X-Ray Production: Contents

1. X-Ray Production: Contents X-Ray Production - Full X-Ray Production - Review X-Ray Production - Quiz Raphex Questions

2. X-Ray Production Details Brian Hargreaves bah@stanford.edu

3. Goal: Produce X-ray Spectrum - + X-Rays

4. Background Charge (C): 1 electron has 1.6x10-19 C Voltage (V): “Electric potential” Energy (J): Charge of 1C accelerated through 1V will have kinetic energy of 1J Current (A): Charge/time (C/s) Power (W): Energy usage rate (W=VxA)

5. Anode Characteristics • Larger Nuclei (Larger Atomic Number, Z) • Greater probability of interactions • Higher X-ray production efficiency • Electron shells • Energy level differences determine energy of characteristic X-rays • Heat Capacity • High heat capacity to not melt(!) • May join to higher heat-capacity material with lower z to conduct away heat.

6. Atomic Structure Electron Shells K L M Nucleus Increasing Binding Energy

7. Electromagnetic Radiation • Energy = hc/l • Increasing energy: • Radio waves • Visible light • Ultraviolet • X-Rays • Gamma Rays • Remember: • 1eV is the energy of an electron through 1 V • 1KeV ~ 1KV, etc.

8. Bremsstrahlung X-ray Generation Brehmsstrahlung = “Braking Radiation” Target Intensity Ee

9. Filtration Effects Ee Ee • Low-energy X-rays more likely to interact: • Filtered in tube housing • Often used for mammography (soft-tissue contrast)

10. Bremsstrahlung X-ray Generation • Bremsstrahlung X-rays: produced by braking interactions • Typical spectrum • Limited by Electron Energy • Low-energy spectrum filtered • Decreasing Electron Energydecreases horizontal extent and height • Increasing Electron Flow (current)only increases height Intensity Energy

11. Characteristic X-Rays Intensity Energy Electron Shells Characteristic X-rays: • incoming electron ejects a bound electron • vacancy is filled by electron from outer shell • Transition results in X-ray emission • Only occurs if incoming electron energy > binding energy • Always at discrete frequencies • Characteristic of target element X-Ray K L M

12. X-Ray Tubes

13. Goal: Produce X-ray Spectrum - + • Heat filament – boil off electrons • Accelerate from cathode to anode • Focus on focal spot • At Anode, produce X-rays: • Brehmsstrahlung • Characteristic • Also produce heat (main limitation)

14. Tube Voltages and Currents Tube Current - - - - - - - - - - - - - - - - - - - + - Cathode (-) Anode (+) Tube Voltage Filament Current Dependence Tube Current Tube Voltage

15. Cathode Review Filament current enables thermionic emission Space-charge limited Increase Tube Voltage to overcome space charge, maximize tube current Further Voltage increases increase kinetic energy, but not tube current Increasing filament current increases tube current Focusing cup directs electrons to focal spot

16. Transformers - Basics Vin Vout • Ratio of #turns changes voltage • Essential to create the huge voltages to produce X-rays

17. Transformers More Ripple Intensity Less Ripple Energy Decreasing Ripple: • Half-wave rectification • Full-wave rectification • 3-phase • 6 or 12 pulse • High-frequency Inverter

18. Target Considerations • Materials: • High atomic number (Z) for more X-rays • High heat capacity / low melting point • Molybdenum gives low-energy characteristic X-rays

19. Rotating Anodes Electron Beam X-rays X-rays Heat is radiated from rotating anode.

20. Focal Spot Low angle • small effective focal spot • small FOV Higher angle • Better heat dissipation • Larger FOV, effective focal spot Heel Effect • Smaller effective spot toward anode

21. Heel Effect Incoming Electrons • Greater attenuation toward anode • Greater attenuation in low energies

22. Heat / Rating Charts • kW rating: maximum power for 0.1s and 100keV • Anode heating - short term (dissipate via conduction) • Rotating anodes reduce heating (dissipate via radiation) • Housing cooling - longer term

23. Heat Charts Given Focal Spot Size, Rotation Speed Lines showing Maximum Current (mA) 10mA Maximum Voltage (kVp) 20mA 30mA Maximum Exposure Time

24. The Key Points… Increase Tube Voltage Intensity Increase Tube Current Energy • Ee ~ Electron Energy (eV) • Vp = 1 kV … Ee = 1 keV • Ee above binding energy for characteristic X-rays

25. X-Ray Production: Contents X-Ray Production - Full X-Ray Production - Review X-Ray Production - Quiz Raphex Questions

26. X-Ray Production Review Brian Hargreaves bah@stanford.edu

27. Goal: Produce X-ray Spectrum - + X-Rays

28. X-Ray Production Concepts: Warmup - + - +

29. X-Ray Production Concepts: Warmup - + Tube Voltage Target Filament Focal Spot Heat Tube Current Transformer Filament Current Focusing Cup Heel Effect Bremsstrahlung X-Ray Spectrum X-Ray Beam Increased Tube Current (mA) Characteristic Lines Unfiltered Spectrum Decreased Tube Voltage (kV) Energy (kV)

30. X-Ray Generation Cathode Filament Anode (target) Tube Voltage (kVp) Tube Current (mA) Focal spot Atomic number How does it work?? Anode angle Field of View Bremsstrahlung spectrum Characteristic X-rays Transformers Heating Rotating anodes

31. X-Ray Generation: Cathode • Transformer generates tube voltage (kVp) • 100 kVp results in electron enery up to 100 keV • Thermionic emission of electrons from Filament • Increased filament current increases tube current (mA) • Electrons accelerated toward anode • Focusing cup directs electrons at focal spot • Electrons hit anode target and produce X-rays Anode Cathode Electrons - Tube Voltage +

32. Tube Voltages and Currents Tube Current - - - - - - - - - - - - - - - - - - - + - Cathode (-) Anode (+) Tube Voltage Filament Current Dependence Tube Current Tube Voltage

33. Bremsstrahlung X-ray Generation • Bremsstrahlung X-rays: produced by braking interactions • Typical spectrum • Limited by kVp • Low-energy spectrum filtered • Decreasing kVp decreases horizontal extent and height • Increasing mAonly increases height Intensity Energy

34. Characteristic X-Rays Intensity Energy Electron Shells Characteristic X-rays: • incoming electron ejects a bound electron • vacancy is filled by electron from outer shell • Transition results in X-ray emission • Only occurs if incoming electron energy > binding energy • Always at discrete frequencies • Characteristic of target element X-Ray K L M

35. Target Considerations • Materials: • High atomic number (Z) for more X-rays • High heat capacity • Molybdenum gives low-energy characteristic X-rays • Heat/Rating Charts • kW rating: maximum power for 0.1s and 100keV • Anode heating - short term (dissipate via conduction) • Rotating anodes reduce heating (dissipate via radiation) • Housing cooling - longer term

36. Focal Spot Low angle • small effective focal spot • small FOV Higher angle • Better heat dissipation (spread over more anode area) • Larger FOV, effective focal spot Heel Effect • Smaller effective spot toward anode

37. Transformers More Ripple Intensity Less Ripple Energy Decreasing Ripple: • Half-wave rectification • Full-wave rectification • 3-phase • 6 or 12 pulse • High-frequency

38. The Key Points… Increase Tube Voltage Intensity Increase Tube Current Energy • Ee ~ Electron Energy (eV) • Vp = 1 kV … Ee = 1 keV • Ee above binding energy for characteristic X-rays

39. Quiz A View this as a presentation to show questions, then answers • What produces the high voltage in the tube? (Transformer) • In the tube, electrons go from ____ to ___ ? Cathode, Anode • A 100 kV tube voltage produces electrons of energy _____ ? 100 keV • Electrons are emitted by increasing the ___? Filament Current

40. Quiz B • How can you increase the tube current? Increase filament current (and electron emission) • At 1mA and 120 kVp, what is the maximum X-ray energy? 120 keV • What is the function of the focusing cup? Focus electron beam on target to keep focal spot small • Is graphite a good choice for target material? No. Low atomic number / small nucleus

41. Quiz C • Why is the Bremsstrahlung spectrum small at low X-ray energy? Filtration effects • How does oscillation in the tube voltage affect the spectrum? Tends to shift to the left (more time at lower kVp) • A larger target angle increases effective focal spot size and increases ______? Field-of-view

42. Quiz D • Why is a rotating anode used? Distribute heat on full circumference. • Why is molybdenum used as a target? Low characteristic X-rays • About what fraction of the electron beam is converted to X-rays? About 1% • What is the heel effect? Smaller, harder, less intense focal spot toward anode

43. Quiz E Electron Shells Binding Energies: K Shell: 105 keV L Shell: 10 keV M Shell: 8 keV Nucleus K L M • What minimum tube voltage produces characteristic X rays? 105 kV • What is the energy of these X rays? 95 keV and 97 keV

44. Quiz F • How does doubling tube current affect the X-ray spectrum? Shifts up (increases intensity at all energies). • At very low voltages _________ prevents electrons from reaching the anode. Space charge limit • At high tube voltages, how does increasing tube voltage affect tube current? Very little

45. Quiz G • Tube housing heats and cools more or less quickly than the anode target? Less quickly • A rotating anode cools by ____ and ____ Conduction and radiation • The tube current that can be sustained at 100 kVp for 0.1 seconds determines the ___ of the tube kW rating / power rating

46. View this as a presentation to show questions, then answers 2007

48. (Note: Consider an X-Ray of the given energy – can you have Bremsstrahlung and/or Characteristic X-rays at that energy?) B C D B