Brand X-ray Tube Co., Inc.- X-ray Tubes FAQ Page
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X-ray Tube FAQ Page
This page is meant as a general information page for people that would like some information on the x-ray tube and its components and the physics that apply to stationary anode x-ray tubes. It is intended to present these ideas in terms everyone can understand, while still being useful to professionals in the field. This may make this discussion overly simplistic for some of you and overly complex for others. We hope that you find this page useful. For links to other x-ray related information please visit our links page. If you have specific questions about the physics that apply to stationary anode x-ray tubes or questions of a more advanced nature please contact engineering.. Please direct questions, comments, or suggestions regarding this website to the webmaster.
X-ray Tube Components:
Cathode: The 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 (the anode).
Anode: The 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. 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. The electrons then impact a target (most commonly made of tungsten, but this target can also be molybdenum, palladium, silver or other material). This causes the release of the potential energy built up by the acceleration of the electrons comprising the electron beam. TMost 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.
Glass envelope: The above components are sealed into a glass envelope. This allows for gases and other impuritites 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.
Common X-ray Tube Terminology:
Focal Spot Size: Focal spot size essentially 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. However, the size of the focal spot that is possible 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. These topics are discussed below.
People often assume 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 for the reasons outlined below in the mAs section.
mAs: Milliampseconds. mA is an abbreviation for milliamps. Milliampseconds is a function of Amps applied (mA) and the amount of time the Amps are applied in seconds(S). mA+s= Milliampseconds. For example, 1 amp applied for 1/100 of a second is 1000 mA x 1/100 S or 10 MAS. This gives an idea of the amount of x-rays generated by a given tube over a given exposure time. It should be noted that 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. At higher mA levels focal spot size will necessarily be increased, sacrificing resolution, making this relationship a direct tradeoff between these two factors.
kVp: 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 By accelerating an electron through a 1000V potential field the electron has one kilo electron Volts (1 KeV of energy). If this electron gives up 100% of its energy upon striking the anode and this energy is converted into x-ray (not heat) the resulting x-ray would be a 1 KeV x-ray. This is why most stationary tubes are limited to a maximum kV operating voltage. Increasing the kV levels may cause an excessive amount of heat to be given off as the electron beam strikes the target, causing tube malfunction and component degradation. This is also why modern tubes for use 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. This rotation of the anode allows for operation at higher peak voltage (kVp) and milliamp (mA) ratings.
Duty Cycle: The 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. Also of importance is at what mA level and kV level the tube will be run at for a given exposure time. This critical information allows us to compute the energy (or load) being exerted on a given tube under a given set of operating characteristics. We can then use existing heat storage and dissipation rates for a given tube to determine whether or not a tube will function properly in a given application at the required mA level, kV level and exposure time necessary for proper performance.
A central issue in determining what x-ray tube will satisfy your requirements is determining the amount of coverage required. Coverage is the area covered by the x-ray beam when it gets to its intended target area. 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.
This coverage is computed with the formula:
TAN(degrees of target) * Distance between anode and target of the beam * 2.
For example, we have a 12 degree target angle and the distance between the anode and the surface to be x-rayed is 200 mm. This gives us:
TAN(12) * 200 * 2 = .212556562 * 200 * 2 = 85.02 mm of coverage.
If you are seeking to determine what minimum target angle that will satisfy your coverage requirement you would use the following formula:
TAN(X) * distance * 2 = Necessary Coverage
For example, for an application that you are developing, it is determined that you will need 1 inch of coverage at a distance of 4 inches. This gives us:
TAN(X) * 4 * 2 = 1
Isolating the variable gives us: TAN(X) = 0.125
The question now is: What is the angle whose tangent function is 0.125?
Using the inverse tangent function we get:
Plugging this back into our formula for coverage we get:
TAN(7.125) * 4 * 2 = .999997682 inches of coverage at 4 inches distance. This discrepancy is due to rounding error. In actuality, the optimal angle would in fact be 7.126. In this instance it would be highly recommended to round up to 8 degrees to assure proper beam coverage. It is necessary for practical production purposes to work in whole angles and it is always recommended to round up. A little bit of extra coverage will rarely be detrimental, as the scope of the beam can be reduced, if necessary, by using a radiation shield with the tube or by sizing the hole in the tube housing that the beam is projected through. Less than adequate coverage will give inadequate examination and be unworkable for a given application.
Please check with us to verify your necessary beam coverage or target angle requirements. We will be happy to verify or compute them for you.
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. 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 x-ray. Energy that is converted to heat is radiated through the anode, the remaining energy is given off as an x-ray. 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. 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 approximately 5,000 times 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. If a person would hold his or her hand to a conventional light source that person would notice some light passing through flesh of the hand. By exciting the electrons and increasing their energy by 5,000 times, an 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.