The X-ray Tube



The X-ray Tube





Introduction


Chapter 1 provided a general discussion of the x-ray tube head assembly and the function of the major parts of the design. Chapter 4 discussed the components of the x-ray circuit and the events that lead to the production of x-rays in the x-ray tube. This chapter examines the x-ray tube itself (Figure 5-1), its general construction, and how it works. The sole purpose for manipulating electricity in an x-ray circuit is to create the environment in the x-ray tube necessary for x-ray production. The need for the radiographer to understand the x-ray tube is twofold. First, as was described in Chapter 4, the radiographer must have a basic understanding of how the tube works to competently and safely formulate exposure techniques and minimize patient radiation dose. Second, such an understanding is critical to extending the life of the tube and to avoid damaging it.



A thorough discussion of each part of the x-ray tube is presented here, including how each selection made at the control panel affects the corresponding part of the x-ray tube. Several procedures and considerations to protect the tube are also presented here. Finally, quality-control considerations to extend the life of the tube are discussed. The safe operation and proper maintenance of the x-ray unit rests with the radiographer; appropriate operation and maintenance of the x-ray unit stems from the knowledge of how it works.



General Tube Construction


Housing


The general construction of the tube head assembly is discussed first. Recall that the x-ray tube is situated in a protective housing that provides solid, stable mechanical support. This housing is a lead-lined metal structure that also serves as an electrical insulator and thermal cushion for the tube itself (Figure 5-2). X-ray production is a rather inefficient process and much of the electrical energy that goes into it is converted to heat. The design of the housing incorporates an oil bath and cooling fans to help dissipate heat away from the tube, protecting it from thermal damage. The tube is immersed in the oil bath, which draws heat away from the tube. The cooling fans circulate air around the assembly, which also helps dissipate heat. Because of the large current and voltage needed to produce x-rays, electrical insulation is necessary. Two large electrical cables enter the housing and are securely attached to the x-ray tube through special high-voltage receptacles. Finally, although x-rays are perceived as being produced and traveling in one direction out through the collimator to the patient and image receptor, this is not the case. X-rays are produced isotropically (in all directions) and another role of the housing is to absorb most of the photons traveling in directions other than toward the patient. This is called leakage radiation and the housing design reduces this radiation to less than 100 mR/hr, as required by regulation.



Two notes of caution about the housing are necessary: First, with extended “on” times, the housing can become rather hot. This is most likely to occur with fluoroscopic units, and most permanently installed units have the tube located under the table that limits the possibility of contact. But mobile fluoroscopic units can be easily touched; caution should be used when they are involved in very long cases in which heating is considerable. Second, the high-voltage cables are not “handles.” Some radiographers develop the bad habit of using them as such, but doing so poses a risk to the radiographer and potential for damage to the equipment.




X-ray Tube


Although there are several specialty designs of the x-ray tube, they do have basic components in common. This text focuses on the design used for general medical radiography. The general-purpose x-ray tube is an electronic vacuum tube that consists of an anode, a cathode, and an induction motor all encased in a glass or metal enclosure (envelope). Figure 5-3 provides a labeled illustration of this design. Recall that the anode is the positive end of the tube and the cathode is the negative end of the tube. The anode incorporates an anode target and an induction motor, half of which is inside and half of which is outside the protective enclosure; the anode is discussed in detail shortly. The cathode consists of the focusing cup and filament with its supporting wires.



The main purpose of the enclosure is to maintain a vacuum within the tube. Because the production of x-rays involves the interaction between filament electrons and the anode target, if any air were present, the electrons from the air would contribute to the electron stream, causing arcing and damage to the tube. The glass envelope variety is generally made of borosilicate glass because it is very heat resistant. However, as these tubes age, vaporized tungsten from the filament deposits on the inside of the glass (called “sun tanning” because of the bronze discoloration of the glass), which causes problems with arcing and damage. The metal envelope variety provides a constant electric potential between the electron stream from the cathode and the enclosure, thereby avoiding the arcing problem and extending tube life. Both enclosure types have a specially designed target window for the desired exit point of the x-rays produced. The target window is fashioned to minimally interfere with (absorb) the x-rays. It is usually about 5 cm square and is a place on the enclosure that has been made thinner than the rest. This thinned section reduces the amount of absorption by the enclosure.




Anode


The anode is the positive end of the tube. It provides the target for electron interaction to produce x-rays and is an electrical and thermal conductor. Remember that electricity is flowing through the x-ray tube and the electrons flowing from cathode to anode are a part of that flow of electricity. Some of the electrons interact with the target to produce x-rays (see Chapter 6), and the rest continue as current flow through the x-ray circuit. Remember, too, that a tremendous amount of heat is also generated during the process and the anode is designed to dissipate this heat.


There are two designs for the anode. One is the stationary anode (Figure 5-4, A). This is basically a tungsten button embedded in a copper rod. It is called stationary because the target does not move. Stationary anodes were used in old tube designs and may still be found in dental x-ray units or those requiring very small exposure techniques. The primary disadvantage of this design is that, because the electrons always hit the same small target area, heat builds up rapidly and can damage the tube. This problem limits the exposure technique factors that can be used and this limitation spurred the development of rotating-anode designs.



The rotating-anode design is used in general-purpose tubes today (Figure 5-4, B

Feb 27, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on The X-ray Tube
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