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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
Resident Physics Lectures
  • Christensen, Chapter 3B

X-Ray Generator Circuit

George David

Associate Professor

Medical College of Georgia

Department of Radiology

x ray generator
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
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

x ray circuit

+

X-ray Circuit

High Voltage Transformer

Rectifier Circuit

Timer

Circuit

Auto-

trans-former

Line

mA selector

Filament Transformer

slide5

+

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
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

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
line voltage compensation

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
line voltage compensation9
Line Voltage Compensation
  • Compensation may be
    • automatic
      • most new & high end equipment
    • manual
      • user must make adjustment

Line

Line

Compensation

slide10

+

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

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

generator voltages
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

high voltage circuit
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
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
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
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

+

-

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 circuit18

+

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

-

-

halfwave rectified circuit

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

fullwave rectifier
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)

fullwave rectifier21

Voltage applied to tube

(also mA waveform)

+

-

X

X

X

X

-

+

Fullwave Rectifier

First Half Cycle

Second Half Cycle

full wave rectification
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

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
three phase generators

Single Phase Power

Three Phase Power

Three-Phase Generators
  • Commercial power generally delivered as 3 phase
  • phases 120o apart
three phase generators25

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
three phase generators26

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
3 f generator circuits

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

Ripple

ripple example
Ripple Example

80 kVp

72 kVp

Ripple = 80 - 72 = 8 kVp

OR

8 / 80 = .1 = 10%

ripple typical values

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

three phase transforming
Three Phase Transforming
  • 3 coils can be hooked up in 2 ways

Delta

Wye

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

Primary

Secondary

3 phase generator33

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

3 phase generator34
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 generator35
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

slide36

+

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.
slide37

+

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)

ma selection

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

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 rating
Generator kilowatt (kW) Rating
  • measured under load
  • kW rating changes with kVp
  • Standard
    • measure at 100 kVp
generator kw rating
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 rating42
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

1 f vs 3 f generators
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)
1 f vs 3 f generators44
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
Exposure Time Control
  • mechanical
    • obsolete
  • electronic, measuring
      • time (crystal)
      • power line pulses
  • automatic (phototiming)
    • terminates exposure based on radiation received by receptor
phototiming geometry

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

Electric Current

Photon

Solid State Detectors
  • PN semiconductor junction generates current when struck by radiation
  • small
  • fast response
  • little beam attenuation
phototiming fields
Phototiming Fields
  • 1, 2, or 3
  • fields may be selected individually or in combination
  • proper positioning critical
phototiming notes
Phototiming Notes
  • must be calibrated for particular film-screen system
  • some generators allow selection from several preset film/screen combinations
phototiming notes52
Phototiming 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)