X ray production & emission


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X ray production & emission

  1. 1. X-Ray Production
  2. 2. Objectives:  X-ray tube interactions  Characteristic and Bremsstrahlung X- rays  X-ray emission spectrum
  3. 3. X-ray Imaging System  PRINCIPAL PARTS  Operating Console  High-voltage generator  X-ray tube  PRIMARY FUNCTION  The system is designed to provide a large number of e- at cathode with high kinetic energy focused to a small target at anode.
  4. 4. Radiographic Equipment X-ray Tube Construction G F E D C A B
  5. 5. How “X-rays” are created  Power is sent to x-ray tube via cables  mA (milliamperage) is sent to filament on cathode side.  Filament heats up – electrons are produced  Negative charge
  6. 6. How “X-rays” are created  Positive voltage (kVp) is applied to anode  Negative electrons are attracted across the tube to the positive anode.  Electrons slow down and finally come to rest  Electron beam is focused from the cathode to the anode target by the focusing cup
  7. 7.  The distance between filament and the x- ray tube target is 1 cm.  Velocity of electron is raised from zero............half the speed of light
  8. 8. E- traveling from cathode to anode  Projectile electron interacts with the orbital electron of the target atom. This interaction results 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.
  9. 9. Tube Interactions  Heat (99%)  x-rays (1%)  X-rays = Characteristic Bremsstrahlung
  10. 10. Heat  Most kinetic energy of projectile e- is converted into heat – 99%  Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize  Outer shell electrons are simply raised to an excited/ higher energy level.
  11. 11. Heat production  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 generation
  12. 12. Heat is an excitation rather than an ionization
  13. 13. Heat production  Production of heat in the anode increases directly with increasing x-ray tube current  Doubling the x-ray tube current doubles the heat produced  Increasing kVp will also increase heat production
  14. 14.  Efficiency of x-ray production is independent of the tube current  Efficiency of x-ray production increases with increasing kVp.  At 60 kvp.........0.5%  At 100 kVp.......1%  At 20 MV..........70%
  15. 15. Characteristic Radiation  Projectile electron interact with inner shell electron  Projectile e- with energy high enough to totally remove an inner-shell electron of the target atom e.g. tungsten  Characteristic x-rays are produced when outer-shell e- fills an inner-shell
  16. 16. Only K-characteristic x-rays of tungsten are useful for imaging
  17. 17. Bremsstrahlung Radiation  Bremsstrahlung is produced by projectile e- interacting with the nucleus of a target atom
  18. 18. Bremsstrahlung Radiation  A projectile e- that completely avoids the orbital e- as it passes through a target atom may come close enough to the nucleus of the atom to come under the influence of its electric field  projectile e- kinetic energy to EM energy  electrostatic force
  19. 19. Bremsstrahlung Radiations  As the projectile electro 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.
  20. 20. Bremsstrahlung is a German word meaning “slowed-down Radiation”
  21. 21. X-ray energy  Characteristic x-rays have very specific energies. K-characteristic x-rays require a tube potential of a least 70 kVp  Bremsstrahlung x-rays that are produced can have any energy level up to the set kVp value. Brems can be produced at any projectile e- value
  22. 22. Discrete spectrum  Contains only specific values
  23. 23. Characteristic X-ray Spectrum  Characteristic has discrete energies based on the e- binding energies of tungsten  Characteristic x-ray photons can have 1 of 15 different energies and no others
  24. 24. Characteristic x-ray emission spectrum
  25. 25. Continuous Spectrum  Contains all possible values
  26. 26. Bremsstrahlung X-ray Spectrum  Brems x-rays have a range of energies and form a continuous emission spectrum
  27. 27. Factors Affecting the x-ray emission spectrum  Tube current,  Tube voltage,  Added filtration,  Target material,  Voltage waveform  The general shape of an emission spectrum is always the same, but the position along the energy axis can change
  28. 28. Quality  The farther to the right the higher the effective energy or quality
  29. 29. Quantity  The more values in the curve, the higher the x-ray intensity or quantity
  30. 30. mAs  A change in mA results in the amplitude change of the x-ray emission spectrum at all energies  The shape of the curve will remain the same
  31. 31. mA increase from 200 to 400
  32. 32. kVp  A change in voltage peak affects both the amplitude and the position of the x-ray emission spectrum
  33. 33. Filtration  Adding filtration is called hardening the x- ray beam because of the increase in average energy  Filtration more effectively absorb low- energy x-rays than high energy x-rays  Characteristic spectrum is not affected & the maximum energy of x-ray emission is not affected
  34. 34. Filtration  Adding filtration to the useful beam reduces the x-ray beam intensity while increasing the average energy (higher quality)  Lowering the amplitude and shifting to the right
  35. 35. What A does this graph indicate?
  36. 36. Target Material  The atomic number of the target affects both the quantity and quality of x-rays  Increasing the target atomic number increases the efficiency of x-ray production and the energy of characteristic and bremsstrhlung x-rays
  37. 37. Target material
  38. 38. Voltage Waveform  5 voltage waveforms: half-wave rectification, full-wave rectification, 3- phase/6-pulse, 3-phase/12-pulse, and high-frequency.  Maintaining high voltage potential
  39. 39. Voltage generators
  40. 40. Factors affecting X-Ray beam quality and quantity An increase in Results in Current(mAs) An increase in quantity; no change in quality Voltage (kVp) An increase in quantity and quality Added filtration A decease in quantity and an increase in quality Target atomic number(Z) An increase in quantity and quality Voltage ripple A decrease in quantity and quality