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X-ray tube is a source of high energy X-ray photons, which are used to expose patient or limb for medical imaging and diagnosis. X-ray tubes are produced with focuses of different sizes, sometimes with two or three different focuses in one enclosure. X-ray tubes are characterized by maximum energy (kVp), power capabilities with each focus and heat capacity for the anode.
1. Inside the X-ray tube, as with any electronic vacuum tube, there is a cathode, which emits electrons into the vacuum and an anode to collect the electrons, establishing a flow of electrical current (knows as the beam) through the x-ray tube.
2. A high voltage power source, 30 to 150 kilovolts (kV), is connected across the cathode and anode to accelerate the electrons. The X-ray spectrum depends on the anode material and the accelerating voltage.
3. The energy generated by the electron beam is a result of excitation of atoms, which free their electrons from orbit. These electrons are now free to become part of the electron beam.
4. This beam is then accelerated through a high voltage field, gaining speed and energy until the electrons strike the target, where this energy is converted into heat and given off as an x-ray.
5. This energy is approximately 0.1% - 2% of the total amount of energy produced by the electron beam. This x-ray is energy in the form of an electromagnetic wave.
6. The main difference between an x-ray photon and that of a visible light photon lies is the energy of each photon. An x-ray photon has much higher energy than the energy of an ordinary light photon. This allows the x-ray photon to pass more readily through materials than would a regular light photon.
7. By exciting the electrons and increasing their energy, x-ray passes more freely through flesh and other materials than would an ordinary light photon. This free passage through flesh and other materials is what allows x-ray to be such a useful diagnostic tool in medical and other imaging disciplines.
8. The x-ray tube envelope components are sealed into a glass or other material envelope. This allows for gases and other impurities to be pumped out of the tube, creating the vacuum necessary for proper performance. The x-ray creation process must occur in a vacuum, so as not to disrupt the electron beam, and also to allow for proper "filament" performance and durability.
9. The x-ray tube cathode acts to excite electrons to the point where they become free from their parent atom and are then able to become part of the electron beam. The cathode acts as a negative electrode and propels the free elections, in the form of an electron beam, towards the positive electrode.
10. The shape and the size of the focal spot of the x-ray tube depend on the "filament" (the active part of the cathode).Focal spot size is a measurement of the resolution that will be afforded by a particular x-ray tube. In general, the smaller the focal spot size, the better the resolution. This often leads to requests for the smallest focal spot size possible.
11. The size of the focal spot is contingent upon the mA level for the application, the kV for the application, duty cycle, necessary beam coverage and target angle of the tube.
12. It is often assumed that the smaller the focal spot size, the "better" the tube. While it is true that improved resolution is afforded by small focal spot sizes, we must keep in mind that by reducing focal spot size, it will be necessary to run at lower mA and/or kV levels relative to focal spot size.
13. The x-ray tube anode acts as a positive electrode, attracting the free electrons and accelerating the electrons through the electromagnetic field that exists between the anode and cathode.
14. This acts to increase the velocity of the electrons, building potential energy. The higher the kV rating, the greater the speed at which the electrons are propelled through the gap between the cathode and anode.
15. The electrons then impact a target (most commonly made of tungsten, but this target can also be molybdenum, palladium, silver or other material), causing the release of the potential energy built up by the acceleration of the electrons comprising the electron beam.
16. Most of this energy is converted to heat and is radiated by the copper portions of the anode. The remainder is refracted off of the target in the form of high energy photons, or x-rays, forming the x-ray beam.
17. The x-ray tube kVp (kilovolts peak) is a measurement of the energy applied to the electrons, which accelerates them through the high voltage field that exists between the cathode and the anode.
18. By accelerating an electron through a 1000V potential field, the electron has one kilo electron Volts (1 KeV of energy). Increasing the kV levels may cause an excessive amount of heat to be given off as the electron beam strikes the target, causing x-ray tube malfunction and component degradation.
19. This is also why modern x-ray tubes used in most CT machines have an anode which rotates during operation, as these applications require much higher levels of kV and mA to accomplish their required imaging operations. By rotating the anode, the heat generated by the electron beam is distributed, rather than focused on a stationary point on the anode.
20. Rotation of the anode allows for operation at higher peak voltage (kVp) and milliamp (mA) ratings.
21. The x-ray tube mAs (Milliampseconds) is a function of Amps applied (mA) and the amount of time the Amps are applied in seconds. This gives an idea of the amount of x-rays generated by a given x-ray tube over a given exposure time.
22. For tube selection, you should determine the minimum level of mA that will satisfy your requirements. The concern here is that at too high an mA with a small focal spot, the electron beam is focused on too small of an area to properly handle the amount of heat generated by the energy conversion process that occurs as the electrons strike the target. This will cause the target material to melt and/or crack, causing tube malfunction.
23. At higher mA levels, focal spot size will necessarily be increased, sacrificing resolution, making this relationship a direct tradeoff between these two factors.
24. The x-ray duty cycle is how long each exposure will be and how long will be given in between exposures for cooling. If a unit will be operated continuously, with no cooling intervals, this is termed continuous duty.
25. Also of importance is at what mA level and kV level the x-ray tube will be run at for a given exposure time. This critical information allows computation of the energy being exerted on a given x-ray tube under a given set of operating characteristics.
26. Existing heat storage and dissipation rates for a given x-ray tube can determine whether or not an x-ray tube will function properly in a given application at the required mA level, kV level and exposure time necessary for proper performance.
27. The x-ray beam coverage area is a central issue in determining what x-ray tube will satisfy your requirements in determining the amount of coverage required.
28. Coverage is contingent upon the angle of the anode of the x-ray tube and the distance between the anode and the intended target of the beam.
29. It is always preferable to use the original type of the x-ray tube, or a tube type recommended by the manufacturer of the equipment.
30. The shelf life of the x-ray tube can be limited to one year or so. Always check production date of the x-ray tube.