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Physics of X-ray production by Dr. Aryan
Wilhelm Conrad Röentgen was the first scientist to observe and record X-rays, first finding them on November 8, 1895. The name X-ray was given to indicate that it was unknown. Various modifications regarding the production and use of X-rays has been made since then
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Physics of X-ray production by Dr. Aryan
- 1. Production of X-rays Presentedby: Dr. Anish Dhakal Resident MD Radiodiagnosis, KUSMS 13th May, 2024
- 2. History of X-rays Wilhelm Conrad Röentgen was the first scientist to observe and record X-rays, first finding them on November 8, 1895. Nobel Prize in 1901 The name X-ray was given to indicate that it was unknown. Various modifications regarding the production and use of X-rays has been made since then
- 4. What are X-rays? X-rays are member of a group of radiations known as electromagnetic radiations which can act either as a wave or a particle depending upon circumstances. X-rays have the following characteristics: 1. They have very short wavelength (0.1 to 1 Angstrom) 2. They travel at the speed of light 3. They can cause ionization 4. They upon falling in certain materials visible light will be emitted called fluorescence 5. They produce a magnetic field at right angle to the electric field along path of propagation.
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- 7. Glass enclosure Itis necessary to seal the two electrodes of the x-ray tube in a vacuum The purpose of the vacuum in the modern x-ray tube is to allow the number and speed of the acceler ated electrons to be controlled independently Hence the quality and quantity of the x-rays produced are improvised.
- 8. The connectingwires must be sealed into the glass wall of the x-ray tube. During op eration of the x-ray tube, both the glass and the connecting wires are heated to high temperatures. Because of differences in their coefficients of expansion, most metals expand more than glass when heated. This difference in expansion would cause the glass-metal seal to break and would destroy the vacuum in the tube if special precau tions were not taken. Because of this prob lem, special alloys having approximately the same coefficients of linear expansion as Pyrex glass, are generally used in X-ray tubes Aging tubes acquire a bronze colored sunburn.
- 9. Cathode Tungsten filament (Reasonsto use it) A pure tungsten filament must be heated to a temperature of at least 2200° C to emit a useful number of electrons (thermions). Tungsten is not as efficient an emitting ma terial as other materials. Can be drawn into a thin wire that is quite strong, has a high melting point (3370° C), and has little tendency to vaporize; thus, such a filament has a rea sonably long life expectancy.
- 10. Thermionic emission:Emission of electrons resulting from the absorption of thermal energy Edison effect: The electron cloud surrounding the filament, produced by thermionic emission Space charge: Electrons emitted from the tungsten filament form a small cloud in the immediate vicinity of the filament. This collection of negatively charged electrons forms what is called the space charge. The tendency of the space charge to limit the emission of more elec trons from the filament is called the space charge effect
- 11. Cathode Vaporization ofthe tungsten filament when it is heated acts to shorten the life of an x-ray tube, because the filament will break if it becomes too thin. The filament should therefore never be heated for longer periods than necessary. Many modern x-ray circuits con tain an automatic filament-boosting circuit. When the X-ray circuit is turned on, but no exposure is being made, a "standby" cur rent heats the filament to a value corre sponding to low current, commonly about 5 mA. This amount of filament heating is all that is required for fluoroscopy. Automatic filament boosting circuit prevent vaporization and hence increases life of the tubes.
- 12. Cathode When electronsleave the fil ament the loss of negative charges causes the filament to acquire a positive charge The filament then attracts some emitted electrons back to itself. When a filament is heated to its emission temperature, a state of equilibrium is quickly reached. In equi librium the number of electrons returning to the filament is equal to the number of electrons being emitted As a result, the number of electrons in the space charge remains constant, with the actual number depending on filament temperature Electron current across an x-ray tube is in one direction only (always cathode to anode)
- 13. Focusing cup Becauseof the forces of mutual repulsion and the large number of electrons, this electron stream would tend to spread itself out and result in bombard ment of an unacceptably large area on the anode of the x-ray tube This is prevented by a structure called the cathode focusing cup, which surrounds the filament. When the x-ray tube is con ducting, the focusing cup is maintained at the same negative potential as the filament. The focusing cup is designed so that its electrical forces cause the electron stream to converge onto the target anode in the required size and shape. The focusing cup is usually made of nickel
- 15. Anode Stationary anode The anode of a sta tionary anode x-ray tube consists of a small plate of tungsten, 2- or 3-mm thick, that is embedded in a large mass of copper. The tungsten plate is square or rectangular in shape, with each dimension usually being greater than 1 cm. The anode angle is usu ally 15 to 20°. Tungsten is chosen as the target material for several reasons. It has a high atomic number (74), which makes it more efficient for the production of x rays. In addition, because of its high melting point, it is able to withstand the high temperature pro duced.
- 16. The rathersmall tungsten target must be bonded to the much larger copper por tion of the anode to facilitate heat dissipation Despite its good thermal char acteristics, tungsten cannot withstand the heat of repeated exposures. Copper is a better conductor of heat than tungsten, so the massive copper anode acts to increase the total thermal capacity of the anode and to speed its rate of cooling. The actual size of the tungsten target is considerably larger than the area bom barded by the electron stream. This is necessary because of the relatively low melting point of copper (1070° C). A single x-ray exposure may raise the tem perature of the bombarded area of the tungsten target by 1000° C or more
- 17. Parts of StationaryAnode X- ray Tube 1. Glass Enclosure (Pyrex Glass), oil and lead housing 2. Cathode – Has three parts a. Filament: Tungsten b. Connecting wires c. Metallic Focusing cup 3. Stationary Anode – Tungsten with Copper
- 18. The focalspot is the area of the tungsten target (anode) that is bombarded by elec trons from the cathode. Most of the energy of the electrons is converted into heat, with less than 1% being converted into X-rays Because the heat is uniformly distrib uted over the focal spot, a large focal spot allows the accumulation of larger amounts of heat before damage to the tungsten tar get occurs. The Problem of Heat dissipation and Spatial resolution
- 19. Before therotating anode was developed, another design was incorporated into x-ray tube targets to allow a large area for heating while maintaining a small focal spot. This design is known as the line-focus principle By angling the target, we can make the effective area of the target much smaller than the actual area of electron interaction
- 20. A lesser anodeangle will reduce the size of the effective focal spot and while a larger anode angle will increase the size of the effective focal spot.
- 21. Concept: A large focalspot is useful to protect the tungsten target as the heat accumulates and dissipates within the area of focal spot A small focal spot is required to achieve good radiographic image quality.
- 23. Anode angle Anodeangle is the angle made by the focal spot with the line perpendicular to the electron beam from the cathode. The electron stream bom bards the target, the surface of which is inclined so that it forms an angle with the plane perpendicular to the incident beam. The anode angle differs according to in dividual tube design and may vary from 6 to 20°. As the angle of the anode is made smaller, the apparent focal spot also becomes smaller. But, decreasing anode angle will increase the anode heel effect
- 24. Anode Heel Effect The intensity of the x-ray beam that leaves the x-ray tube is not uniform throughout all portions of the beam. The intensity of the beam depends on the angle at which the x rays are emitted from the focal spot. This variation is termed the “ anode heel effect." The intensity of the beam toward the anode side of the tube is less than that which angles toward the cathode. The decreased intensity of the xray beam that is emitted more nearly parallel to the surface of the angled target is caused by the absorption of some of the xray photons by the target itself. At a 40-in. target-film distance, the anode end of the film will receive a relative ex posure of 73% and the cathode end will receive a relative exposure of 105%. Thus, there is about a 30% difference in the in tensity of exposure between the two ends of the film.
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- 26. Clinical Significance ofHeel Effect 1. The intensity of film exposure on the anode side of the X-ray tube is significantly less than that on the cathode side of the tube - The thicker parts should be placed toward the cathode (filament) side of the x-ray tube. 2. The heel effect is less noticeable when larger focus film distances are used. 3. The heel effect will be less for smaller films
- 27. For general diagnostic radiographydone at a 40-in. focus-film distance, the anode angle is usually no smaller than 15°. Focal spot size is expressed in terms of the apparent or projected focal spot; sizes of 0.3, 0.6, 1.0, and 1.2 mm are commonly employed.
- 28. Parts of RotatingAnode X-ray Tube 1. Glass Enclosure (Pyrex Glass), oil and housing (lead) 2. Cathode – has three parts a. Filament - Tungsten b. Connecting wires c. Metallic Focusing cup 3. Rotating Anode – Tungsten (90%) and Rhenium (10%) 4. Anode Stem – Molybdenum 5. Induction Motor – Stator and Rotor
- 29. Rotating Anode Theanode of a rotating anode tube con sists of a large disc of tungsten, or an alloy of tungsten, which theoretically rotates at a speed of about 3600 revolutions per min ute when an exposure is being made. The tungsten disc has a beveled edge. The angle of the bevel may vary from 6 to 20°. The bevel is used to take advantage of the line focus principle. The purpose of the rotating anode is to spread the heat produced during an exposure over a large area of the anode. The diameter of the tungsten disc de termines the total length of the target track, and obviously affects the maximum per missible loading of the anode. Typical disc diameters measure 75, 100, or 125 mm.
- 30. Anode It has beenfound that an alloy of about 90% tungsten and 10% rhenium (a heavy metal with good thermal ca pacity) produces an anode that is more re sistant to surface roughening and has a higher thermal capacity than an anode of pure tungsten. Three modifications of the tube help to overcome the problem associated with this increased velocity – The length of the anode stem is made as short as possible to decrease the inertia of the anode The anode assembly rotates on two sets of bearings, which are placed as far apart as pos sible The inertia of the anode is reduced by decreasing the weight of the anode itself
- 31. Anode Heat gen eratedin a solid tungsten disc is dissipated by radiating through the vacuum to the wall of the tube, and then into the sur rounding oil and tube housing. (heat is dissipated in a stationary anode by absorption and conductivity is provided by the massive copper anode). In the rotating anode tube, absorption of heat by the anode assembly is undesirable because heat absorbed by the bearings of the anode as sembly would cause them to expand and bind.
- 32. Because ofthis problem the stem, which connects the tungsten tar get to the remainder of the anode assem bly, is made of molybdenum. Molybdenum has a high melting point (2600° C) and is a poor heat conductor. Thus, the molyb denum stem provides a partial heat barrier between the tungsten disc and the bearings of the anode assembly.
- 35. Housing of theX-ray Tubes The tube housing is lined with lead and serves to absorb primary and secondary X-rays that would otherwise produce high intensity of radiation around the tube, resulting in the needless exposure of the patients and personnel, as well as excessive film fogging. Inside the housing mineral oil is used to prevent short circuiting between the grounding wires and the tube. The leakage radiation measured at a distance of 1 meter from the source shall not exceed 100 mR (milliroentgens) in an hour when the tube is operated at its max imum continuous rated current for the maximum rated tube potential.
- 36. Control of theX-ray Tubes Conventional X-rays tube had two electrodes, cathode and anode. A third electrode has been used now a days in a special setting of X-ray tube called the Grid Controlled X-ray Tube. A grid controlled X-ray tube is simply the modification of the focusing cup used in the cathode of X-ray tube with a more negative voltage relative to the filament. It functions to cut off the tube current when required and hence control the X-ray tube
- 37. Metallic/Ceramic X-ray tubes This tube has a metal casing instead of the usual glass envelope, and has three ceramic insulators. Two insulators provide insulation for the two (positive and nega tive) high-voltage cables, and one supports the anode stem. The anode ro tates on an axle which has bearings at each end to provide greater stability and reduce stress on the shaft. This additional support allows use of a more massive anode, with anode weights of 2000 g possible. Anodes in conventional x-ray tubes are generally limited to no more than 700 g. Ceramic insulators are used to insulate the high voltage parts of the x- ray tube from the metal tube envelope. Aluminum oxide is a commonly used ceramic insula tor. Using metal as the x-ray tube enclosure (instead of glass) offers several advantages, the three most important being: 1. Less off-focus radiation 2. Longer tube life with high tube cur rents 3. Higher tube loading
- 38. Off-focus radiation Emissionof x-ray photons which originate outside of the anode focal spot Off-focus radia tion is produced by an x-ray tube when high-speed electrons interact with metal surfaces other than the focal track of the anode (usually other parts of the anode). The main source of off-focus electrons is electron backscatter from the anode. These scattered electrons may then strike the anode a second time and produce x-rays from areas other than the anode focal track. Off-focus radiation may be partly con trolled by placing the collimator, or a lead diaphragm, as close to the x- ray tube as possible A small port close to the anode will stop more of the widely spread offfocus beam than will a similar sized port placed farther from the anode.
- 39. Off-focus radiation Themetal enclosure decreases off-focus radiation by attracting off-focus electrons to the grounded metal wall of the x-ray tube Since the metal enclosure is at zero potential (i.e. it is grounded), the enclo sure is relatively positive as compared to the electrons, which are at a negative po tential. Electrons striking the metal wall may produce x- rays, but the low atomic number metal will produce few and low-energy x-rays.
- 40. Metallic/Ceramic x-ray tubes Longerhalf life Deposition of tungsten (from the anode) on the glass wall of x-ray tubes eventually builds up enough to act as an electrode and cause arcing between the glass and fila ment. Tungsten deposition is of greatest concern when high tube currents (high ma) are used. A metal enclosed x-ray tube has its metal enclosure connected to ground, and deposition of tungsten will not alter this grounding. For this reason, the useful life of a metal-enclosed x-ray tube will be greater than that of a glass tube, especially when used for high tube cur rents, such as in angiography.
- 41. High tube loading Themore massive metal anode of this tube allows significantly higher tube currents (more mA) to be used because of the larger heat storage ca pacity of the anode. This allows a higher mA setting for single exposures In addition, there is increased capacity for serial exposures because of better cooling that results from more efficient transfer of heat to the oil through the metal enclosure
- 42. References: Christensen’s Physicsof Diagnostic Radiology, 4th edition
- 43.
Editor's Notes
- #2 Fluoroscopic Imaging was used during the 1st production of the X-rays.
- #4 Mechanical and electromagnetic waves with long wavelengths contain less energy than waves with short wavelengths.
- #7 Pyrex. / (ˈpaɪrɛks) / noun. trademark. any of a variety of borosilicate glasses that have low coefficients of expansion, making them suitable for heat-resistant glassware used in cookery and chemical apparatus. Pyrex, (trademark), a type of glass and glassware that is resistant to heat, chemicals, and electricity.
- #9 Ductile (of a metal) able to be drawn out into a thin wire. able to be deformed without losing toughness; pliable, not brittle.
- #24 Anode heel effect refers to the lower field intensity towards the anode in comparison to the cathode due to lower x-ray emissions from the target material at angles perpendicular to the electron beam. Basic concept The conversion of the electron beam into x-rays doesn’t simply occur at the surface of the target material but deep within it. Because x-rays are produced deep in the target material they must traverse back out of it before they can proceed to the target field. More target material needs to be traversed at emission angles that are perpendicular to the electron beam (closer to the anode) than at those more parallel to it (closer to the cathode). This increase in material leads to more resorption of the x-rays by the target material resulting in fewer x-rays reaching the field at angles perpendicular to the electron beam. It also means that the x-rays emitted to angles closer to the incident beam travel through less target material and fewer are resorbed. The end result is that the field intensity towards the cathode is more than that towards the anode. Factors anode angle: by increasing the angle, the amount of target material perpendicular to the anode is decreased resulting in less resorption of x-rays produced. target-to-film distance: increase in distance reduces heel effect by allowing more divergence of the beam which produces a more uniform image. field size: the field will be more uniform at the centre (i.e. smaller field size) due to the collimator absorbing the peripheral variations. positioning: by aligning higher attenuating material towards the cathode and lower attenuating material towards the anode the resulting field is more uniform
- #29 Stationary Vs Rotating Anode?
- #30 Good thermal capacity and more resistant to roughening.
- #31 Wall of the tube – Oil - Housing
- #35 Use of Oil – Prevent Short Circuiting between the grounding wires and the tube.
- #37 Aluminum Oxide – Ceramic insulator.
- #40 Tube arcing occurs when there is a short-circuit within the tube, typically from the cathode to the tube envelope. The result is a temporary loss of x-ray output and a localised artifact. A number of causes of tube arcing are recognised 1: insulator surface flashover insulator breakdown vacuum flashover most common due to particulate impurities or gas within the tube new tubes are more prone to this problem due to residual gas A small amount of tube arching is not uncommon and modern scanners have automated processes to remove the artifact from the final images 1,2.