The cathode is the negative part of X-ray tube. It is a spiral-shaped filament made of tungsten and generally traces of thorium. The electrons are emitted from the cathode by a thermal emission mechanism: the cathode is brought to high temperature by an electric current. The electrons released by high temperature are subjected to potential difference and are precipitated on the anode with a high speed. The filament is mounted in a hollow piece called concentration, whose purpose is to focus the electrons on the anode.
On some devices, the cathode is composed of two filaments of different sizes. The large filament increases the electron flow and therefore the production of x-rays while the small filament can focus an electron beam than on a smaller area of the anode, thereby improving the sharpness of the image. Depending on the amount of electrons needed to produce X-rays and desired by the imperatives of sharpness of the image desired, the large or small filament may be selected.
The anode is the positive part of X-ray tube and corresponds to the target. This is where x-rays are produced when the electrons accelerated by the potential difference between two electrodes collide with the anode. Surface bombardment of electrons on the anode is called the home and its size is a determinant of the fineness of the image. X-rays are emitted in all directions from the home, but x-rays are partially stopped by the anode itself. The largest concentration of x-ray which is reflected in a direction perpendicular to the surface of the anode: this is called reflective anode. The anode surface is oblique to the direction of the electron beam so as to permit more x-rays can exit the tube.
The x-ray production is very inefficient since the yield of X-ray tubes in radiology is about 1%. A large amount of heat is generated along the x-ray, which poses significant technological problems that limit and in any event, the amount of x-rays produced. The anode is usually made of tungsten because tungsten has a high atomic number (Z = 74), which promotes performance, but also a high melting temperature (3410 degrees).
Overheating, which can cause melting of the home depends on the concentration of electrons on the anode and energy. The x-ray flux produced in the tube depends on household size (which itself depends on the size of the electron beam and therefore the size of the cathode), the electron flow (measured in mA) and the electron energy (measured in kV). The overheating of the anode limits the electrical power (kW) used to produce x-rays.
When a large amount of x-ray is necessary, the technology of the rotating anode spreads the heat over a larger area while maintaining the same size home. The anode is then composed of a disc mounted on an axis of rotation. The speed of rotation is usually 3 to 9000 revolutions per minute.
The turning anode
The advantages of the rotating anode is an increase in the amount of x-ray products, and thus may increase the power of the generator. The devices are equipped with rotating anode power than 15-20 kW. The other major advantage and can reduce the size of homes to low power, thus improving the sharpness of the image. When taking of the radiograph, a time of initiation of the anode must be respected, and the outbreak is always done in 2 stages.
X-ray tube is surrounded by several protective enclosures to ensure electrical protection, thermal and mechanical tube while ensuring user protection against radiation leaks.
X-ray tube is surrounded by a glass bulb whose function is to provide electrical insulation, to evacuate the heat generated and to ensure a vacuum as perfect as possible. In the absence of voids electrical phenomena occurring parasites unacceptable. The bulb is usually made of glass or a combination of glass and metal or ceramic and metal. The bulb is immersed in oil, which contributes to the cooling system. The whole is enclosed in a metal sheath, ensure the removal of heat, mechanical protection of the tube, and an X-ray absorption effects.
The protective casing lets out an x-ray output window. X-ray the less energetic, which will not contribute to the formation of the image, but which may have biological effects are eliminated by a filter of aluminum there is talk of beam hardening, because the average energy of the beam x-ray increases after filtration. The size of x-ray beam is then adjusted by the use of diaphragms. A light beam is used to simulate the position of x-ray beam before taking the radiograph.
The high voltage generator has the task of producing a current in the same direction between the cathode and the anode (the return of electrons on the cathode would be catastrophic), as constant as possible and voltage adjustable between 50 and 120 kV. A conventional generator is composed of a rectifier and transformer. The rectifier diodes usually consist of meetings so that the current flows always in the same direction. The processor provides the processing of a current of 220 or 380 volts with a current of high voltage from 50 to 120 kVolt. The generator is a centerpiece of the X-ray machine as it affects its operation, performance and reliability.
Most generators fitted to the X-ray equipment are Veterinarians who single-phase generators operating from a single phase 220 or 380 Volts. The current is converted to the voltage desired by the operator through a transformer. The current is rectified by reversing the flow direction of the negative part of the cycle through a diode bridge. Single-phase generators are old technology and suffer from a lack of reliability and low efficiency in the production of x-ray Indeed, the voltage varies widely during the exposure, from 0 to maximum voltage, resulting in a discontinuous production of x-ray Moreover, these generators are heavy and bulky.
The three-phase generators operating on the same principle as the single-phase generators, but with a three-phase current, which can severely limit the voltage fluctuations during the exhibition. These generators perform better and are more reliable.
However, they are rarely used in veterinary medicine because they are expensive and require special wiring.
The high frequency generators are the newest high-voltage generators used in radiology. They represent an important technological advance. The 60 Hz current is converted into a current of high frequency (20-300 kHz) and used to charge a capacitor. Developed tension this time varies depending on the current frequency, which allows fine tuning and more reliable output voltage. These generators have a better yield, greater reliability, and are less bulky than the others. They tend to displace the last few years the other generators.