Resident Physics Lectures Christensen, Chapter 3B X-Ray Generator Circuit George David Associate...
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Transcript of Resident Physics Lectures Christensen, Chapter 3B X-Ray Generator Circuit George David Associate...
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)