X-Ray Production Lecture PDF

X-ray production Dr. Nahla Atallah  TABLE OF CONTENTS  After the completion of this chapter, the student should be able to do the following: 1.Discuss the interactions between projectile electrons and the x-ray tube target. 2. Identify characteristic and bremsstrahlung...

X-ray production Dr. Nahla Atallah  TABLE OF CONTENTS  After the completion of this chapter, the student should be able to do the following: 1.Discuss the interactions between projectile electrons and the x-ray tube target. 2. Identify characteristic and bremsstrahlung x- rays.  ELECTRON TARGET INTERACTIONS  The x-ray imaging system description emphasizes that its primary function is to accelerate electrons from the cathode to the anode in the x-ray tube.  The three principal parts of an x-ray imaging system— the operating console, the x-ray tube, and the high- voltage generator—are designed to provide a large number of electrons with high kinetic energy focused onto a small spot on the anode  Stationary objects have no kinetic energy; objects in motion have kinetic energy proportional to their mass and to the square of their velocity. The kinetic energy equation follows. For example, a 1000-kg automobile has four times follows: the kinetic energy of a 250-kg motorcycle travelling at the same speed (Figure 7-1). If the motorcycle were to double its velocity, however, it would have the same kinetic energy as the automobile. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  In determining the magnitude of the kinetic energy of a projectile, velocity is more important than mass.  In an x-ray tube the projectile is the electron.  All electrons have the same mass; therefore, electron kinetic energy is increased by raising the kVp.  As electron kinetic energy is increased, both the intensity (quantity) and the energy (quality) of the x- ray beam are increased.  The modern x-ray imaging system is remarkable.  It conveys to the x-ray tube target an enormous number of electrons at a precisely controlled kinetic energy.  At 100 mA, for example, 6 × 1017 electrons travel from the cathode to the anode of the x-ray tube every second.  In an x-ray imaging system operating at 70 kVp, each electron arrives at the target with a maximum kinetic energy of 70 keV. Because there are 1.6 × 10−16 J per keV, this energy is equivalent to the following: O0-0-==  When this energy is inserted into the expression for kinetic energy and calculations are performed to determine the velocity of the electrons, the result is a :  The distance between the filament and the x- ray tube target is only approximately 1 cm.  It is not difficult to imagine the intensity of the accelerating force required to raise the velocity of electrons from zero to half the speed of light in so short a distance.  Electrons travelling from cathode to anode constitute the x-ray tube current. When these electrons hit the heavy metal atoms of the x-ray tube target, they transfer their kinetic energy to the target atoms.  These interactions occur within a very small depth of penetration into the target.  the projectile electrons slow down and finally come nearly to rest, The electron from the cathode interacts with the orbital electrons or the nuclear field of target atoms. These interactions result in the conversion of electron kinetic energy into thermal energy (heat) and electromagnetic energy in the form of infrared radiation (also heat) and x-rays.  Anode Heat  Most of the kinetic energy of electrons is converted into heat (Figure 7-2).  The electrons interact with the outer shell electrons of the target atoms but do not transfer sufficient energy to these outer-shell electrons to ionize them.  Rather, the outer-shell electrons are simply raised to an excited, or higher, energy level. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  The outer-shell electrons immediately drop back to their normal energy level with the emission of infrared radiation.  The constant excitation and return of outer shell electrons are responsible for most of the heat generated in the anodes of x-ray tubes.  Only approximately 1% of electron kinetic energy is used for the production of x-radiation  Therefore, sophisticated as it is, the x-ray imaging system is not very efficient.  The production of heat in the anode increases directly with increasing x-ray tube current.  Doubling the x-ray tube current doubles the heat produced.  Heat production also increases directly with increasing kVp, at least in the diagnostic range  Although the relationship between varying kVp and varying heat production is approximate, it is sufficiently exact to allow the computation of heat units for use with anode cooling charts.  The efficiency of x-ray production is independent of the tube current.  The efficiency of x-ray production increases with increasing kVp  Characteristic Radiation  If the electron interacts with an inner-shell electron of the target atom rather than with an outer-shell electron, characteristic x-rays can be produced.  Characteristic x-rays result when the interaction is sufficiently violent to ionize the target atom through total removal of an inner-shell electron.  Figure 7-3 illustrates how characteristic x-rays are produced.  When the cathode electron ionizes a target atom by removing a K-shell electron, a temporary electron void is produced in the K shell.  This is an unnatural state for the target atom, and it is corrected when an outer-shell electron falls into the void in the K shell.  The transition of an orbital electron from an outer shell to an inner shell is accompanied by the emission of an x-ray.  The x-ray has energy equal to the difference in the binding energies of the orbital electrons involved.  By the same procedure, the energy of x-rays resulting from M- to-K, N-to-K, O-to-K, and P-to-K transitions can be calculated.  Tungsten, for example, has electrons in shells out to the P shell, and when a K-shell electron is ionized, its position can be filled with electrons from any of the outer shells.  All of these x-rays are called K x -rays because they result from electron transitions into the K shell.  Similar characteristic x-rays are produced when the target atom is ionized by removal of electrons from shells other than the K shell.  Note that Figure 7-3 does not show the production of x- rays resulting from ionization of an L-shell electron.  Such a diagram would show the removal of an L-shell electron by the projectile electron.  The vacancy in the L shell would be filled by an electron from any of the outer shells.  X-rays resulting from electron transitions to the L shell are called L x-rays and have much less energy than K x-rays because the binding energy of an L-shell electron is much lower than that of a K-shell electron.  Similarly, M-characteristic x-rays, N-characteristic x-rays, and even O-characteristic x-rays can be produced in a tungsten target.  Figure 7-4 illustrates the electron configuration and Table 7-1 summarizes the production of characteristic x-rays in tungsten.  Although many characteristic x-rays can be produced, these can be produced only at specific energies, equal to the differences in electron-binding energies for the various electron transitions.  Except for K x-rays, all of the characteristic x-rays have very low energy.  The L x-rays, with approximately 12 keV of energy, penetrate only a few centimetres into soft tissue. Consequently, they are useless as diagnostic x-rays, as are all the other low-energy characteristic x-rays.  The last column in Table 7-1 shows the effective energy for each of the characteristic x-rays of tungsten.  Because the electron binding energy for every element is different, the energy of characteristic x-rays produced in the various elements is also different. The effective energy of characteristic x-rays increases with increasing atomic number of the target element. The production of heat and characteristic x- rays involves interactions between the projectile electrons and the electrons of x-ray tube target atoms  Bremsstrahlung Radiation  A third type of interaction in which the projectile electron can lose its kinetic energy is an interaction with the nuclear field of a target atom. In this type of interaction the kinetic energy of the projectile electron is also converted into electromagnetic energy.  A projectile electron that completely avoids the orbital electrons as it passes through a target atom may come sufficiently close to the nucleus of the atom to come under the influence of its electric field (Figure 7-5).  Because the electron is negatively charged and the nucleus is positively charged, there is an electrostatic force of attraction  The closer the projectile electron gets to the nucleus, the more it is influenced by the electric field of the nucleus.  This field is very strong because the nucleus contains many protons and the distance between the nucleus and projectile electron is very small.  As the projectile electron passes by the nucleus, it is slowed down and changes its course, leaving with reduced kinetic energy in a different direction.  This loss of kinetic energy reappears as an x-ray. These types of x-rays are called bremsstrahlung x-rays. Bremsstrahlung is a German word that means“ slowed-down radiation.” Bremsstrahlung x-rays can be considered radiation that results from the braking of cathode electrons by the nucleus.  For example, when an x-ray imaging system is operated at 70 kVp, electrons from the cathode have kinetic energies from zero to 70 keV.  An electron with kinetic energy of 70 keV can lose all, none, or any intermediate level of that kinetic energy in a bremsstrahlung interaction. Therefore the bremsstrahlung x-ray produced can have any energy up to 70 keV.  This is different from the production of characteristic x-rays, which have very specific energies  Figure 7-5 illustrates how one can consider the production of such a wide range of energies through the bremsstrahlung interaction.  A low-energy bremsstrahlung x-ray results when the electron is barely influenced by the nucleus A maximumenergy x-ray occurs when the electron loses all its kinetic energy and simply drifts away from the nucleus. Bremsstrahlung x-rays with energies between these two extremes occur more frequently.