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Resident Physics Lectures

Resident Physics Lectures. Christensen, Chapter 2B Tube Ratings. George David Associate Professor Department of Radiology Medical College of Georgia. Heat Units. A unit of energy Single Phase Definition Kilovoltage X tube current X exposure time kVp X mA X sec

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Resident Physics Lectures

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  1. Resident Physics Lectures • Christensen, Chapter 2B Tube Ratings George David Associate Professor Department of Radiology Medical College of Georgia

  2. Heat Units • A unit of energy • Single Phase Definition • Kilovoltage X tube current X exposure time • kVp X mA X sec • Three Phase (constant potential/high frequency) Definition • 1.35 X Kilovoltage X tube current X exposure time • 1.35 X kVp X mA X sec

  3. Heat Units • 70 kVp • 200 mA • 0.25 second Heat Units Single Phase Exposure • 70 X 200 X 0.25 = 3500 heat units ?

  4. Heat Units Heat Units Three Phase Exposure • 60 kVp • 100 mA • 0.1 second • 60 X 100 X 0.1 X 1.35 = 810 heat units ?

  5. Heat is the Enemy X-Ray Tube Heat

  6. Tube Rating Chart • Indicates load limit tube can safely accept • Based upon • Tube construction • High VoltageWaveform

  7. Tube, Target, & Ratings • surface area bombarded by electrons • focal spot size (actual) • target angle • anode diameter • Melting point • Heat transfer • Anode rotation speed

  8. Tube Rating Charts • single exposure • multiple rapid exposure (angiographic) capability

  9. Single Exposure Rating Charts • Tube specific • Incorporated in virtually all generators • prevents illegal single exposures • Better ratings (more heat allowed) for • Large focal spot • High speed anode rotation

  10. Typical Single-Exposure Tube Rating Chart • shows maximum exposure time for single exposure at given kV & mA

  11. Example • What is the maximum exposure time at 90 kVp & 300 mA?

  12. Example • What is the maximum exposure time at 120 kVp & 400 mA? Can’t do 120 kVp at 400 mA for any exposure time. ?

  13. Single Exposure Rating Charts • Actually 8 charts combining: • generator • single phase (1F) • three phase (3F) • focal spot • small • large • anode speed • standard (3400 rpm) • high (9600 rpm) 1FSFS3400 RPM 3FSFS3400 RPM 1FLFS3400 RPM 3FLFS3400 RPM 1FSFS9600 RPM 3FSFS9600 RPM 1FLFS9600 RPM 3FLFS9600 RPM

  14. On-Board Tube Rating Charts • Checks to see if legal exposure at low-speed rotation. • Automatically switches to high speed anode rotation as needed • Locks out illegal exposures Safe at3400rpm? Safe at9600rpm? No No No ExposureAllowed Yes Yes Allow 3400 rpm Exposure Allow 9600 rpm Exposure

  15. Kilowatt Rating • Ability of x-ray tube to make single exposure of reasonable duration (usually .1 sec.) • Found on tube rating chart • standard assumptions • Use 0.1 sec. exposure time • Three phase chart • high speed rotor rotation

  16. Kilowatt Rating (cont.) • Units 1 watt = 1 volt X 1 amp 1 watt = 1 kilovolt X 1 mA 1 kilowatt (kW) = 1 kilovolt X 1 mA / 1000 • kW rating for a standard 0.1 sec exposure kW rating = kVp X mA / 1000 • use maximum mA at given kVp @ .1 sec

  17. Kilowatt Rating (cont.) • 100 kVp exposure usually used • For a 100 kVp, .1 sec exposure kW rating = mA / 10 Use maximum mA at 100 kVp, .1 sec. • Each focal spot has its own kW rating X Interpolate! ~32 kW

  18. Anode Thermal Characteristics Chart • 2 charts in one • cooling curve in absence of heating • anode heating • for continuous heat input (fluoroscopy)

  19. Continuous Heating - Fluoroscopy • Fluoro almostalways single phase • Find appropriate curve • HU/sec = kVp X mA • Follow from current heat to right for fluoro time

  20. Continuous Heating - Fluoroscopy • Technique • 100 kVp • 6 mA • 600 HU/sec • Start with50,000 HU • Fluoro for3 minutes ~105,000 HU x x 3 minutes

  21. Cooling • Start on cooling curve with current heat units • 110,000 for this example • Cool for2 minutes x x ~40,000 HU x 2 minutes

  22. Angiographic Rating Chart • Provides maximum heat units per exposure for given # of • exposures per second • total exposures Total # of Exposures Exposurespersecond Maximum Load in Peak kV X mA X sec.

  23. Example 90 X 100 = 9000 (Maximum Load) • How many total exposures can be done at • 90 kVp • 100 mAs • 3 frames / sec. Total # of Exposures 13 Exposurespersecond Maximum Load in Peak kV X mA X sec.

  24. Tube Rating Considerations • ability of tube to withstand multiple exposures during several hours of heavy use depends upon • anode storage / cooling curves • housing storage / cooling curves • housing cooling can be improved with • fans • oil / water circulators

  25. Tube Damage Warning

  26. Anode Damage • heat capacity exceeded • melted spots on anode • thermal shock (high mA on cold anode) • can cause cracks in anode (tube death)

  27. Protecting the Anode • Tube warm-up • Eliminates thermal shock from high mA exposures on cold anode • Warm-up needed whenever tube cold • once in the morning not sufficient if tube not used for several hours

  28. + arcing High Voltage Arcs • electrons move from filament to tube housing instead of to anode • can be caused by filament evaporation • deposition of filament on glass envelope as result of • high filament currents • long filament boost time • reduce by not holding first trigger longer than needed • very short exposure with instantaneously very high mA • Generator often drops off line

  29. Tube Insert Damage • Bearing Damage • prevents proper rotation of anode • anode can run too slow • anode can stop • results in thermal damage to anode (melted spots) • Filament break • renders one focal spot completely inoperative

  30. Reducing Tube Wear: Lower mA 80 kVp 500 mA, 0.1 sec 80 kVp 100 mA, 0.5 sec or • Both exposure are 50 mAs • Same radiation to image receptor • Same dose to patient Don’t smoke that tube

  31. Reducing Tube Wear: Lower mA 80 kVp 500 mA, 0.1 sec 80 kVp 100 mA, 0.5 sec or • Low mA reduces tube wear • filament temperatures lower • reduces filament evaporation Don’t smoke that tube

  32. Reducing Tube Wear: Lower mA 80 kVp 500 mA, 0.1 sec 80 kVp 100 mA, 0.5 sec or • use lowest mA (and largest focal spot) consistent with patient motion considerations • Large focal spot allows higher mA to be used Don’t smoke that tube

  33. Reducing Tube Wear: Raise kVp 70 kVp 100 mAs 90 kVp 40 mAs or • High kVp exposures require less heat units for same film density • higher kVp more penetrating • High kVp also reduces patient exposure • More penetrating beam • BUT higher kVp reduces contrast • Use highest kVp consistent with required contrast Don’t smoke that tube

  34. Reducing Tube Wear • Reduce use of high speed anode rotation • use longer times instead of higher kV and/or mA • High speed rotation greatly increases bearing wear • generators automatically select high speed for high combinations of kV & mA • BUT longer exposure times • increase exposure time & patient motion • use lowest mA consistent with patient motion considerations Don’t smoke that tube

  35. Reducing Tube Wear • Reduce first trigger holding time • Reduces bearing wear • Reduces tube rotation time • Reduces filament evaporation • filament evaporation can lead to tube arcing • Holding first trigger sometimes necessary • synchronizing breathing for children Don’t smoke that tube

  36. Oil Leaks • May be accompanied by air bubble in housing • Eventually causes high voltage arcing • Requires immediate service attention

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