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Resident Physics Lectures

Resident Physics Lectures. Christensen, Chapter 3B X-Ray Generator Circuit. George David Associate Professor Medical College of Georgia Department of Radiology. X-Ray Generator. Supplies electrical power to x-ray tube high voltage between anode & cathode filament voltage

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Resident Physics Lectures

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  1. Resident Physics Lectures • Christensen, Chapter 3B X-Ray Generator Circuit George David Associate Professor Medical College of Georgia Department of Radiology

  2. X-Ray Generator • Supplies electrical power to x-ray tube • high voltage between anode & cathode • filament voltage • Controls exposure timing • Turns exposure on and off • High voltage switched on and off • Filament heated before exposure

  3. Generator Components • control console • kVp adjust • mA adjust • time adjust • transformer • high voltage (step up) • filament • low voltage (step down) • electronics cabinet • support circuitry or mAs adjust

  4. + X-ray Circuit High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer

  5. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer Line Incoming line voltage connected to generator through a circuit breaker. Typ. 220-240 volt AC single phase 240, 480 volt AC three phase

  6. Incoming Power • Line affects generator performance • diameter of wire • length or wire • other devices sharing branch circuit • Resistance of power line wires can reduce generator voltage during exposure affecting • power available to x-ray tube • calibration

  7. Generator Incoming Power Line Circuit Breaker Circuit Breaker • Generator connected to power line through a circuit breaker • Limits current from power line to generator • Allows generator to be disconnected from power line

  8. Generator Incoming Power Line Circuit Breaker Line Voltage Compensation • Incoming voltage can vary during day • Generators need to correct for changes in line voltage • power line fluctuations affect calibration

  9. Line Voltage Compensation • Compensation may be • automatic • most new & high end equipment • manual • user must make adjustment Line Line Compensation

  10. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer Autotransformer • High voltage Transformer has fixed ratio • Autotransformer has variable ratio • Autotransformer needed to provide variable kilovoltage to tube

  11. Line Line Compensation Autotransformer major kV selector to high voltage transformer primary Timer Circuit minor kV selector to filament transformer primary mA regulator Autotransformer does line compensation & kVp selection

  12. Generator Voltages • Input line voltage • single or three phase • 115 - 480 Volts AC • Autotransformer • provides variable voltage to primary of high voltage transformer 1f 3f Auto Transformer High Voltage Transformer Timer Circuit Power Line

  13. High Voltage Circuit • Supplies high voltage for x-ray tube • Step-up transformer • primary from autotransformer • secondary to rectifier circuit • mA monitored at center grounded point of secondary Auto- transformer Rectifier Circuit mA High Voltage Transformer

  14. High Voltage Transformer • Grounded metal box • filled with oil • electrical insulator • Function • increases or decreases alternating voltage • Also contains rectifier circuit • changes alternating current into direct current

  15. Self (tube) Rectified Circuit • X-Ray tube acts as rectifier • Current only flows from cathode to anode • cathode is source of free electrons • Rarely seen Secondary of High Voltage Transformer Voltage applied to tube mA waveform

  16. Self-rectification Disadvantages Wasted Used • hot anode can emit electrons • accelerate & can destroy filament • half of electrical cycle wasted Voltage applied to x-ray tube mA waveform X-Rays Produced

  17. + - Halfwave Rectifier Circuit • X-ray tube connected to secondary of high voltage transformer through diode rectifiers • Alternating voltage applied to secondary of high voltage transformer Voltage applied to tube

  18. + First Half Cycle: Diodes closed Voltage applied to tube Tube current (mA) results - - X Second Half Cycle: Diodes open No voltage applied to tube No tube current (mA) + Halfwave Rectifier Circuit - -

  19. Applied to X-ray Tube Output of High Tension Transformer Applied to x-ray tube Blocked (not used) Halfwave Rectified Circuit • 60 pulses per second • only positive half cycle of high tension transformer used • inefficient • negative half cycle wasted Secondary of High Voltage Transformer

  20. Fullwave Rectifier • Four diodes • 120 pulses/second • exposure times half of halfwave circuit Secondary of High Voltage Transformer Voltage applied to tube (also mA waveform)

  21. Voltage applied to tube (also mA waveform) + - X X X X - + Fullwave Rectifier First Half Cycle Second Half Cycle

  22. Full-Wave Rectification • Rectifiers • Four diode “bridge” configuration used with single phase • both + & - half cycle of high tension transformer used • efficient • circuit reverses negative half cycle & applies to x-ray tube Tube Output of High Tension Transformer Applied to X-ray Tube

  23. Applied to X-ray Tube Radiation Waveform Pulsed Radiation • single phase input power results in pulsed radiation • Disadvantages • intensity only significant when voltage is near peak • low voltage heats target and produces low-energy photons • absorbed in tube, filter, or patient • can contribute to dose

  24. Single Phase Power Three Phase Power Three-Phase Generators • Commercial power generally delivered as 3 phase • phases 120o apart

  25. Rectified Input 3 Phase Voltage To X-Ray Tube Three-Phase Generators • Rectifier circuit • Inverts negative voltage • sends highest of 3 phases to x-ray tube

  26. Single Phase Power Three Phase Output Three-Phase Generators • much higher tube ratings than single phase • more efficient than single phase • shorter exposures • lower exposure

  27. Three Phase Output 3f Generator Circuits • pulses • number of peaks per 1/60 second (16.6 msec) power line cycle • windings • 3 primary coils (one for each phase) • 3 or 6 secondary • with 6 secondaries, 2 secondary coils induced per primary

  28. Ripple • variation of kilovoltage from maximum • usually expressed as percentage of maximum kV Ripple

  29. Ripple Example 80 kVp 72 kVp Ripple = 80 - 72 = 8 kVp OR 8 / 80 = .1 = 10%

  30. Three Phase Output Ripple Typical Values • single phase • always 100 % (kV ranges from zero to maximum) • three phase • 4-13% • constant potential • 0 % • Medium / high frequency • very low; approx 0. Single Phase Output Constant Potential or High Frequency Output

  31. Three Phase Transforming • 3 coils can be hooked up in 2 ways Delta Wye

  32. 3-phase generator • Primary windings • generally delta • Secondary windings • may be delta or wye Primary Secondary

  33. Ripple Three Phase Output 3-phase generator • Six pulse six rectifier • one primary delta • one secondary wye • six rectifiers • One on each side of each secondary coil • 13.5% ripple Primary Secondary

  34. 3 Phase Generator • 6-Pulse Twelve Rectifier • 1 delta primary • 2 wye secondaries • 6 secondary windings • two diodes per winding • 13.5% ripple Primary Ripple Secondary Secondary Three Phase Output

  35. 3 Phase Generator • 12-Pulse Twelve Rectifier • 1 delta primary • 2 secondaries, 1 wye, 1 secondary • 30o phase difference between secondaries • 6 secondary windings • 2 diodes per winding • 3.5% ripple Primary Ripple Secondary Secondary Three Phase Output

  36. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer mA regulator • Circuitry for mA selection • Adjusts mA on the fly during exposure.

  37. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA selector Filament Transformer Filament Transformer Steps down AC voltage from Autotransformer & mA selector to smaller AC voltage required by filament (8-12 volts typical)

  38. Line Line Compensation mA selection • Allows selection from available discrete mA stations. • Applies correct voltage to primary of filament transformer. to filament transformer primary 10 mA 25 mA mA stabilizer 50 mA 100 mA 200 mA 300 mA 400 mA

  39. mA Stabilization During Exposure • On first trigger • mA regulator supplies anticipated voltage to filament transformer primary • mA monitored during exposure • Corrections made to filament voltage during exposure as necessary • if mA low, filament voltage boosted • if mA high, filament voltage lowered

  40. Generator kilowatt (kW) Rating • measured under load • kW rating changes with kVp • Standard • measure at 100 kVp

  41. Generator kW Rating • three phase • kV X mA / 1000 • mAmax / 10 at 100 kVp 1000 mA @ 70 kVp 800 mA @ 80 kVp 600 mA @ 100 kVp 300 mA @ 120 kVp 600 / 10 = 60 kW

  42. Generator kW Rating • single phase • kV X mA X 0.7 / 1000 • mAmax X 0.7 / 10 at 100 kVp 600 mA @ 70 kVp500 mA @ 80 kVp400 mA @ 100 kVp 250 mA @ 120 kVp 400 X 0.7 / 10 = 28 kW

  43. 1f vs. 3f Generators 1f 3f • Typical home & small business power • inexpensive • transformer windings • 1 primary coil • 1 secondary coil • Industrial power • expensive • transformer windings • 3 primary coils • one for each phase • 6 secondary coils • 2 secondary coils induced per primary)

  44. 1f vs. 3f Generators 1f 3f • 100% ripple • 8 ms minimum exp. Time • 1/120th second • lower output intensity • puts less heat in tube for same technique • 4-13% ripple • higher average kVp • slightly less patient exposure • <=1 ms minimum exp. time • higher output intensity • puts more heat in tube

  45. Exposure Time Control • mechanical • obsolete • electronic, measuring • time (crystal) • power line pulses • automatic (phototiming) • terminates exposure based on radiation received by receptor

  46. Grid Film Entrance type Sensor Exit type Sensor Phototiming Geometry • entrance type • detector in front of film • detector must be essentially invisible • exit type • detector behind film • obsolete except for mammography • detector visible because of high contrast image

  47. Phototiming Radiation Detectors • screen & photomultiplier tubes (PM Tubes) • obsolete • ionization chambers • solid-state detectors

  48. Ionization Chambers • Almost always entrance type • Notes • thin parallel aluminum plates are electrodes • voltage applied between plates • collect ions produced by radiation in air between electrodes • collected ions produce electric current Photon + - + -

  49. Electric Current Photon Solid State Detectors • PN semiconductor junction generates current when struck by radiation • small • fast response • little beam attenuation

  50. Phototiming Fields • 1, 2, or 3 • fields may be selected individually or in combination • proper positioning critical

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