Resident Physics LecturesResident Physics Lectures

• Christensen, Chapter 3B

X-Ray Generator Circuit

George DavidAssociate ProfessorMedical College of GeorgiaDepartment of Radiology

X-Ray GeneratorX-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 ComponentsGenerator 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

X-ray CircuitX-ray Circuit

LineAuto-trans-former

High Voltage

Transformer

Filament Transformer

Rectifier Circuit

TimerCircuit

+

mA selector

LineAuto-trans-former

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

+

Incoming line voltage connected to generator through a circuit breaker.circuit breaker.Typ. 220-240 volt AC single phase240, 480 volt AC three phase

Line

Filament Transformer

mA selector

Incoming PowerIncoming 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 BreakerCircuit 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

CircuitBreaker

Line Voltage CompensationLine 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

CircuitBreaker

Line

Line Voltage CompensationLine Voltage Compensation

• Compensation may be automatic

» most new & high end equipment

manual» user must make adjustment

LineCompensation

LineAuto-trans-former

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

+

•High voltage Transformer has fixed ratio•Autotransformer has variable ratio•Autotransformer needed to provide variable kilovoltage to tube

Autotransformer

Filament Transformer

mA regulator

AutotransformerAutotransformer

Line

LineCompensation

TimerCircuit

to high voltage transformer

primary

to filament transformer

primarymA

regulator

major kV selector

minor kV selector

Autotransformer does line compensation & kVp selection

Generator VoltagesGenerator Voltages

• Input line voltage single or three phase 115 - 480 Volts AC

• Autotransformer provides variable voltage to primary of high

voltage transformer

1

PowerLine

AutoTransformer

High VoltageTransformer

TimerCircuit

High Voltage CircuitHigh 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

RectifierCircuitmA

High Voltage Transformer

High Voltage TransformerHigh 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 CircuitSelf (tube) Rectified Circuit

Secondary of High Voltage Transformer

mA waveform

•X-Ray tube acts as rectifier•Current only flows from cathode to anode

•cathode is source of free electrons•Rarely seen

Voltage applied to tube

Self-rectification Disadvantages

Self-rectification Disadvantages

• hot anode can emit electrons accelerate & can destroy

filament

• half of electrical cycle wasted

Voltage applied tox-ray tube

mA waveform

UsedWasted

X-Rays Produced

Halfwave Rectifier CircuitHalfwave 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 CircuitHalfwave Rectifier Circuit

+

-

X Second Half Cycle:Diodes openNo voltage applied to tubeNo tube current (mA)

+

-

First Half Cycle:Diodes closedVoltage applied to tubeTube current (mA) results

-

-

Halfwave Rectified CircuitHalfwave Rectified Circuit

Secondary of High Voltage Transformer

•60 pulses per second•only positive half cycle of high tension transformer used

•inefficient•negative half cycle wasted

Blocked (not used)

Applied to x-ray tube

Output of High Tension Transformer Applied to X-ray Tube

Fullwave RectifierFullwave 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 RectifierFullwave Rectifier

+

-X

X

First Half Cycle Second Half Cycle

Voltage applied to tube(also mA waveform)

X

X

+

-

Full-Wave RectificationFull-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

Applied to X-ray TubeOutput of High Tension Transformer

Tube

Pulsed RadiationPulsed 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 GeneratorsThree-Phase Generators

• Commercial power generally delivered as 3 phase

• phases 120o apart

Single Phase Power Three Phase Power

Three-Phase GeneratorsThree-Phase Generators• Rectifier circuit

Inverts negative voltage sends highest of 3 phases to x-ray tube

To X-Ray Tube

Input 3 Phase VoltageRectified

Three-Phase GeneratorsThree-Phase Generators• much higher tube ratings than

single phase• more efficient than single phase

shorter exposures lower exposure

Three Phase OutputSingle Phase Power

3 Generator Circuits3 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

Three Phase Output

RippleRipple

• variation of kilovoltage from maximum• usually expressed as percentage of

maximum kV

Ripple

Ripple ExampleRipple Example

Ripple = 80 - 72 = 8 kVpOR

8 / 80 = .1 = 10%

80 kVp

72 kVp

Ripple Typical ValuesRipple 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.

Three Phase Output

Single Phase Output

Constant Potential or High Frequency Output

Three Phase TransformingThree Phase Transforming

• 3 coils can be hooked up in 2 ways

Delta Wye

3-phase generator3-phase generator

• Primary windings generally delta

• Secondary windings may be delta or wye

Primary

Secondary

3-phase generator3-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

Three Phase Output

Ripple

Primary

Secondary

3 Phase Generator3 Phase Generator

• 6-Pulse Twelve Rectifier 1 delta primary 2 wye secondaries

» 6 secondary windings

• two diodes per winding

13.5% ripple

Three Phase Output

Ripple

Primary

SecondarySecondary

3 Phase Generator3 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

Three Phase Output

Ripple

Primary

SecondarySecondary

LineAuto-trans-former

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

+

•Circuitry for mA selection•Adjusts mA on the fly during exposure.

mA regulator

Filament Transformer

mA regulator

LineAuto-trans-former

High Voltage

Transformer

Rectifier Circuit

TimerCircuit

+

Steps down AC voltage from Autotransformer & mA selector to smaller AC voltage required by filament (8-12 volts typical)

Filament Transformer

Filament Transformer

mA selector

mA selectionmA selection

Line

LineCompensation

to filament transformer

primarymAstabilizer

•Allows selection from available discrete mA stations.•Applies correct voltage to primary of filament transformer.

10 mA

25 mA

50 mA100 mA

200 mA

300 mA

400 mA

mA Stabilization During Exposure

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) RatingGenerator kilowatt (kW) Rating

• measured under load

• kW rating changes with kVp

• Standard measure at 100 kVp

Generator kW RatingGenerator 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 RatingGenerator 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 kVp250 mA @ 120 kVp

400 X 0.7 / 10 = 28 kW

1 vs. 3 Generators1 vs. 3 Generators

• 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 3

1 vs. 3 Generators1 vs. 3 Generators

• 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

1 3

Exposure Time ControlExposure Time Control

• mechanical obsolete

• electronic, measuring» time (crystal)

» power line pulses

• automatic (phototimingphototiming) terminates exposure based on radiation received by

receptor

Phototiming GeometryPhototiming 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

Exit typeSensor

Grid

FilmEntrance typeSensor

Phototiming Radiation DetectorsPhototiming Radiation Detectors

• screen & photomultiplier tubes (PM Tubes)

» obsolete

• ionization chambers

• solid-state detectors

Ionization ChambersIonization 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 DetectorsSolid State Detectors• PN semiconductor junction

generates current when struck by radiation

• small

• fast response

• little beam attenuation

Photon Electric Current

Phototiming FieldsPhototiming Fields• 1, 2, or 3

• fields may be selected individually or in combination

• proper positioning critical

Phototiming NotesPhototiming Notes

• must be calibrated for particular film-screen system

• some generators allow selection from several preset film/screen combinations

Phototiming NotesPhototiming 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)