Resident Physics Lectures - Weebly

Resident Physics Lectures - Weebly

Resident Resident Physics Physics Lectures Lectures Christensen, Chapter 3B X-Ray Generator Circuit George David Associate Professor Medical College of Georgia Department of Radiology X-Ray X-Ray Generator Generator Supplies electrical power to xray 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 Generator Components Components control console kVp adjust mA adjust time adjust or mAs adjust

transformer high voltage (step up) filament low voltage (step down) electronics cabinet support circuitry X-ray X-ray Circuit Circuit Timer Circuit Autotransformer mA selector

Rectifier Circuit + Line High Voltage Transformer Filament Transformer Timer Circuit Rectifier Circuit

Autotransformer mA selector + Line High Voltage Transformer 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 Incoming Power 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

Circuit Breaker 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 Line Voltage Voltage Compensation

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 Line Voltage Voltage Compensation Compensation Compensation may be automatic

most new & high end equipment manual user must make adjustment Line Line Compensation Timer Circuit Autotransformer mA regulator Rectifier Circuit

+ Line High Voltage Transformer Filament Transformer Autotransformer High voltage Transformer has fixed ratio Autotransformer has variable ratio Autotransformer needed to provide variable kilovoltage to tube Autotransformer Autotransformer major kV

selector Timer Circuit Line minor kV selector to high voltage transformer primary to filament transformer primary mA regulator

Line Compensation Autotransformer does line compensation & kVp selection Generator Generator Voltages Voltages Input line voltage single or three phase 115 - 480 Volts AC 1 Autotransformer provides variable voltage to primary of high voltage transformer Auto

Transformer Power Line High Voltage Transformer Timer Circuit High High Voltage Voltage Circuit 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 Autotransformer High Voltage Transformer mA Rectifier Circuit High High Voltage Voltage Transformer 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 Self (tube) (tube) Rectified Rectified Circuit 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 Self-rectification Disadvantages Disadvantages Wasted hot anode can emit electrons accelerate & can destroy filament half of electrical cycle wasted

Used Voltage applied to x-ray tube mA waveform X-Rays Produced Halfwave Halfwave Rectifier Rectifier Circuit 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 Halfwave Rectifier Rectifier Circuit 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 Halfwave Rectified Rectified Circuit Circuit 60 pulses per second only positive half cycle of high tension transformer used

inefficient negative half cycle wasted Secondary of High Voltage Transformer Output of High Tension Transformer Applied to x-ray tube Blocked (not used) Applied to X-ray Tube Fullwave Fullwave Rectifier Rectifier Four diodes 120 pulses/second exposure times half of halfwave circuit

Secondary of High Voltage Transformer Voltage applied to tube (also mA waveform) Fullwave Fullwave Rectifier Rectifier Voltage applied to tube (also mA waveform) First Half Cycle + Second Half Cycle

X X X - X + Full-Wave Full-Wave Rectification 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 Output of High Tension Transformer Applied to X-ray Tube Tube Pulsed Pulsed Radiation 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 Three-Phase Generators Generators Commercial power generally delivered as 3 phase phases 120o apart Single Phase Power Three Phase Power Three-Phase Three-Phase Generators

Generators Rectifier circuit Inverts negative voltage sends highest of 3 phases to x-ray tube Input 3 Phase Voltage To X-Ray Tube Rectified Three-Phase Three-Phase Generators Generators much higher tube ratings than single phase more efficient than single phase shorter exposures lower exposure

Single Phase Power Three Phase Output 3 3 Generator Generator Circuits 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

Three Phase Output Ripple Ripple variation of kilovoltage from maximum usually expressed as percentage of maximum kV Ripple Ripple Ripple Example Example 80 kVp 72 kVp Ripple = 80 - 72 = 8 kVp OR 8 / 80 = .1 = 10%

Ripple Ripple Typical Typical Values Values single phase always 100 % (kV ranges from zero to maximum) Single Phase Output three phase 4-13% constant potential Three Phase Output 0 %

Medium / high frequency very low; approx 0. Constant Potential or High Frequency Output Three Three Phase Phase Transforming Transforming 3 coils can be hooked up in 2 ways Delta Wye

3-phase 3-phase generator generator Primary windings generally delta Secondary windings may be delta or wye Primary Secondary 3-phase 3-phase generator generator Six pulse six rectifier one primary delta one secondary wye

six rectifiers Primary One on each side of each secondary coil 13.5% ripple Ripple Three Phase Output Secondary 33 Phase Phase Generator Generator 6-Pulse Twelve Rectifier 1 delta primary

2 wye secondaries 6 secondary windings two diodes per winding Primary 13.5% ripple Ripple Secondary Three Phase Output Secondary 33 Phase Phase Generator 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 Primary 3.5% ripple Ripple Secondary Three Phase Output Secondary Timer Circuit

Autotransformer mA regulator Rectifier Circuit + Line High Voltage Transformer Filament Transformer

mA regulator Circuitry for mA selection Adjusts mA on the fly during exposure. Timer Circuit Rectifier Circuit Autotransformer mA selector + Line

High Voltage Transformer Filament Transformer Filament Transformer Steps down AC voltage from Autotransformer & mA selector to smaller AC voltage required by filament (8-12 volts typical) mA mA selection selection Allows selection from available discrete mA stations. Applies correct voltage to primary of

filament transformer. Line 10 mA 25 mA 50 mA 100 mA 200 mA 300 mA 400 mA Line Compensation mA stabilizer to filament transformer primary

mA mA Stabilization Stabilization During During Exposure 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 Generator kilowatt

kilowatt (kW) (kW) Rating Rating measured under load kW rating changes with kVp Standard measure at 100 kVp Generator Generator kW kW Rating 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 Generator kW kW Rating 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 1 1 vs. vs. 3

3 Generators Generators 1 3 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) 1 1 vs. vs. 3 3 Generators Generators 1 3 100% 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 Exposure Time Time Control Control mechanical obsolete electronic, measuring time (crystal) power line pulses automatic (phototiming) phototiming

terminates exposure based on radiation received by receptor Phototiming Phototiming Geometry 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 Grid

Film Entrance type Sensor Exit type Sensor Phototiming Phototiming Radiation Radiation Detectors Detectors screen & photomultiplier tubes (PM Tubes) obsolete ionization chambers solid-state detectors Ionization

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

Phototiming Phototiming Fields Fields 1, 2, or 3 fields may be selected individually or in combination proper positioning critical Phototiming Phototiming Notes Notes must be calibrated for particular film-screen system some generators allow selection from several preset film/screen combinations

Phototiming Phototiming Notes Notes phototimer must correct for 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)

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