Transcript Resident Physics Lectures

Rad T 290
Generators
Generator 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
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
Line 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
Circuit
Breaker
Line Voltage Compensation
• Compensation may be
Automatic
» Almost everything today
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
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 Voltages
• Input line voltage
single or three phase
115 - 480 Volts AC
1f
• Autotransformer
provides variable voltage to primary of high
voltage transformer
Auto
Transformer
Power
Line
3f
High Voltage
Transformer
Timer
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
Autotransformer
High Voltage
Transformer
mA
Rectifier
Circuit
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
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 Rectifier
Voltage applied to tube
(also mA waveform)
First Half Cycle
+
Second Half Cycle
-
X
X
X
X
+
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
Output of High Tension Transformer
Applied to X-ray Tube
Tube
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
Applied to X-ray Tube
Radiation Waveform
Three-Phase Generators
• Commercial power generally
delivered as 3 phase
• phases 120o apart
Single Phase Power
Three Phase Power
Three-Phase 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 Generators
• much higher tube ratings than
single phase
• more efficient than single phase
shorter exposures
lower exposure
Single Phase Power
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
Three Phase Output
Three Phase Transforming
• 3 coils can be hooked up in 2
ways
Delta
Wye
3-phase generator
• Primary windings
generally delta
• Secondary windings
may be delta or wye
Primary
Secondary
3 Phase 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
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
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 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 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
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)
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
• electronic, measuring
» time (crystal)
» power line pulses
• automatic (phototiming)
terminates exposure based on radiation received by
receptor
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
Grid
Film
Entrance type
Sensor
Exit type
Sensor
Phototiming Radiation Detectors
• ionization chambers
• solid-state detectors
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
• PN semiconductor junction
generates current when struck
by radiation
• small
• fast response
• little beam attenuation
Photon
Electric
Current
Phototiming Fields
• 1, 2, or 3
• fields may be selected
individually or in combination
• proper positioning critical
Phototiming Notes
• must be calibrated for particular
film-screen system
• some generators allow selection
from several preset film/screen
combinations
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)