RMI 213: Principles of Medical Imaging Lecture 9 - PDF

RMI 213 Principles of medical imaging Lecture 9 X-ray Production Slide 1 fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Discuss the interactions between pro...

RMI 213 Principles of medical imaging Lecture 9 X-ray Production Slide 1 fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Discuss the interactions between projectile electrons and the x-ray tube target 2. Identify characteristic and Bremsstrahlung x-rays 3. Describe the x-ray emission spectrum 4. Explain how mAs, kVP , added filtration, target material, and voltage ripple affect the x-ray emission spectrum Slide 2 fchs.ac.ae Prescribed Text Bushong, S.C., Radiologic Science for Technologists, 10th edition, Mosby/Elsevier; St Louis, 2012, pages 123-135. Notes: 1. Each lecture in this course will relate very closely to a specific set of pages in the above text. It is strongly recommended that students read the pages indicated prior to coming to the lecture. 2. The students outcomes listed at the commencement of each lecture are essentially those found in the prescribed text for the relevant chapter. Slide 3 fchs.ac.ae Electron Target Interactions The basic principle in generating x-rays from a tube is to accelerate the electrons to high energy, kinetic energy, and then deposit this energy into the target material(Anode) In a 70 kVP tube each electron gains a kinetic energy of KEelectron = 70  1000  1.6  10-19 = 1.12  10-14 J Put another way, this is 70,000 kV per electron which is significant at the atomic level! Operated at 100 mA this means 6  1017 of these electrons striking the target every second At the target the electron speed is about ½ the speed of light Slide 4 fchs.ac.ae Projectile Electrons Electrons accelerated to such high speed are referred to as projectile electrons On colliding with the target they transfer their KE to the target atoms They only ‘hit’ the atoms at or very close to the surface Having lost this energy they slowly find their way out of the target through the conducting circuitry This electron KE is converted to heat energy (about 99%) and x-radiation energy (about 1%) Slide 5 fchs.ac.ae Anode Heat Projectile electrons interact with outer shell atomic electrons; These are raised to higher orbit level and, falling back, release heat energy as infrared radiation (also heat) and x-rays. Doubling the electron current simply doubles the heat energy output Bushong, Figure 7-2, page 126 Slide 6 fchs.ac.ae Characteristic Radiation Suppose a projectile electron interacts with inner shell electrons closer to the nucleus An atomic electron, e.g. from the K shell, is ejected from the atom; the atom becomes ionized (not the electron as stated in the figure!) An outer shell electron fills this vacancy Energy is released: an x-ray is produced Bushong, Figure 7-3, page 126 Slide 7 fchs.ac.ae Characteristic Radiation If the outer shell electron was from the K-shell, then the x-ray produced will be a K x-ray If the outer shell electron was from the L-shell, an L x-ray is produced; We can also have M x-rays, N x-rays, etc. x-rays will have energies specific to the electron energy levels of the target atom x-rays produced from specific transitions into the inner shells are called characteristic x-rays – characteristic of the atom Slide 8 fchs.ac.ae Characteristic Radiation With a tungsten target, only K x-rays are emitted The L (and lower energy M, N, etc.) x-rays have low energies and are usually absorbed by the target material itself Tungsten Target Bushong, Table 7-1, page 127 Slide 9 fchs.ac.ae Bremsstrahlung Radiation Electromagnetic radiation produced by the deceleration of a charged particle e- after passing through the electric and magnetic fields of a nucleus. Electrons that ‘avoid’ atomic electrons interact instead with the electric field of the nucleus As they slow down, they lose energy, a fraction or all at once, but in smaller increments with progressively greater probability This continuous spectrum is referred to a the Bremsstrahlung (or ‘braking’) radiation At low energies these x-rays are simply absorbed by target materials and do not escape to be part of the beam that passes through the tube window Slide 10 fchs.ac.ae X-ray Emission Spectrum The spectrum of x-ray energies that exit the window is a probability distribution function related to all the interactions that occur as the projectile electrons lose their KE This leads to: A Bremsstrahlung component, Discrete, characteristic and a characteristic component, all reduced in probability at low Continuous, Bremsstrahlung energies by target and window absorption Bushong, Figure 7-8, page 129 Slide 11 fchs.ac.ae Characteristic x-ray Spectrum Characteristic x-rays arise from the discrete energy level differences of the stable electron states in the atom, the binding energy levels For tungsten, these values are shown opposite, but the L x-rays never escape the tube Bushong, Figure 7-9, page 129 Slide 12 fchs.ac.ae Characteristic x-ray Spectrum The diagram shows typical spectra from tungsten and molybdenum tubes with an applied voltage of about 90 kVP Bushong, Figure 7-10, page 129 Slide 13 fchs.ac.ae Bremsstrahlung x-ray Spectrum This can tell you the kVP applied to the tube where the probability falls to zero The general shape is the same for all tubes and all target materials The sharp decrease at low energies is due to absorption of the x-rays by the target, the window and oil (if present in the tube housing) Bushong, Figure 7-10, page 129 Slide 14 fchs.ac.ae Factors Affecting the x-ray Emission Spectrum The total number of x-rays is defined as the area under the spectrum curve The greater the area under the curve, the greater the beam quantity (more x-rays) The higher the kVP the better the quality (the penetrating ability of the beam) So we can examine the following changes: mA (beam current alone) for fixed kVP and exposure time mAs (current times time) for a fixed kVP kVP (tube voltage) for fixed mA and exposure time Slide 15 fchs.ac.ae Effect of mA and mAs Doubling the current simply doubles the total number of x-rays produced, but does not change the spectrum plot shape in any way (see diagram) Similarly, no shape change occurs for an increased exposure time – just ‘more of the same’ Bushong, Figure 7-11, page 131 Slide 16 fchs.ac.ae Effect of kVP Increasing the voltage alters the extent to the high-energy end (the right-hand side, in the diagram below) of the spectrum. This changes beam quality and quantity However, there is no change to the position of the characteristic part of the spectrum Approximately a 15% increase in kVP doubles the x-ray output intensity Bushong, Figure 7-12, page 131 Slide 17 fchs.ac.ae Effect of Added Filtration Most x-ray beams also pass through additional material placed immediately after the tube window, using filters This increases the average energy (the quality) of the beam while also reducing the intensity (the quantity) Filters ‘harden’ the beam; Filters remove the low energy x-rays which contribute to patient dose but not to the image quality Bushong, Figure 7-13, page 132 Slide 18 fchs.ac.ae Effect of Target Material The target material affects both quality and quantity of the beam: Bremsstrahlung radiation increases with atomic number of the target material Characteristic radiation energy also changes with target material Most images are taken using W (tungsten) targets Mammography uses tubes with Mo or Rh target material generally Bushong, Figure 7-14, page 133 Slide 19 fchs.ac.ae Effect of Voltage Variation Different tube voltage schemes change the ripple amplitude and the ripple frequency 3-phase tube power supplies result in a more constant, more efficient output than do single- phase supplies The maximum x-ray output is associated with the peaks of the voltage waveform Bushong, Figure 7-15, page 133 Slide 20 fchs.ac.ae Summary Check that you can satisfy the learning outcomes for this lecture Go over calculations/exercises undertaken during the lecture Make sure you can define/describe the following terms: ionization characteristic and Bremsstrahlung radiation electromagnetic radiation spectrum change with change of kVP, mA and s filtering beam hardening x-ray beam contribution to image and to dose Slide 21 fchs.ac.ae