X-Ray Production & Emission

1. X-Ray Production & Emission

2. Objectives: • Review x-ray production requirements • X-ray tube interactions • X-ray emission spectrum

3. Principle Parts of the X-ray Imaging System • Operating Console • High-voltage generator • X-ray tube • The system is designed to provide a large number of e- with high kinetic energy focused to a small target

4. E- traveling from cathode to anode • Projectile e- interacts with the orbital e- of the target atom. This interaction results in the conversion of e- _______ energy into ________ energy and ________ energy.

5. Tube Interactions • 3 possible tube interactions • Tube interactions are generated from _____ slamming into ________? • Heat (99%), EM energy as infrared radiation (heat) & x-rays (1%) • X-rays = Characteristic (20%) or Bremsstrahlung (80%)

6. 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

7. Heat is an excitationrather than an ionization

8. Heat production • Production of heat in the anode increases directly with increasing x-ray tube current & kVp • Doubling the x-ray tube current doubles the heat produced • Increasing kVp will also increase heat production

9. Characteristic Radiation – 2 steps • Projectile e- with high enough energy to totally remove an inner-shell electron of the tungsten target • Characteristic x-rays are produced when outer-shell e- fills an inner-shell void • All tube interactions result in a loss of kinetic energy from the projectile e-

10. It is called characteristic because it is characteristic of the target element in the energy of the photon produced

11. Only K-characteristic x-rays of tungsten are useful for imaging

12. Bremsstrahlung Radiation • Heat & Characteristic produces EM energy by e- interacting with tungsten atoms e- of the target material • Bremsstrahlung is produced by e- interacting with the nucleus of a target tungsten atom

13. Bremsstrahlung Radiation • A projectile e- that completely avoids the orbital e- as it passes through a target atom may pass close enough to the nucleus of the atom to convert some of the projectile e- kinetic energy to EM energy • Because of the electrostatic force?

14. Bremsstrahlung is a german word meaning slowed-down radiation

15. 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

16. Discrete spectrum • Contains only specific values

17. Continuous Spectrum • Contains all possible values

18. 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

19. Characteristic x-ray emission spectrum

20. Bremsstrahlung X-ray Spectrum • Brems x-rays have a range of energies and form a continuous emission spectrum

21. 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

22. Quality • The farther to the right the higher the effective energy or quality

23. Quantity • The more values in the curve, the higher the x-ray intensity or quantity

24. mAs • A change in mA or s or both results in the amplitude change of the x-ray emission spectrum at all energies • The shape of the curve will remain the same

25. mA increase from 200 to 400

26. kVp • A change in voltage peak affects both the amplitude and the position of the x-ray emission spectrum

27. Filtration • Adding filtration is called hardening the x-ray beam because of the increase in average energy • Characteristic spectrum is not affected & the maximum energy of x-ray emission is not affected

28. Filtration • Adding filtration to the useful beam reduces the x-ray beam intensity while increasing the average energy • Added filtration is an increase in the average energy of the x-ray beam (higher quality) with a reduction in x-ray quantity • Lowering the amplitude and shifting to the right

29. What kVp does this graph indicate?

30. 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 bremsstrahlung x-rays

31. Target material

32. 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

33. Voltage generators

34. X-ray Quantity or Intensity • What units of measurement is used for radiation exposure or exposure in air? • Milliampere-seconds (mAs) – x-ray quantity is proportional to mAs • Kilovolt Peak (kVp) – If kVp were doubled the x-ray intensity would increase by a factor of four or kVp2

35. X-ray Quantity or Intensity • Distance – x-ray intensity varies inversely with the square of the distance from the x-ray target • When SID is increased, mAs must be increased by SID2to maintain constant OD

36. Filtration • 1 to 3 mm of aluminum (Al) added to the primary beam to reduce the number of low-energy x-rays that reach the patient, reducing patient dose • Filtration reduces the quantity of x-rays in the low-energy range

37. Reducing low-energy photons

38. X-ray Quality or Penetrability • As the energy of an x-ray beam is increased, the penetrability is also increased • High-energy photons are able to penetrate tissue farther than low-energy photons • High-quality = high-penetrability • Low-quality = low-penetrability

39. HVL = Half-Value Layer • What is the HVL • HVL is affected by the kVp and added filtration in the useful beam • Photon quality is also influenced by kVp & filtration • HVL is affected by kVp

40. HVL • In radiography, the quality of the x-rays is measured by the HVL • The HVL is a characteristic of the useful x-ray beam • A diagnostic x-ray beam usually has an HVL of 3 to 5 mm Al

41. HVL • 3 to 5 mm Al = to 3 to 6 cm of soft tissue • HVL is determined experimentally and a design specification of the equipment

42. X-ray Quality • Kilovolt Peak (kVp) = increasing the kVp increased photon quality and the HVL

43. Types of Filtration • Diagnostic x-ray beams have two filtration components – inherent filtration and added filtration • Inherent filtration – The glass enclosure of the tube (the window) – approximately 0.5 mm Al equivalent

44. Added Filtration • 1 or 2 mm sheet of aluminum between the tube housing and the collimator • The collimator contributes an additional 1mm Al equivalent added filtration

46. Compensating filter • A filter usually made of Al, but plastic can be used to maintain OD when patient anatomy varies greatly in thickness • Are useful in maintaining image quality. They are not radiation protection devices

47. Wedge filter

48. Compensating Filter • What is an aspect of the tube design that works as a compensating filter? • What causes this?

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