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Resident Physics Lectures

Resident Physics Lectures. Christensen, Chapter 3 X-Ray Generators. George David Associate Professor of Radiology. Transformer Construction. Transformers have 2 coils of wire no electrical contact between coils When electric current passed through one coil

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Resident Physics Lectures

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  1. Resident Physics Lectures • Christensen, Chapter 3 X-Ray Generators George David Associate Professor of Radiology

  2. Transformer Construction • Transformers have 2 coils of wire • no electrical contact between coils • When electric current passed through one coil • magnetic field develops around first coil • second coil near enough to feel magnetic field Magnetic Field Current Flow

  3. Incoming AC Power Primary Coil Secondary Coil(s) Transformer Coil Designations • primary • coil to which power is applied • secondary • coil which feels magnetic field of primary coil

  4. Transformer Coils • When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil • power is induced in secondary coil • no physical connection Incoming AC Power Primary Coil Secondary Coil(s)

  5. NS = 850 • NP = 1200 Turns Ratio Definition • number of windings of secondary coil divided by number of windings of primary coil • 850 / 1200 for transformer below

  6. Transformer Theory • Transformers alter both voltage & current of AC waveforms • Voltage in secondary can be > or < voltage in primary Input AC Voltage & Current Output AC Voltage & Current

  7. Transformer Law • Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------- # Prim. Coils Prim. Voltage

  8. NS = 850 • NP = 1200 Transformer Law # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ----------------- # Prim. Coils Prim Voltage NS VS ----- = ----- NP VP 240 VAC ? V 850 ? ------ = ----- 1200 240 If VPRIM = 240 Volts then VSEC = 170 Volts

  9. Transformer Types • Step down Transformer • # primary coils > # secondary coils • primary voltage > secondary voltage • Step up Transformer • # primary coils < # secondary coils • primary voltage < secondary voltage

  10. Autotransformer Taps • Only one winding • incoming AC voltage connected across coils • primary • Output voltage proportional to # coils between taps • secondary Input NS NP Primary Secondary

  11. Autotransformer • Voltage law for autotransformers same as for transformers • Secondary voltage adjustable by moving to a different tap • changes # secondary coils NS Input NS NP NS VS ----- = ----- NP VP Primary

  12. Input 50 100 Primary Autotransformer 80 Input 50 100 20 Primary

  13. Rectification • Changes alternating current output of high voltage transformer to direct current • allows current flow in one direction only • x-ray tube is a rectifier because current will not flow from anode to cathode • no source of free electrons at anode

  14. Diodes are One Way Streets for Electric Current • Allows current to flow cathode to anode • Blocks current from flowing anode to cathode No Yes C A C A

  15. 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

  16. 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

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

  18. + 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

  19. 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

  20. + 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

  21. 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

  22. 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

  23. 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

  24. + 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 - -

  25. 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)

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

  27. 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

  28. 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

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

  30. 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

  31. 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

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

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

  34. 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

  35. + High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer Timer • Starts & stops exposure • Turns transformer primary (low voltage) on & off

  36. Exposure Timing • Manual • Operator sets time • Automatic (Phototimed) • Equipment measures exposure • Terminates exposure when designated exposure is measured

  37. Grid Recptor 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 film mammography • detector visible because of high contrast image

  38. Ionization Chambers • thin parallel aluminum plates are electrodes • voltage applied between plates • radiation ionizes air • ions collected in air between electrodes • collected ions produce electric current Photon + - + -

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

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

  41. + Primary switching High Voltage Transformer Rectifier Circuit Timer Circuit Auto- trans-former Line mA regulator Filament Transformer • Switching (timing) almost always done using low voltage • High voltage switching used only in high end applications • Angio • Cardiac Cath

  42. Primary Switch Types • solid state • silicon-controlled rectifiers (SCR’s or thyristers) • turned on by voltage pulse • Can only be turned off by interrupting current through it

  43. + High Voltage Transformer Rectifier Circuit Timer Circuit Secondary switching Auto- trans-former Line mA regulator Filament Transformer • High voltage switched, not filament • Requirement • fast • smooth

  44. + 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.

  45. + 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)

  46. Power Storage Generators • Application • Remote locations • Inadequate power from power line • Outlet inaccessible • Types • Battery-powered generators • Capacitor discharge generators

  47. Battery-Powered Generators • Batteries used for • x-ray • transport • Independent of power line during exposure • Disadvantages • Batteries must be charged • Batteries maintenance • Heavy • Battery’s DC converted to AC for high voltage transformer to operate

  48. Medium (or high) Frequency Generators • higher frequency square wave voltage sent to primary of high voltage transformer • Conventional generators use power line’s sine wave • very efficient • transformer & generator very small • some transformers integral with x-ray tube head

  49. Medium FrequencyGenerator Operation • incoming AC converted to DC DC AC First Step

  50. Medium FrequencyGenerator Operation • Pulsating DC smooth to constant voltage Pulsating DC Constant DC Second Step

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