|
 |
 |
 |
 |
|
|
|
|
|
|
The hot cathode tube commonly in use today differs markedly from the cathode ray tube (Hittorf-Crookers’ tube) with which Rontgen was working at the time of his most important discovery of the x-ray. The earlier tube consisted of a partially evacuated glass envelope that contained two electrodes maintained at a high difference of potential. The electrons required for the production of x-rays were generated through the action of positive ions that resulted from partial ionization of the rarefied gas striking the metallic cathode with sufficient energy to cause release of electrons. The free electrons were then attracted and accelerated by the electrical field in the tube and made to strike the tube wall opposite the cathode. The deceleration of the electrons as they struck the glass wall produced x-rays. Thus, the first x-ray tubes produced x-rays through the ionic bombardment of the cathode producing a cathode ray that struck the glass wall of the tube producing x-rays.
It was not possible to control the number of electrons flowing across the gap within the tube and the number of electrons produced was relatively small. The degree of evacuation of gas from the tube varied. Therefore, a low intensity of x-radiation was produced by the cathode ray tube and it was impossible to closely control the amount or quality of the radiation produced. Because the area of interaction between electrons and glass tube wall was relatively large, it was not possible to effectively direct the x-ray beam and a radiograph of poor detail resulted.
The quality of the x-ray tube was improved by placing a piece of metal within the glass envelope and focusing the electron beam through use of a concave surface of the cathode. Early in the development of x-ray tubes, the importance of the type of metal that was to be used as a target for the electron beam was recognized. Tungsten placed within a copper block utilized the desirable qualities of the tungsten in production of x-rays and utilized the copper to quickly dissipate the heat produced by the interaction of the cathode beam on the target.
Gas tubes continued to be used even though the production of the x-ray beam was unreliable and the in intensity of x-ray production was low. Reliability of production was partially corrected by the development of an x-ray tube with a device that controlled the gas pressure within the tube.
Important changes in tube construction were introduced by W. D. Coolidge in 1913 when he developed a ductile tungsten that led to the production of the hot filament tube by The General Electric Company. This phenomenon, called the Edison Effect, results from subjecting electrons to sufficient heat so that thermal agitation
overcomes the binding energy of the electrons and allows them to escape the atoms to which they were bound. Because of the production of electrons through the heating of a filament, it no longer was necessary to have gas within the tube. Thus, the Coolidge tube could be completely evacuated, controlling the number of electrons available to strike the target and reducing the possible interactions with gas molecules. The number of x-rays produced could then be controlled. This type of x-ray tube is used in most x-ray machines today (Table 3-1).
The purpose of the x-ray tube today is the production of a controlled x-ray beam. The tube must be sensitive to controls placed on it by the x-ray machine so that both the number and penetrating power of the radiation produced can be accurately controlled.
'The x-ray tube is composed of parts that are better understood if consideration is given to what is needed for the production of x-rays. This includes:
The purpose of the cathode is:
The cathode is a part of both the high and low tension circuits of the x-ray machine. The cathode is sometimes referred to as a "filament" since this accurately describes in part its physical appearance. Tungsten is used in construction of the cathode filament
|
|
Gas filled |
Evacuated |
Inconsistent source of electrons |
Controlled source of electrons |
Entire wall of glass tube used for target |
Utilize a small metal target |
No device for electron focusing |
Focusing device used to direct electrons |
Cathode shape not critical |
Cathode assumes filament form |
Heat production not critical |
Anode heat production significant |
because of its high melting point (3370° C) and a vapor pressure that prevents the wire from vaporizing at the high temperatures imposed. The composition of most filaments today has been changed to a tungsten-rhenium alloy. The wire forming the filament is small, only about 0.2 mm in diameter. One or two filaments are found in modern x-ray tubes (Fig. 3-1).
The filament is heated by the low-tension circuit of the x-ray machine. The number of electrons made available is determined by the temperature of the filament. When a metal is heated, electrons are held less tightly by the nucleus of the atoms of the metal, and when the energy level exceeds the binding energy, a cloud of
electrons is then available to travel to the anode when a difference in potential (voltage) is expressed across the tube. The process by which the electrons are liberated from the filament is referred to as thermionic emission.
The filament sits in a separate metallic focusing cup usually made of molybdenum (Fig. 3-2). Because of the tendency for particles of equal charge to repel each other, it would be possible for the electron beam to spread as it crossed the tube and bombard an undesirably large target area on the anode. By maintaining the focusing cup at the same negative potential as is the heated filament, the repulsive charge from the focusing cup tends to direct the electrons toward a relatively small area on the target.
Filament serves as a source of electrons and is positioned within the focusing cup which directs the flow of the electrons toward the anode.
 |
The construction of the anode may vary greatly and represents one of the major differences that exists between types of x-ray tubes and the equipment in which they are placed. Basic construction of the anode consists of a target material placed on the surface of a larger cylinder or disc. It is estimated that more than 99% of the energy in the electron beam is converted to heat energy at the time of its interaction with the target . The ability of the target material to withstand high temperatures and the speed with which heat can be dissipated by the anode are therefore of great importance. If the atomic number of the target material is sufficiently high, it will favorably influence the efficiency of production of x-rays. Because tungsten satisfies these requirements, it has long served as a target material of choice.Tungsten has
The rhenium-tungsten target has replaced the solid tungsten target because of improved ability to dissipate heat. The physical form of the tungsten target and the manner in which heat is dissipated differ greatly in the two types of anode construction (Table 3-2).
Molybdenum is also used as a target material. Since it is a low atomic number, Bremsstramung production is less efficient and therefore characteristic radiation becomes more important. Molybdenum anode tubes take advantage of this factor and when used in the range of 40 kVp they produce a high percentage of characteristic radiation between 17.9 and 19.5 keV which is highly desirable for mammography.
There are two basic types of x-ray tubes dependent on the character of the anode. The difference in construction determines the maximum level of production of X-rays. Tubes with stationary anodes are found in dental units or small portable units. Tubes with rotating anodes are used in equipment of larger capacity.
Stationary Anode (Fig. 3-3) The anode consists of a small plate of tungsten embedded in the end of a copper cylinder. Despite the high melting point of tungsten, a copper cylinder is still needed to dissipate the great amount of heat generated during an exposure. Copper is a better conductor of heat and has a relatively high melt ing point (1070° C). The tungsten block sits on the end of the copper cylinder with the surface of the tungsten plate at a predetermined angle. This is usually from 15 to 22.5 degrees.
The size of the tungsten plate exceeds the size of the electron beam.
 |
This is necessary to avoid the electron beam striking the surface of the cylinder and causing melting of the copper, since it has a relatively lower melting point. One of the major technical achievements that made production of high heat load x-ray tubes was the discovery of a type of bonding that would hold the tungsten plate to the copper cylinder during the production of temperatures in excess of 1000°C, recognizing the different coefficient of expansion between the two metals.
Rotating Anode (Fig. 3-4) A limiting factor relative to heat production was soon detected in use of the stationary anode. Generators were quickly developed that could produce x-ray beams whose interaction with the target far exceeded the temperature limits placed by the stationary anode.
Rotation of the anode at high speeds was not accomplished without overcoming many difficulties, since rotation of the anode at high speeds requires excellent bearing surfaces. Because of the heat generated during exposures, the bearings must operate over a wide
range of temperatures. In the stationary anode it was possible to dissipate through the anode a large part of the heat produced. Because of the low tolerance of the bearings to heat, they must be
 |
 |
protected in the rotating anode from high heat. The molybdenum stem provides a barrier to heat transfer between the anode and the bearings of the anode assembly. Molybdenum has a high melting point (2600°C) and is a poor conductor of heat. Another problem to overcome was discovery of a satisfactory method of lubrication of vacuum-enclosed bearing surfaces.
By placing target material on the surface of a rotating disc, size of the target surface was greatly increased (Fig. 3-5). The value of a rotating anode is the distribution of heat produced at the time of interaction of the electron beam with the target over a much larger amount of metal. Since heat produced is the limiting factor in the size of the exposure that could be obtained through use of a
stationary anode, development of a rotating anode opened the field of radiology to much higher exposures produced in much shorter exposure times. The diameter of the anode disc varies and obviously determines the total length of the tungsten target track. Diameters found in typical tubes are 75-100 mm (3 to 5 inches).
The useful life of a rotating anode tube is often determined by the life of the anode bearings. The speed of rotation of the anode varies with a speed of 3,000 to 3,600 revolutions per minute being common. High speed rotating anodes have been developed that achieve speeds of 8,000 to 10,500 rpm. The switch used in making an exposure with a rotating anode tube usually has two stages. The first stage brings the anode to full speed of rotation and the second stage triggers the flow of electrons across the tube and the
resulting production of x-rays. Some times a foot switch and some hand switches have only one stage and it must be remembered that following depression of such a switch, there is a slight delay before exposure while the anode reaches full speed. The "whir ring" noise of the anode may be loud enough to disturb some animals. This is a condition that should be evaluated before the purchase of an x-ray unit.
The angle of the surface of the anode disc can vary as it does in stationary anode tubes. The angle can be decreased to as low as 10° with a range of 17° to 10°. It is also possible to have the anode disc with two target foci placed in tandem on separate tracks beveled at different angles (biangled tube). Newer anode discs are constructed as a bimetal unit of tungsten and molybdenum that are sintered together. The surface directed toward the cathode
__________________________________________________________
__________________________________________________________
is of tungsten as in conventional rotating anode tubes, while the rest is made of molybdenum. Because of the weight of molybdenum is only half that of tungsten, many of the problems of high speed rotation of the heavy anode disc are thus eliminated.
The rotating anode disc plus the anode stem are a part of the anode that rotates and are often referred to as a "rotor". This is actually a part of the electromagnetic induction motor that creates anode rotation and is formed of bars of copper and soft iron. The other portion of the electrical motor consists of the stationary windings positioned outside the glass envelope and is referred to as the "stator" or that part that does not rotate. It is formed of a series of electromagnets equally spaced around the neck of the x-ray tube (Table 3-3).
The area of the surface of the target that is bombarded by electrons during an exposure is called the focal spot(Fig 3-6). The size of the focal spot is affected by the size of the filament which is determined at the time of tube construction. When viewed perpendicularly to the surface of the target, the size of the focal spot is referred to as the "projected focal spot" or "effective focal spot". The actual focal spot tends to be rectangular in shape while the projected focal spot is more nearly square in shape. In addition to a different shape of the projected versus the actual focal spot, there is also a most important difference in area.
 |
 |
The focal spot on the target surface is that area that is bombarded by the electrons in the cathode beam. The size of the electron beam is determined by:
Since heat is distributed nearly equal throughout the focal spot, a larger focal spot can accommodate the amount of energy released at the time of interaction of the electron beam with the target,thus offering greater protection to the target. However, the size of the focal spot is important in determining the resulting detail, or shadow sharpness, on the radiograph. The smaller the focal spot size, the sharper is the image on the radiograph. By utilizing the angle of the target surface, it is possible to influence the apparent size of the focal spot. This principle is referred to as the line-focus principle and is important in increasing the quality of the radiograph (Fig 3-7)
At a 200 target angle, the area of the effective focal spot is only 1/3 as large as the actual focal spot size. Appreciate that 00 target angle would provide the smallest possible effective focal spot size. However, the film coverage by the x-ray beam would be limited because x-rays cannot penetrate through the anode disc and the beam is cut-off or limited in size by the anode surface (Fig. 3-8). This is referred to as "hell cut-off", and will be discussed in greater detail.
In a stationary anode tube, the focal spot must be larger to accommodate the heat produced. Common focal spot sizes are 1.5, 2.9, 3.2,4.2, and 4.5 cm in size. The advertised size is that of the effective local spot and is usually printed on the tube housing. A rotating anode tube often has a smaller projected focal spot size than a stationary anode tube and usually incorporates two filaments with two different focal spotsizes available in rotating anode tubes. Some of these are 0.3 and 1.0,0.3 and 1.2, 0.3 and 1.5, 0.3 and 2.0, 0.6 and 1.6, 0.6 and 2.0, 0.8 and 1.8, 1.0 and 2.0, 1.2 and 2.0, 1.5 and 1.5, 2.0 and 2.0 mm. Tubes with 1.0 and 2.0 mm focal spots are in common usage. The smaller focal spot is used for lower heat
 |
producing exposures and takes advantage of the use of the smaller focal spot size to produce a radiograph of greater detail. The larger focal spot is needed in larger heat producing exposures required to penetrate a larger animal or to make repeated exposures in a shorter time interval. It is possible that both filaments produce electron beams that bombard the same target track or it may be that the anode is constructed to have two target tracks. In which case, one track has a smaller radius and is closer to the center of the anode disc. The biangled tubes are constructed to have two target paths that allow for the different target angles. This combination of two focal spot sizes and different target angles gives the greatest opportunity for the widest range of possibilities of different effective focal spot sizes within a given x-ray tube (Table 3-4).
The National Electrical Manufacturers ‘ Association standard (NEMA standard XR5-1974) stipulates acceptable tolerances in x-ray focal spots and specifies how they are to be measured. Focal spots cannot be smaller than the nominal size, but for focal spots of less than 0.8 millimeters, the tolerance is plus 50 percent; of 0.8 through 1.5 mm, plus 40 percent; and of greater than 1.5 mm, plus 30 percent. Hence, a nominal 0.6 mm focal spot may actually measure 0.9 mm and still be within acceptable standards.
------------------------------------------------------------------
------------------------------------------------------------------
 |
The apparent size of the focal spot on an x-ray tube may be measured with a pinhole x-ray camera (Fig. 3-9). A hole with a diameter of a few hundredths of a millimeter is drilled in a plate opaque to x-rays. The plate is positioned between the x-ray tube and an x-ray film. An exposure is made and the resulting image is measured on the exposed film. Any measurement of the focal spot is equal to the corresponding measurement in the exposed image times the ratio of the distance between the tube and the opaque plate divided by the distance between the opaque plate and the x-ray film.
There is presently a move to abandon this long-established method of determining focal spot size through use of a pinhole camera and substituting a resolution chart method. If different methods are to be used to estimate focal spot size, it is important to use proper terms to report these findings. Effective focal spot size (Feff) is the size if the focal spots as seen from the image plane in the central x-ray beam. This is synonymous with pinhole size. Use of resolution charts to measure the resolving capacity of the focal spots takes into account the combined effects of both size and intensity distribution of the radiation. The result is therefore, an equivalent focal spot size (Feq) and not the effective focal spot size. (Milne, 1974).
After determination of the size of the effective focal spot size, it is possible to determine the actual physical width of the focal spot by knowing the anode angle. The Feff measurements are required to calculate the heat load and to determine magnification of small objects. The Feq values indicate resolving power and are of more value to the practicing radiologist.
Focal spots can also be imaged by means of a star test pattern. A star-test pattern can be viewed as an object to be radiographed at a specific distance from the focal spot and specific distance from the film. Comparison of these two distances determines the magnification factor as in the pinhole camera. The star test pattern can also be viewed as consisting of many line-pair test patterns with the diameter of the star corresponding to a line-pair of different size. The stars have alternating spokes of lead and plastic. Magnification of the radiograph of the star test pattern includes information on the energy distribution in the focal spot. This information may be more valuable to the radiologist because of the interest in the effect the focal spot has on the image rather than the image of the focal spot as would be determined by the pinhole camera. Increasing mA settings cause increase filament temperature and have been shown to affect the space charge around the filament and produce a doubling in size of the effective focal spot. (Spiegler and Breckinridge, 1972).
 |
In past discussions it has been assumed that the intensify of the x-ray beam was uniform throughout the field of radiation. This, intact, is not the case and a higher flux of x-rays is present in that part of the field toward the cathode end of the tube (Fig. 3-10).
Low-energy x-rays generated in a tungsten target are attenuated severely during their escape from the target. For targets mounted at a small angle, the attenuation is greater for x-rays emerging along the anode side of the beam than for those emerging along the anode side of the nearest the cathode. This inconsistency in beam intensity is referred to as the heel effect. Heel effect is noticeable in diagnostic radiography because of the easy absorption of low energy photons generated and because of the steep target angles used. The resulting radiograph of an object of equal density and thickness will be darker on that part of the radiograph near the cathode and lighter on that part of the radiograph near the anode.
The cathode end of the tube housing is usually identified by a "negative" sign. The level of x-rays reaches a maximum of 105% intensity at an angle of 120 toward the cathode, while only 75% intensity is present at an angle of 120 toward the anode. These figures assume an intensity of 100% at the central angle. This inequality intensity can be used to the advantage of the radiographer by positioning the heavier or more dense part of the patient on the end of the table under the cathode. The portion of the x-ray beam with the greatest intensity will then pass through the portion of the body with the greatest density.
There are three important aspects of the heel effect that must be considered in diagnostic radiography. One is that the intensity of the beam is stronger toward the cathode, so if the patient is unequal in thickness, the more dense or thicker portion of the body should be positioned toward the cathode. A second aspect is that the heel effect will be much less noticeable if the focal-film distance is large. Conversely, if the focal-film distance is shortened, the effect of the unequal intensity within the x-ray beam will be noticed greatly. The third aspect is that the heel effect will be noticed when exposing a large film while not as obvious when exposing a small film (Fig. 3-11).
An example of the beneficial use of this phenomenon is the examination of the abdomen of the dog. The cranial portion of the abdomen contains the liver and is, therefore, large and has a relatively higher tissue density than the caudal portion of the abdomen. If the head of the dog is positioned at the end of the table under the cathode, the higher intensity portion of the beam will be directed toward the more dense cranial abdomen.
 |
The radiographer should also appreciate that he may not completely ignore use of the heel effect. If a body part to be examined has unequal tissue thickness or equal density and the heel effect is not used to your advantage, it will consequently work to a disadvantage. Thinner, less dense portions to be radiographed will be in that part of the x-ray field with a higher intensity of radiation, thus magnifying differences in film density throughout the radiograph.
X-rays may also originate from another region in the tube, other than the focal spot. They are referred to as "off-focus" x-rays or stem radiation, implying that they have resulted from electrons interacting with tube parts outside the focal spot. They cause some loss of image resolution on the radiograph because they become one of the causes of radiation outside the primary beam that contributes to fogging of the radiograph. Off-focus radiation or extra-focal radiation has been measured and reported to be close to 30 percent of the exiting beam in the event of an improperly coned tube (Sideband, 1979).
One of the methods available to control off-focus radiations is to place an aperature diaphragm as close to the focal spot as is possible. The other method involves tube construction and requires the embedding of the target in a low Z material such as graphite so that the resulting off-focus x-rays produced are of low energy and are absorbed by the tube window (Mattsson, 1955: Wagner, et al., 1974).
Glass housing has the purpose of providing an envelope within which a vacuum can be maintained. The vacuum permits independent control of both the number of electrons that constitutes an electron beam and the speed of flow of the electrons. The vacuum eliminates the possibility of collisions between molecules of air and accelerated electrons. In addition, removal of air prevents deterioration of the filament by oxidation.
The tube housing provides:
Causes of Radiographic Tube Failure or Malfunction
The cost of operating the radiology portion of a practice has risen in the past few years. One way to reduce these expenses is to extend the useful life of the radiographic tube. Almost 95% of the damaged tubes returned to manufacturers have been damaged as a result of technical error (Thompson, 1983). These common errors lead to:
For the most part, these types of damaged are controllable, and their control could significantly extend the useful life of the x-ray tube.
Tube malfunction or failure can occur in a number of different ways. A discussion of each component part of the tube follows with a specific description of malfunction of that part. Where possible, recommendations are given that will insure longer tube life.
X-ray tubes cost from $2,500 to $19,000 to replace at today’s prices. Because of these high replacement costs, it behooves you to understand major causes of tube failure. Often a technologist never learns why a tube has failed or how the failure might have been prevented. Tubes can fail fail for many reasons, but the two most common are filament evaporation (deposit) and thermal overloading. Other causes that are partially related are
Cathode Failure One of the most common causes of tube failure is filament evaporation. That is a function of both time and temperature.
________________________________________________
_________________________________________________________
Therefore, the higher the temperature of the filament and the longer it stays at that temperature, the more chance there is for evaporation (Table 3-5).
OIder x-ray units place a maximum heat on the filament at all times the machine's "on-off" switch is at the "on" position. For this reason some care must be given to not leave the machine switches activated for prolonged periods of time. Larger capacity tubes can accept the high heat produced at the filament for only a short period of time.Therefore, newer machines use a "stand-by" current to preheat the filament at a low level when the switch is in the "on" position prior to making the exposure, the switch is moved to a pre-exposure position and the current in the low-tension circuit is markedly increased and the filament is brought to a high temperature that produces a cloud of electrons sufficient for the exposure required. This type of switch provides some protection for the filament. However, it is still important not to leave the switch in the position in which the filament is at maximum heat for any longer than is necessary. Consider the following potentially damaging situations.
A technician positions the patient, sets the technique and pushes the switch to the "on" position. Suddenly the technician recognizes that the selected radiographic technique is incorrect and decides to change the miniamperage setting. The switch is released to the "off" position. After the change in technique, the switch is again pressed to "on" and the exposure made. Another common problem occurs when a technician sets a technique, positions the patient, pushes the switch to "on" and notes that the animal has moved. The switch is released to the "off" position, the animal repositioned and the switch pushed to "on". Again the animal moves slightly and the exposure is further delayed. This procedure goes on for several minutes before the final exposure is made.
each time the switch is moved to the "Off" position, the filament is boosted to maximum temperature. This type of abuse to the filament causes about 33% of all tube failures. Consider a filament to have an operating life of 500 minutes at maximum temperature. If minimum boost times are assumed, each exposure might be made within 2 seconds. That would mean that a total of 15,000 exposures would be anticipated. However, if it was necessary to hold the switch at the boost temperature for an average of 10 seconds before each exposure could be made, the number of exposures would be cut to 3,000. Another aspect of the "boost and hold" method of exposures is the transfer of the heat generated to the tube housing where the thermal overload can cause failure of the anode dealing
