefits of radiography
Wide availability of the method and ease of research.
Most studies do not require special patient preparation.
Relatively low cost of research.
The images can be used for consultation with another specialist or in another institution (unlike ultrasound images, where a second examination is necessary, since the images obtained are operator-dependent).
Static image - the complexity of assessing the function of the body.
The presence of ionizing radiation that can have a harmful effect on the patient.
The information content of classical radiography is much lower than such modern methods of medical imaging as CT, MRI, etc. Ordinary x-ray images reflect the projection layering of complex anatomical structures, that is, their summation x-ray shadow, in contrast to the layered series of images obtained by modern tomographic methods.
Without the use of contrast agents, radiography is not informative enough to analyze changes in soft tissues that differ little in density (for example, when studying the abdominal organs).
Physical bases of roentgenoscopy. Disadvantages and advantages of the method
In modern conditions, the use of a fluorescent screen is not justified due to its low luminosity, which makes it necessary to conduct research in a well-darkened room and after a long adaptation of the researcher to the dark (10-15 minutes) to distinguish a low-intensity image.
Now fluorescent screens are used in the design of X-ray image intensifier, which increases the brightness (glow) of the primary image by about 5,000 times. With the help of an electron-optical converter, the image appears on the monitor screen, which significantly improves the quality of diagnostics, does not require darkening of the X-ray room.
Advantages of fluoroscopy
The main advantage over radiography is the fact of the study in real time. This allows you to evaluate not only the structure of the organ, but also its displacement, contractility or extensibility, the passage of a contrast agent, and fullness. The method also allows you to quickly assess the localization of some changes, due to the rotation of the object of study during transillumination (multi-projection study).
Fluoroscopy allows you to control the implementation of some instrumental procedures - catheter placement, angioplasty (see angiography), fistulography.
The resulting images can be placed on a regular CD or network storage.
With the advent of digital technologies, 3 main disadvantages inherent in traditional fluoroscopy have disappeared:
Relatively high radiation dose compared to radiography - modern low-dose devices have left this disadvantage in the past. The use of pulsed scan modes further reduces the dose load by up to 90%.
Low spatial resolution - on modern digital devices, the resolution in scopy mode is only slightly inferior to the resolution in radiographic mode. In this case, the ability to observe the functional state of individual organs (heart, lungs, stomach, intestines) "in dynamics" is of decisive importance.
The impossibility of documenting research - digital imaging technologies make it possible to save research materials, both frame-by-frame and as a video sequence.
Fluoroscopy is performed mainly in the X-ray diagnosis of diseases of the internal organs located in the abdominal and chest cavities, according to the plan that the radiologist draws up before the start of the study. Sometimes, the so-called survey fluoroscopy is used to recognize traumatic bone injuries, to clarify the area to be radiographed.
Contrast fluoroscopic examination
Artificial contrast greatly expands the possibilities of X-ray examination of organs and systems where tissue densities are approximately the same (for example, the abdominal cavity, whose organs transmit X-rays to approximately the same extent and therefore have low contrast). This is achieved by introducing into the lumen of the stomach or intestines an aqueous suspension of barium sulfate, which does not dissolve in digestive juices, is not absorbed by the stomach or intestines and is excreted naturally in a completely unchanged form. The main advantage of barium suspension is that, passing through the esophagus, stomach and intestines, coats their inner walls and gives a complete picture of the nature of elevations, depressions and other features of their mucous membrane on the screen or film. The study of the internal relief of the esophagus, stomach and intestines contributes to the recognition of a number of diseases of these organs. With more tight filling, it is possible to determine the shape, size, position and function of the organ under study.
Mammography - the basics of the method, indications. Advantages of digital mammography over film.
Mammography- chapter medical diagnostics, engaged in non-invasive researchmammary gland, mainly female, which is carried out with the aim of:
1. prophylactic examination (screening) of healthy women to detect early, non-palpable forms of breast cancer;
2. differential diagnosis between cancer and benign dyshormonal hyperplasia (FAM) of the breast;
3. assessment of the growth of the primary tumor (single node or multicentric cancerous foci);
4.Dynamic dispensary monitoring of the state of the mammary glands after surgery.
The following methods of radiation diagnostics of breast cancer have been introduced into medical practice: mammography, ultrasound, computed tomography, magnetic resonance imaging, color and power Doppler, mammography-guided stereotaxic biopsy, and thermography.
X-ray mammography
Currently, in the world, in the vast majority of cases, X-ray projection mammography, film (analogue) or digital, is used to diagnose female breast cancer (BC).
The procedure takes no more than 10 minutes. For the shot, the chest should be fixed between two planks and slightly compressed. The picture is taken in two projections so that you can accurately determine the location of the neoplasm, if it is found. Because symmetry is one of the diagnostic factors, both breasts should always be examined.
MRI mammography
Complaints about retraction or bulging of any part of the gland
Discharge from the nipple, changing its shape
Soreness of the mammary gland, its swelling, resizing
As a preventive screening method, mammography is prescribed for all women aged 40 years and older, or women who are at risk.
Benign breast tumors (particularly fibroadenoma)
Inflammatory processes (mastitis)
Mastopathy
Tumors of the genital organs
Diseases of the endocrine glands (thyroid, pancreas)
Infertility
Obesity
History of breast surgery
Advantages of digital mammography over film:
Reduction of dose loads during X-ray studies;
Improving the efficiency of research, allowing to identify previously inaccessible pathological processes (possibility of digital computer image processing);
Possibilities of using telecommunication networks for transmission of images for the purpose of remote consultation;
Achievement of economic effect during mass research.
