X-ray Production



X-ray Production




This chapter is about x-ray tube structure and function and how these factors affect the primary x-ray beam. The electric factors that control x-ray production are introduced in this chapter. Chapters 4 and 5 contain a tremendous amount of detail, and most of it is probably unfamiliar to you. Although it is all interrelated and is presented in a logical order, you may feel a bit overwhelmed if you try to comprehend it too quickly. Do not attempt to assimilate it all at once. When this material is taken in small bites and reviewed as needed, the entire process of creating and controlling x-rays will gradually come into focus.


Roentgen discovered x-rays while working with a Crookes tube (Fig. 5-1), a cathode ray tube that was the forerunner of the fluorescent tube and the neon light. These tubes were used in physics laboratories in the late nineteenth century for the investigation of electricity. In 1913, the General Electric Company introduced the Coolidge tube (Fig. 5-2), a “hot cathode tube” that was the prototype for modern x-ray tubes.





X-Ray Tube


Fig. 5-3 illustrates a simple x-ray tube with its principal parts labeled. There are four essential requirements for the production of x-rays: (1) a vacuum, (2) a source of electrons, (3) a target, and (4) a high potential difference (voltage) between the electron source and the target.



A Pyrex glass envelope forms the basic structure of the x-ray tube. It is made of strong, heat-resistant glass and contains both the source of electrons and the target. The air is removed from the glass envelope to form a near-perfect vacuum so that gas molecules will not interfere with the process of x-ray production. The tube is fitted on both ends with connections for the electric supply.


The source of electrons is a filament at one end of the tube. The filament consists of a small coil of tungsten wire. Tungsten (chemical symbol W) is a metal element; it is a large atom with 74 electrons in orbit around its nucleus. An electric current flows through the filament to heat it. An advantage of using tungsten is that it has a high melting point, which enables it to last through thousands of exposures. As explained in Chapter 4, heat speeds up the movement of the electrons in their orbits and increases their distance from the nucleus. Electrons in the outermost orbital shells move so far from the nucleus that they are no longer held in orbit but are flung out of the atom, forming an “electron cloud” around the filament (Fig. 5-4). This process is called thermionic emission. The electron cloud is called a space charge and is the source of free (in air) electrons for x-ray production.



At the opposite end of the tube is the anode (also referred to as the target), a hard, smooth, slanted metal surface that is also made of tungsten. The electrons are directed toward the target, which is the place where x-rays are generated.


A high-voltage electric source provides acceleration of the electrons. A large step-up transformer supplies the voltage (40 to 125 kVp) required for x-ray production. The two ends of the x-ray tube are connected in the transformer circuit so that the filament end is negative and the target end is positive during an exposure. The positive, target end of the tube is called the anode; the negative, filament end is called the cathode.


The high positive electric potential at the target attracts the negatively charged electrons of the space charge, which move rapidly across the tube, forming an electron stream. When these fast-moving electrons collide with the target, the kinetic energy of their motion is converted into a different form of energy. The great majority of this kinetic energy (>99%) is converted into heat, and only a small amount is converted into the energy form that we know as x-rays (Fig. 5-5).




Bremsstrahlung and Characteristic Radiation


X-rays are produced at the target as a result of either a sudden deceleration or an absorption of the electron stream. These interactions of electrons with tungsten atoms may occur in one of two ways (Box 5-1).




Bremsstrahlung Radiation


X-rays are produced when an incoming electron misses all the electrons in the tungsten atom, gets very close to the nucleus, and then suddenly slows down and abruptly changes direction. As a result, the electron loses energy. This sudden energy change is converted into an x-ray photon (Fig. 5-6). The photon produced is a small “bundle” of electromagnetic energy. X-rays created by this interaction are called bremsstrahlung radiation. Bremsstrahlung is a German word that means braking or slowing. The short term “brems” is often used instead of the long word. Every x-ray exposure will contain photons produced from bremsstrahlung interactions in the anode. Below 70 kVp, 100% of the photons in the x-ray beam are from bremsstrahlung interactions. Above 70 kVp, about 85% of the beam is bremsstrahlung. Therefore it is evident that the majority of all x-ray photons produced are from the bremsstrahlung interactions.




Characteristic Radiation


X-rays are also produced when an incoming electron collides with the K-shell (inner shell) electron of the tungsten atom and ejects it out of orbit. Both the incoming electron and the K-shell electron are removed. The void in the K-shell is filled with an electron from any of the other orbits. X-rays created by this interaction are called characteristic radiation (Fig. 5-7). Below 70 kVp, there are no characteristic photons produced. Above 70 kVp, about 15% of the beam is characteristic. This is because the binding energy of a K-shell is 69.5 and it takes at least a 70-kVp exposure to eject this electron.



In terms of producing x-ray images, there is no difference between a bremsstrahlung and characteristic photon. They are simply produced by different interactions of the incoming electrons in the anode. Technically, this cannot be controlled. The primary x-ray beam is made up of both bremsstrahlung and characteristic radiation.


The wavelength and energy of the x-ray beam is said to be heterogeneous. This means that it is made up of many different wavelengths and energies. X-ray energy is measured in kiloelectron volts (keV).



Characteristics of the Cathode and the Anode


Cathode


Although it is essential to have at least one filament for x-ray production, modern multipurpose x-ray tubes are dual-focus tubes (Fig. 5-8). They contain two filaments, one large and one small. Only one filament is used at a time.



Each filament is situated in a hollow area in the cathode called a focusing cup (Fig. 5-9). The focusing cup has a slight negative charge. The shape of the focusing cup and its negative electric charge cause the electrons to be repelled in the direction of a very precise area on the target called the focal spot

Mar 7, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on X-ray Production
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