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Rad T 290
Generators
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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
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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
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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
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Line
Line Voltage CompensationLine Voltage Compensation
• Compensation may be Automatic
» Almost everything today
Manual» user must make adjustment
LineCompensation
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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
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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
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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
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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
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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
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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)
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Fullwave RectifierFullwave Rectifier
+
-X
X
First Half Cycle Second Half Cycle
Voltage applied to tube(also mA waveform)
X
X
+
-
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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
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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
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Three-Phase GeneratorsThree-Phase Generators
• Commercial power generally delivered as 3 phase
• phases 120o apart
Single Phase Power Three Phase Power
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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
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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
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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
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Three Phase TransformingThree Phase Transforming
• 3 coils can be hooked up in 2 ways
Delta Wye
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3-phase generator3-phase generator
• Primary windings generally delta
• Secondary windings may be delta or wye
Primary
Secondary
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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
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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
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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
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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
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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
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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
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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
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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
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Exposure Time ControlExposure Time Control
• electronic, measuring» time (crystal)
» power line pulses
• automatic (phototimingphototiming) terminates exposure based on radiation received by
receptor
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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
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Phototiming Radiation DetectorsPhototiming Radiation Detectors
• ionization chambers
• solid-state detectors
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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
-
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Solid State DetectorsSolid State Detectors• PN semiconductor junction
generates current when struck by radiation
• small
• fast response
• little beam attenuation
Photon Electric Current
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Phototiming FieldsPhototiming Fields• 1, 2, or 3
• fields may be selected individually or in combination
• proper positioning critical
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Phototiming NotesPhototiming Notes
• must be calibrated for particular film-screen system
• some generators allow selection from several preset film/screen combinations
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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)