X-Ray Emission Lecture PDF

X-Ray Emission Dr. Nahla Atallah  TABLE OF CONTENTS  After the completion of this Lecture, the student should be able to do the following: 1. Describe the x-ray emission spectrum. 2. Discuss factors that affect X-Ray Spectrum emission.  X-RAY EMISSION SPECTRUM...

X-Ray Emission Dr. Nahla Atallah  TABLE OF CONTENTS  After the completion of this Lecture, the student should be able to do the following: 1. Describe the x-ray emission spectrum. 2. Discuss factors that affect X-Ray Spectrum emission.  X-RAY EMISSION SPECTRUM  Characteristic X-ray Spectrum:  The discrete energies of characteristic x-rays are characteristic of the differences between electron binding energies in a particular element.  A characteristic x-ray from tungsten, for example, can have 1 of 15 different energies (see Table 7-1) and no others.  A plot of the frequency with which characteristic x- rays are emitted as a function of their energy would look similar to that shown for tungsten in Figure 7-9.  Such a plot is called the characteristic x-ray emission spectrum. Five vertical lines representing K x-rays and four vertical lines representing L x-rays are included. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  The lower energy lines represent characteristic emissions from the outer electron shells.  The relative intensity of the K x-rays is greater than that of the lower energy characteristic x-rays because of the nature of the interaction process.  K x-rays are the only characteristic x-rays of tungsten with sufficient energy to be of value in diagnostic radiology.  Although there are five K x-rays, it is customary to represent them as one, as has been done in Figure 7-10 with a single vertical line, at 69 keV. Bremsstrahlung X-ray Spectrum: If it were possible to measure the energy contained in each bremsstrahlung x-ray emitted from an x-ray tube, one would find that these energies range from the peak electron energy all the way down to zero.  In other words, when an x-ray tube is operated at 90 kVp, bremsstrahlung x-rays with energies up to 90 keV are emitted.  A typical bremsstrahlung x-ray emission spectrum is shown in Figure 7-10.  The general shape of the bremsstrahlung x-ray spectrum is the same for all x-ray imaging systems.  The maximum energy (in keV) of a bremsstrahlung x-ray is numerically equal to the kVp of operation.  The greatest number of x-rays is emitted with energy approximately one third of the maximum energy.  The number of x-rays emitted decreases rapidly at very low energies. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  As described before, the energy of an x-ray is equal to the product of its frequency (f) and Planck’s constant (h). X-ray energy is inversely proportional to its wavelength.  As x-ray wavelength increases, x-ray energy decreases.  The minimum wavelength of x-ray emission corresponds to the maximum x-ray energy, and the maximum x-ray energy is numerically equal to the kVp.  FACTORS AFFECING THE X-RAY EMISSION SPECTRUM  The total number of x-rays emitted from an x-ray tube could be determined by adding together the number of x-rays emitted at each energy over the entire spectrum, a process called integration.  Graphically, the total number of x-rays emitted is equivalent to the area under the curve of the x- ray emission spectrum.  The general shape of an emission spectrum is always the same, but its relative position along the energy axis can change.  The farther to the right a spectrum is, the higher the effective energy or quality of the x-ray beam.  The larger the area under the curve, the higher is the x-ray intensity or quantity.  A number of factors under the control of radiographers influence the size and shape of the x- ray emission spectrum and therefore the quality and quantity of the x-ray beam.  These factors are summarized in Table 7-2.  Effect of mA and mAs  If one changes the current from 200 to 400 mA while all other conditions remain constant, twice as many electrons will flow from the cathode to the anode, and the mAs will be doubled.  This operating change will produce twice as many x- rays at every energy.  In other words the x-ray emission spectrum will be changed in amplitude but not in shape (Figure 7-11).  Effect of kVp  As the kVp is raised, the area under the curve increases to an area approximating the square of the factor by which kVp was increased.  Accordingly, the x-ray quantity increases with the square of this factor.  When kVp is increased, the relative distribution of emitted x-ray energy shifts to the right to a higher average x-ray energy. The maximum energy of x-ray emission always remains numerically equal to the kVp.  Figure 7-12 demonstrates the effect of increasing the kVp while other factors remain constant.  The lower spectrum represents x-ray operation at 72 kVp, and the upper spectrum represents operation at 82 kVp—a 10-kVp (or 15%) increase. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1.  Effect of Added Filtration  Adding filtration to the useful x-ray beam reduces x-ray beam intensity while increasing the average energy.  Thiseffect is shown in Figure 7-13, where an x- ray tube is operated at 95 kVp with 2-mm aluminum (Al) added filtration compared with the same operation with 4-mm Al added filtration.  Added filtration more effectively absorbs low- energy x-rays than high-energy x-rays; therefore, the bremsstrahlung x-ray emission spectrum is reduced further on the left than on the right. Adding filtration is sometimes called hardening the x-ray beam because of the relative increase in average energy. The characteristic spectrum is not affected, nor is the maximum energy of x-ray emission. There is no simple method for calculating the precise changes that occur in x-ray quality and quantity with a change in added filtration.  Effect of Target Material  The atomic number of the target affects both the number (quantity) and the effective energy (quality) of x-rays.  As the atomic number of the target material increases, the efficiency of the production of bremsstrahlung radiation increases, and high- energy x-rays increase in number to a greater extent than low-energy x-rays. The change in the bremsstrahlung x-ray spectrum is not nearly as pronounced as the change in the characteristic spectrum.  After an increase in the atomic number of the target material, the characteristic spectrum is shifted to the right, representing the higher energy characteristic radiation.  This phenomenon is a direct result of the higher electron binding energies associated with increasing atomic number.  These changes are shown schematically in Figure 7-14.  Tungsten is the primary component of x-ray tube targets, but some specialty x-ray tubes use gold as target material. The atomic numbers for tungsten and gold are 74 and 79, respectively.  Molybdenum (Z = 42) and rhodium (Z = 45) are target elements used for mammography. In many dedicated mammography imaging systems, these elements are incorporated separately into the target. The x-ray quantity from such mammography target material is low owing to the inefficiency of x-ray production. This occurs because of the low atomic number of these target elements.  Elements of low atomic number also produce low-energy characteristic x-rays.