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 Controls exposure timing Turns exposure on and off High voltage switched on and off Filament heated before exposure

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

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

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

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

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 Incoming Power Line Generator 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 Incoming Power Line Generator Circuit Breaker

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

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

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

Generator Voltages Input line voltage Autotransformer 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

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

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

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

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

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

Halfwave Rectifier Circuit + - 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 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 Blocked (not used) Applied to x-ray tube Output of High Tension Transformer Applied to X-ray Tube

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)

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

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

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 Applied to X-ray Tube Radiation Waveform

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

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

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

3f Generator Circuits pulses windings 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 Three Phase Output

Ripple variation of kilovoltage from maximum usually expressed as percentage of maximum kV Ripple

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

Constant Potential or High Frequency 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 Three Phase Output Constant Potential or High Frequency Output

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

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

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 Three Phase Output Ripple Secondary

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

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

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.

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)

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

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

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

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

Generator kW Rating single phase kV X mA X 0.7 / 1000 mAmax X 0.7 / 10 at 100 kVp 600 mA @ 70 kVp 500 mA @ 80 kVp 400 mA @ 100 kVp 250 mA @ 120 kVp 400 X 0.7 / 10 = 28 kW

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)

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

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

Phototiming Geometry entrance type exit 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 Exit type Sensor Grid Film Entrance type

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

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 + - + -

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

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

Phototiming Notes must be calibrated for particular film-screen system some generators allow selection from several preset film/screen combinations

Phototiming Notes phototimer must correct for safety rate response kVp response of film/screen system phototiming sensor Higher kVp beam more penetrating Less attenuated by phototimer detector safety exposure limited to 600 mAs if phototimer does not terminate exposure (2000 mAs for < 50 kV)