X-ray radiation and its use in medicine

Position on the electromagnetic wave scale

The energy ranges of X-rays and gamma rays overlap over a wide energy range. Both types of radiation are electromagnetic radiation and, with the same photon energy, are equivalent. The terminological difference lies in the method of occurrence - X-rays are emitted with the participation of electrons (either bound in atoms or free) while gamma radiation is emitted in the processes of de-excitation of atomic nuclei. Photons of characteristic (that is, emitted during transitions in the electron shells of atoms) X-ray radiation have an energy from 10 to 250 keV, which corresponds to radiation with a frequency from 2 1015 to 6 1019 and a wavelength of 0.005-100 (the generally accepted definition of the lower limit of the range of X-rays does not exist on the wavelength scale). Soft X-rays have the lowest photon energy and radiation frequency (and longest wavelength), while hard X-rays have the highest photon energy and radiation frequency (and shortest wavelength). Hard X-rays are used primarily for industrial purposes. The conventional boundary between soft and hard X-ray radiation on the wavelength scale is about 2 Å (≈6 keV)[1].

How to protect yourself from unwanted consequences?

There are three ways to protect yourself from the harmful effects of ionizing radiation:

• Time and intervals between studies - if you do not exceed the recommended standards and scan according to the radiation passport, no harm will be done to the body. The duration of the study is also important, so it is advisable to be examined by professionals who can minimize the time the patient spends in a radioactive environment.
• Personal protection measures - X-rays do not act pointwise, but scatter, therefore the risk of irradiation of neighboring areas increases. That is why during scanning it is recommended to wear special lead aprons that can reflect harmful rays.

Laboratory sources

X-ray tubes

Schematic illustration of an X-ray tube. X - X-rays, K - cathode, A - anode (sometimes called anti-cathode), C - heat sink, Uh
- cathode filament voltage,
Ua
- accelerating voltage, Win - water cooling inlet, Wout - water cooling outlet

X-rays arise from the strong acceleration of charged particles (bremsstrahlung), or from high-energy transitions in the electronic shells of atoms or molecules. Both effects are used in X-ray tubes. The main structural elements of such tubes are a metal cathode and anode (previously also called an anticathode

).
In X-ray tubes, electrons emitted by the cathode are accelerated by the difference in electrical potential between the anode and the cathode (no X-rays are emitted, since the acceleration is too small) and strike the anode, where they are sharply decelerated. In this case, due to bremsstrahlung, X-ray radiation is generated, and at the same time electrons are knocked out from the internal electron shells of the anode atoms. The empty spaces in the shells are occupied by other electrons of the atom. In this case, X-ray radiation is emitted with an energy spectrum characteristic of the anode material (characteristic radiation, frequencies are determined by Moseley's law: ν = A ( Z − B ) , {\displaystyle {\sqrt {\nu }}=A(ZB),} where Z
is the atomic number of the anode element,
A
and
B
are constants for a certain value of the principal quantum number
n
of the electron shell). Currently, anodes are made mainly of ceramics, with the part where the electrons strike being made of molybdenum or copper.

Crookes tube.
During the acceleration-deceleration process, only about 1 kinetic energy of the electron goes into x-ray radiation, 99% of the energy is converted into heat.

Particle accelerators

X-ray radiation can also be produced at charged particle accelerators.
So-called synchrotron radiation occurs when a beam of particles is deflected in a magnetic field, causing them to experience acceleration in a direction perpendicular to their motion. Synchrotron radiation has a continuous spectrum with an upper limit. With appropriately selected parameters (magnetic field strength and particle energy), X-rays can also be obtained in the spectrum of synchrotron radiation. Wavelengths of K-series spectral lines () for a number of anode materials.[2],[3]

Interaction with matter

The wavelength of X-rays is comparable to the size of atoms, so there is no material from which an X-ray lens can be made. In addition, when perpendicularly incident on a surface, X-rays are almost not reflected. Despite this, methods have been found in X-ray optics to construct optical elements for X-rays. In particular, it turned out that diamond reflects them well[4].

X-rays can penetrate matter, and different substances absorb them differently. Absorption of X-rays is their most important property in X-ray photography. The intensity of X-rays decreases exponentially depending on the distance traveled in the absorbing layer ( I = I

0
e-kd
, where
d
is the layer thickness, coefficient
k
is proportional to
Z
³λ³,
Z
is the atomic number of the element, λ is the wavelength).

Absorption occurs as a result of photoabsorption (photoeffect) and Compton scattering:

• Photoabsorption
  to the process of a photon knocking out an electron from the shell of an atom, which requires that the photon energy be greater than a certain minimum value.
  If we consider the probability of an absorption event depending on the photon energy, then when a certain energy is reached, it (the probability) sharply increases to its maximum value. For higher energy values ​​the probability decreases continuously. Because of this dependence, they say that there is an absorption limit
  . The place of the electron knocked out during the act of absorption is taken by another electron, and radiation with a lower photon energy is emitted, the so-called. fluorescence process.
• An X-ray photon can interact not only with bound electrons, but also with free and weakly bound electrons. Scattering of photons by electrons occurs - the so-called. Compton scattering
  . Depending on the scattering angle, the photon wavelength increases by a certain amount and, accordingly, the energy decreases. Compton scattering, compared to photoabsorption, becomes predominant at higher photon energies[5].

Biological effects

X-ray radiation is ionizing. It affects the tissues of living organisms and can cause radiation sickness, radiation burns and malignant tumors. For this reason, protective measures must be taken when working with X-rays. It is believed that the damage is directly proportional to the absorbed dose of radiation. X-ray radiation is a mutagenic factor.

Registration

• Luminescence effect. X-rays can cause some substances to glow (fluorescence). This effect is used in medical diagnostics during fluoroscopy (observation of an image on a fluorescent screen) and x-ray photography (radiography). Medical photographic films are usually used in combination with intensifying screens, which contain x-ray phosphors, which glow under the influence of x-rays and illuminate the photosensitive emulsion. The method of obtaining life-size images is called radiography. With fluorography, the image is obtained on a reduced scale. A luminescent substance (scintillator) can be optically coupled to an electronic detector of light radiation (photomultiplier, photodiode, etc.), the resulting device is called a scintillation detector. It allows you to record individual photons and measure their energy, since the energy of a scintillation flash is proportional to the energy of the absorbed photon.
• Photographic effect. X-rays, just like ordinary light, can directly illuminate a photographic emulsion. However, without a fluorescent layer, this requires 30 to 100 times more exposure (i.e., dose). The advantage of this method (known as screenless radiography) is that the image is sharper.
• In semiconductor detectors, X-rays produce electron-hole pairs at the pn junction of a diode switched in the blocking direction. In this case, a small current flows, the amplitude of which is proportional to the energy and intensity of the incident X-ray radiation. In pulsed mode, it is possible to record individual X-ray photons and measure their energy.
• Individual X-ray photons can also be recorded using gas-filled ionizing radiation detectors (Geiger counter, proportional chamber, etc.).

Possible consequences of radiography

Are x-rays harmful, and what could be the consequences of exceeding the recommended standards? As already mentioned, the hematopoietic organs are the most sensitive to radiation, so the following deviations are possible:

• Minor changes in blood composition after low doses of radiation.
• Leukemia is a decrease in the number of leukocytes and a violation of their structure, due to which the body becomes vulnerable, immunity decreases and interruptions in the functioning of the entire body occur.
• Erythrocytopenia is a drop in the level of red blood cells (red blood cells), which are responsible for transporting oxygen. As a result, organs and tissues begin to experience oxygen starvation.
• Thrombocytopenia is a decrease in the number of platelets, whose function is blood clotting. As a result, the risk of bleeding increases.

In addition, frequent X-rays can cause other pathologies:

• The growth of malignant neoplasms (the skin, bones, mammary glands, ovaries, blood, thyroid gland and lungs are most susceptible to this).
• Premature aging of the skin and the whole body.
• Pathological processes in the lens with subsequent development of cataracts.
• Immunosuppression up to immunodeficiency, as a result of which the body becomes susceptible to various infections.
• Violation of metabolic processes.
• Impotence in men and damage to eggs in women.
• In children there is a violation of physical and mental development.

In order to understand how harmful X-rays are, you should know that ionizing radiation becomes dangerous only with prolonged intense exposure. The use of radiography for diagnostic purposes involves short-term, low-dose radiation. Modern medical equipment is equipped with digital sensors that reduce the level of radiation exposure several times, so diagnostics using X-rays is considered relatively safe even in the case of multiple scans. It was found that a single exposure to digital X-rays increases the risk of developing malignant tumors by no more than 0.001%, which is very little.

Application

• Using X-rays, you can “enlighten” the human body, as a result of which you can get an image of bones, and with modern devices, internal organs (see also radiography and fluoroscopy). This uses the fact that the element calcium ( Z
  = 20), which is found predominantly in bones, has an atomic number that is much greater than the atomic numbers of the elements that make up soft tissues, namely hydrogen (
  Z
  = 1), carbon (
  Z
  = 6) , nitrogen (
  Z
  = 7), oxygen (
  Z
  = 8). In addition to conventional devices that provide a two-dimensional projection of the object under study, there are computed tomographs that allow one to obtain a three-dimensional image of internal organs.

• Detection of defects in products (rails, welds, etc.) using X-ray radiation is called X-ray flaw detection.

• In materials science, crystallography, chemistry and biochemistry, X-rays are used to elucidate the structure of substances at the atomic level using diffraction scattering of X-rays from crystals (X-ray diffraction). A well-known example is the determination of the structure of DNA.

• Using X-rays, the chemical composition of a substance can be determined. In an electron beam microprobe (or in an electron microscope), the analyzed substance is irradiated with electrons, while the atoms are ionized and emit characteristic X-ray radiation. X-rays can be used instead of electrons. This analytical method is called X-ray fluorescence analysis.

• X-ray television introscopes are actively used at airports, allowing one to view the contents of hand luggage and baggage in order to visually detect dangerous objects on the monitor screen.

• X-ray therapy is a section of radiation therapy that covers the theory and practice of the therapeutic use of X-rays generated at a voltage on the X-ray tube of 20-60 and a skin-focal distance of 3-7 cm (short-distance radiotherapy) or at a voltage of 180-400 kV and a skin-focal distance of 30 —150 cm (external radiotherapy). X-ray therapy is carried out mainly for superficial tumors and for some other diseases, including skin diseases (ultrasoft Bucca X-rays).

Natural X-rays

On Earth, electromagnetic radiation in the X-ray range is formed as a result of the ionization of atoms by radiation that occurs during radioactive decay, as a result of the Compton effect of gamma radiation that occurs during nuclear reactions, and also by cosmic radiation. Radioactive decay also leads to the direct emission of X-ray quanta if it causes a rearrangement of the electron shell of the decaying atom (for example, during electron capture). X-ray radiation that occurs on other celestial bodies does not reach the Earth's surface, as it is completely absorbed by the atmosphere. It is studied by satellite X-ray telescopes such as Chandra and XMM-Newton.

What is more harmful - Mantoux or X-ray?

Today, practicing medical specialists pay great attention to the issue of identifying infection of the population with Mycobacterium tuberculosis and diagnosing the disease in the early stages of development.

Children undergo the Mantoux test annually; in adults, diagnosis is carried out using the following studies:

• preventive fluorography;
• plain radiographic examination;
• bacteriological analysis of sputum;
• computed or magnetic resonance imaging.

The Mantoux reaction is the introduction into the child’s body of a small dose of waste products of mycobacterium tuberculosis, which causes an immune response. The degree of reaction of the child’s body corresponds to the presence of infection.

The tuberculin test has a number of negative aspects. The size of the post-injection papule depends on the reactivity of the child’s body. If a small patient has an allergy, a violent immune response is observed - the size of the spot exceeds the permissible 5 mm. A weakened immune system can cause the reaction to be negative even in the presence of infection.

The diagnostic procedure must be carried out in an anti-tuberculosis dispensary. However, it is often carried out in children's institutions where the staff does not have a good command of the technique of performing the test.

In addition, water should not get into the tuberculin injection site, it should not be rubbed or injured, and children do not always comply with such requirements. This leads to a false positive reaction.

Despite the small dose of the diagnostic test, a child with a certain susceptibility may develop clinical manifestations of an allergic reaction:

Low specificity – the accuracy of the method does not exceed 50%. A positive test result is also observed after vaccination against tuberculosis (BCG), which stimulates the production of immune antibodies that provide protection for the body when it “encounters” the causative agent of the disease, infection with non-pathogenic forms of Tuberculosismycobacterium. However, this technique is still widely used by pediatric phthisiatricians because of its simplicity and accessibility.

History of discovery

A photograph (x-ray) of the hand of Albert von Kölliker taken by V. K. Roentgen.
X-rays were discovered by Wilhelm Conrad Roentgen. While studying cathode rays experimentally, on November 8, 1895, he noticed that cardboard coated with barium platinum sulfur dioxide, located near the cathode ray tube, began to glow in a dark room. Over the next few weeks, he studied all the basic properties of the newly discovered radiation, which he called X-rays (“X-rays”). On December 22, 1895, Roentgen made the first public announcement of his discovery at the Physics Institute of the University of Würzburg[6]. On December 28, 1895, Roentgen’s article entitled “On a new type of rays” was published in the journal of the Würzburg Physico-Medical Society [7].

But 8 years earlier, in 1887, Nikola Tesla recorded in his diary entries the results of a study of X-rays and the bremsstrahlung radiation they emitted, but neither Tesla nor his circle attached serious importance to these observations. In addition, even then Tesla suggested the danger of prolonged exposure to X-rays on the human body.

The cathode ray tube that Roentgen used in his experiments was developed by J. Hittorf and W. Crookes. When this tube operates, X-rays are generated. This was shown in experiments by Heinrich Hertz and his student Philipp Lenard through the blackening of photographic plates. However, none of them realized the significance of their discovery and did not publish their results.

For this reason, Roentgen did not know about the discoveries made before him and discovered the rays independently - when observing the fluorescence that occurs during the operation of the cathode ray tube. Roentgen studied X-rays for a little over a year (from November 8, 1895 to March 1897) and published three articles about them, which included a comprehensive description of the new rays. Subsequently, hundreds of works by his followers, then published over the course of 12 years, could neither add nor change anything significant. Roentgen, who had lost interest in X-rays, told his colleagues: “I’ve already written everything, don’t waste your time.” Also contributing to Roentgen's fame was the famous photograph of Albert von Kölliker's hand, which he published in his article (see image on the right). 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. In other countries, the name preferred by Roentgen is used - X-rays, although phrases similar to Russian (English: Roentgen rays, etc.) are also used. In Russia, the rays began to be called “X-rays” on the initiative of V. K. Roentgen’s student, Abram Fedorovich Ioffe.

Signs of human exposure to x-rays

The most common forms of radiation poisoning are considered to be gastrointestinal and bone marrow levels of exposure, at which severe changes in the functioning of the body occur.

The main signs of X-ray exposure are given in the table.

If the above symptoms appear, you should immediately consult a doctor.

Notes

1. ↑ 1 2 Blokhin M.A.
   X-ray radiation // Physical encyclopedia: [in 5 volumes] / Ch. ed. A. M. Prokhorov. - M.: Great Russian Encyclopedia, 1994. - T. 4: Poynting - Robertson - Streamers. - pp. 375-377. — 704 p. — 40,000 copies. — ISBN 5-85270-087-8.
2. CRC Handbook of Chemistry and Physics 75th ed. David R. Lide P.10-227. CRC Press ISBN 0-8493-0475-X
3. Crystallographica, v1.60a. Oxford Cryosystems 1995-1999.
4. Yuri Erin.
   The high reflectivity of diamond in the hard X-ray range has been confirmed. Elements - science news (03/03/2010). Retrieved May 11, 2010. Archived August 27, 2011.
5. X-ray scattering on layered nanosystems with rough interfaces. — Nanosystems, 2012
6. Manolov K., Tyutyunnik V.
   Biography of the atom. Atom - from Cambridge to Hiroshima. — Reworked lane. from Bulgarian. - M.: Mir, 1984. - P. 17-18. — 246 p.
7. W. C. Roentgen.
   Ueber eine neue Art von Strahlen // Sonderabbdruck aus den Sitzungsberichten der Würzburger Physik.-medic. Gesellschaft. — 1895.