X-rays are created by converting electron energy into photons, which takes place in an x-ray tube. The amount (exposure) and quality (spectrum) of radiation can be adjusted by changing the current, voltage and operating time of the device.
Working principle
X-ray tubes (the photo is given in the article) are energy converters. They take it from the network and turn it into other forms - penetrating radiation and heat, the latter being an undesirable by-product. The design of the X-ray tube is such that it maximizes photon production and dissipates heat as quickly as possible.
A tube is a relatively simple device, usually containing two fundamental elements - a cathode and an anode. When current flows from the cathode to the anode, the electrons lose energy, resulting in the generation of X-rays.
Anode
The anode is the component that emitshigh energy photons. This is a relatively massive metal element that is connected to the positive pole of the electrical circuit. Performs two main functions:
- converts electron energy into x-rays,
- dissipates heat.
The anode material is chosen to enhance these functions.
Ideally, most of the electrons should form high-energy photons, not heat. The fraction of their total energy that is converted into X-rays (efficiency) depends on two factors:
- atomic number (Z) of the anode material,
- energy of electrons.
Most X-ray tubes use tungsten as an anode material, which has an atomic number of 74. In addition to having a large Z, this metal has some other characteristics that make it suitable for this purpose. Tungsten is unique in its ability to retain strength when heated, has a high melting point and low evaporation rate.
For many years, the anode was made from pure tungsten. In recent years, an alloy of this metal with rhenium has begun to be used, but only on the surface. The anode itself under the tungsten-rhenium coating is made of a lightweight material that stores heat well. Two such substances are molybdenum and graphite.
X-ray tubes used for mammography are made with a molybdenum coated anode. This material has an intermediate atomic number (Z=42) which generates characteristic photons with energies convenient tofor taking pictures of the chest. Some mammography devices also have a second anode made of rhodium (Z=45). This allows you to increase energy and achieve greater penetration for tight breasts.
The use of rhenium-tungsten alloy improves long-term radiation output - over time, the efficiency of pure tungsten anode devices decreases due to thermal damage to the surface.
Most anodes are shaped like beveled discs and are attached to an electric motor shaft that rotates them at relatively high speeds while emitting X-rays. The purpose of rotation is to remove heat.
Focal spot
Not the entire anode is involved in the generation of X-rays. It occurs on a small area of its surface - a focal spot. The dimensions of the latter are determined by the dimensions of the electron beam coming from the cathode. In most devices, it has a rectangular shape and varies between 0.1-2 mm.
X-ray tubes are designed with a specific focal spot size. The smaller it is, the less blurring and sharper the image, and the larger it is, the better heat dissipation.
Focal spot size is one of the factors to consider when choosing X-ray tubes. Manufacturers produce devices with small focal spots when it is necessary to achieve high resolution and sufficiently low radiation. For example, this is required when examining small and thin parts of the body, as in mammography.
X-ray tubes are mainly produced with two focal spot sizes, large and small, which can be selected by the operator according to the imaging procedure.
Cathode
The main function of the cathode is to generate electrons and collect them into a beam directed at the anode. As a rule, it consists of a small wire spiral (thread) immersed in a cup-shaped depression.
Electrons passing through the circuit usually cannot leave the conductor and go into free space. However, they can do it if they get enough energy. In a process known as thermal emission, heat is used to expel electrons from the cathode. This becomes possible when the pressure in the evacuated X-ray tube reaches 10-6–10-7 mmHg. Art. The filament heats up in the same way as the filament of an incandescent lamp when current is passed through it. The operation of the X-ray tube is accompanied by heating the cathode to the glow temperature with the displacement of part of the electrons from it by thermal energy.
Balloon
The anode and cathode are contained in a hermetically sealed container. The balloon and its contents are often referred to as an insert, which has a limited life and can be replaced. X-ray tubes mostly have glass bulbs, although metal and ceramic bulbs are used for some applications.
The main function of the balloon is to provide support and insulation for the anode and cathode, and to maintain a vacuum. Pressure in the evacuated X-ray tubeat 15°C is 1.2 10-3 Pa. The presence of gases in the cylinder would allow electricity to flow freely through the device, and not just in the form of an electron beam.
Case
The design of the x-ray tube is such that, in addition to enclosing and supporting other components, its body serves as a shield and absorbs radiation, except for the useful beam passing through the window. Its relatively large outer surface dissipates much of the heat generated inside the device. The space between the body and the insert is filled with oil for insulation and cooling.
Chain
An electrical circuit connects the tube to a power source called a generator. The source receives power from the mains and converts alternating current to direct current. The generator also allows you to adjust some circuit parameters:
- KV - voltage or electrical potential;
- MA is the current that flows through the tube;
- S – duration or exposure time, in fractions of a second.
The circuit provides the movement of electrons. They are charged with energy, passing through the generator, and give it to the anode. As they move, two transformations occur:
- potential electrical energy is converted into kinetic energy;
- kinetic, in turn, is converted into x-rays and heat.
Potential
When electrons enter the bulb, they have potential electrical energy, the amount of which is determined by the voltage KV between the anode and cathode. X-ray tube workingunder voltage, to create 1 KV of which each particle must have 1 keV. By adjusting KV, the operator endows each electron with a certain amount of energy.
Kinetics
Low pressure in the evacuated X-ray tube (at 15°C it is 10-6–10-7 mmHg.) allows particles to fly out from the cathode to the anode under the action of thermionic emission and electric force. This force accelerates them, which leads to an increase in speed and kinetic energy and a decrease in potential. When a particle hits the anode, its potential is lost and all of its energy is converted into kinetic energy. A 100-keV electron reaches speeds in excess of half the speed of light. Hitting the surface, the particles slow down very quickly and lose their kinetic energy. It turns into X-rays or heat.
Electrons come into contact with individual atoms of the anode material. Radiation is generated when they interact with orbitals (X-ray photons) and with the nucleus (bremsstrahlung).
Link Energy
Each electron inside an atom has a certain binding energy, which depends on the size of the latter and the level at which the particle is located. The binding energy plays an important role in the generation of characteristic X-rays and is necessary to remove an electron from an atom.
Bremsstrahlung
Bremsstrahlung produces the largest number of photons. Electrons penetrating the anode material and passing near the nucleus are deflected and slowed downthe force of attraction of the atom. Their energy lost during this encounter appears as an X-ray photon.
Spectrum
Only a few photons have an energy close to that of electrons. Most of them are lower. Let us assume that there is a space or field surrounding the nucleus in which the electrons experience a "braking" force. This field can be divided into zones. This gives the field of the nucleus the appearance of a target with an atom in the center. An electron hitting any point of the target experiences deceleration and generates an X-ray photon. Particles hitting closest to the center are the most affected and therefore lose the most energy, producing the highest energy photons. Electrons entering the outer zones experience weaker interactions and generate lower energy quanta. Although the zones have the same width, they have a different area depending on the distance to the core. Since the number of particles falling on a given zone depends on its total area, it is obvious that the outer zones capture more electrons and create more photons. This model can be used to predict the energy spectrum of X-rays.
Emax photons of the main bremsstrahlung spectrum corresponds to Emax electrons. Below this point, as the photon energy decreases, their number increases.
A significant number of low energy photons are absorbed or filtered as they attempt to pass through the anode surface, tube window or filter. Filtration is generally dependent on the composition and thickness of the material through whichthe beam passes through, which determines the final form of the low-energy curve of the spectrum.
KV Influence
The high-energy part of the spectrum is determined by the voltage in X-ray tubes kV (kilovolt). This is because it determines the energy of the electrons reaching the anode, and photons cannot have a potential higher than this. What voltage does the x-ray tube work with? The maximum photon energy corresponds to the maximum applied potential. This voltage may change during exposure due to AC mains current. In this case, the Emax of a photon is determined by the peak voltage of the oscillation period KVp.
Besides the quantum potential, KVp determines the amount of radiation created by a given number of electrons hitting the anode. Since the overall efficiency of bremsstrahlung increases due to an increase in the energy of the bombarding electrons, which is determined by KVp, it follows that KVp affects the efficiency of the device.
Changing KVp usually changes the spectrum. The total area under the energy curve is the number of photons. Without a filter, the spectrum is a triangle, and the amount of radiation is proportional to the square of KV. In the presence of a filter, increasing KV also increases photon penetration, which reduces the percentage of filtered radiation. This leads to an increase in radiation output.
Characteristic radiation
The type of interaction that produces the characteristicradiation, includes the collision of high-speed electrons with orbital ones. Interaction can only occur when the incoming particle has Ek greater than the binding energy in the atom. When this condition is met and a collision occurs, the electron is ejected. In this case, a vacancy remains, which is filled by a particle of a higher energy level. As the electron moves, it gives off energy, which is emitted in the form of an X-ray quantum. This is called characteristic radiation, since the E of a photon is a characteristic of the chemical element from which the anode is made. For example, when an electron from the K-level of tungsten with Ebond=69.5 keV is knocked out, the vacancy is filled by an electron from the L-level with Ebond=10, 2 keV. The characteristic X-ray photon has an energy equal to the difference between these two levels, or 59.3 keV.
In fact, this anode material results in a number of characteristic X-ray energies. This is because electrons at different energy levels (K, L, etc.) can be knocked out by bombarding particles, and vacancies can be filled from different energy levels. Although the filling of L-level vacancies generates photons, their energies are too low to be used in diagnostic imaging. Each characteristic energy is given a designation that indicates the orbital in which the vacancy formed, with an index that indicates the source of electron filling. Index alpha (α) indicates the occupation of an electron from the L-level, and beta (β) indicatesfilling from level M or N.
- Spectrum of tungsten. The characteristic radiation of this metal produces a linear spectrum consisting of several discrete energies, while the bremsstrahlung creates a continuous distribution. The number of photons produced by each characteristic energy differs in that the probability of filling a K-level vacancy depends on the orbital.
- Spectrum of molybdenum. Anodes of this metal used for mammography produce two rather intense characteristic X-ray energies: K-alpha at 17.9 keV, and K-beta at 19.5 keV. The optimal spectrum of X-ray tubes, which allows to achieve the best balance between contrast and radiation dose for medium-sized breasts, is achieved at Eph=20 keV. However, bremsstrahlung is produced at high energies. Mammography equipment uses a molybdenum filter to remove the unwanted part of the spectrum. The filter works on the "K-edge" principle. It absorbs radiation in excess of the binding energy of electrons at the K-level of the molybdenum atom.
- Spectrum of rhodium. Rhodium has an atomic number of 45, while molybdenum has atomic number 42. Therefore, the characteristic X-ray emission of a rhodium anode will have a slightly higher energy than that of molybdenum and is more penetrating. This is used for imaging dense breasts.
Double-surface molybdenum-rhodium anodes allow the operator to select a distribution optimized for different breast sizes and densities.
Effect of KV on the spectrum
The value of KV greatly affects the characteristic radiation, since it will not be produced if KV is less than the energy of the K-level electrons. When KV exceeds this threshold, the amount of radiation is generally proportional to the difference between tube KV and threshold KV.
The energy spectrum of X-ray photons coming out of the instrument is determined by several factors. As a rule, it consists of bremsstrahlung and characteristic interaction quanta.
The relative composition of the spectrum depends on the anode material, KV and filter. In a tube with a tungsten anode, characteristic radiation is not produced at KV< 69.5 keV. At higher CV values used in diagnostic studies, characteristic radiation increases the total radiation by up to 25%. In molybdenum devices, it can make up a large part of the total generation.
Efficiency
Only a small part of the energy delivered by electrons is converted into radiation. The main part is absorbed and converted into heat. Radiation efficiency is defined as the proportion of the total radiated energy from the total electrical energy imparted to the anode. The factors that determine the efficiency of an X-ray tube are the applied voltage KV and the atomic number Z. An example relationship is as follows:
Efficiency=KV x Z x 10-6.
The relationship between efficiency and KV has a specific impact on the practical use of X-ray equipment. Due to the release of heat, the tubes have a certain limit on the amount of electric althe energy they can dissipate. This imposes a limitation on the power of the device. As KV increases, however, the amount of radiation produced per unit of heat increases significantly.
The dependence of the efficiency of X-ray generation on the composition of the anode is only of academic interest, since most devices use tungsten. An exception is molybdenum and rhodium used in mammography. The efficiency of these devices is much lower than tungsten due to their lower atomic number.
Efficiency
The efficiency of an X-ray tube is defined as the amount of exposure, in milliroentgens, delivered to a point in the center of the useful beam at a distance of 1 m from the focal spot for every 1 mAs of electrons passing through the device. Its value expresses the ability of the device to convert the energy of charged particles into x-rays. Allows you to determine the exposure of the patient and the image. Like efficiency, device efficiency depends on a number of factors, including KV, voltage waveform, anode material and surface damage, filter, and time of use.
KV control
KV effectively controls the X-ray tube output. It is generally assumed that the output is proportional to the square of KV. Doubling KV increases exposure by 4x.
Waveform
Waveform describes the way KV changes over time during generationradiation due to the cyclic nature of the power supply. Several different waveforms are used. The general principle is that the less the KV shape changes, the more efficiently X-rays are produced. Modern equipment uses generators with a relatively constant KV.
X-ray tubes: manufacturers
Oxford Instruments produces a variety of devices, including glass devices up to 250 W, 4-80 kV potential, focal spot up to 10 microns and a wide range of anode materials, including Ag, Au, Co, Cr, Cu, Fe, Mo, Pd, Rh, Ti, W.
Varian offers over 400 different types of medical and industrial x-ray tubes. Other well-known manufacturers are Dunlee, GE, Philips, Shimadzu, Siemens, Toshiba, IAE, Hangzhou Wandong, Kailong, etc.
X-ray tubes "Svetlana-Rentgen" are produced in Russia. In addition to traditional devices with a rotating and stationary anode, the company manufactures devices with a cold cathode controlled by the light flux. The advantages of the device are as follows:
- work in continuous and pulse modes;
- inertialessness;
- LED current intensity regulation;
- spectrum purity;
- possibility of obtaining x-rays of varying intensity.
