Download - Resident Physics Lectures
Resident Physics Lectures
Christensen, Chapter 3
X-Ray Generators
George DavidAssociate Professor of Radiology
Transformer Construction
Transformers have 2 coils of wire no electrical contact between coils
When electric current passed through one coil magnetic field develops around first coil second coil near enough to feel magnetic field
Current Flow
Magnetic Field
Transformer Coil Designations
primarycoil to which power is
appliedsecondary
coil which feels magnetic field of primary coil
PrimaryCoil
SecondaryCoil(s)
IncomingAC Power
Transformer Coils
When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil power is induced in secondary coilno physical connection
PrimaryCoil
SecondaryCoil(s)
IncomingAC Power
Turns RatioTurns Ratio Definition
number of windings of secondary coil divided by number of windings of primary coil850 / 1200 for transformer below
NP = 1200 NS = 850
Transformer TheoryTransformers alter both voltage &
current of AC waveformsVoltage in secondary can be > or <
voltage in primary
Input AC Voltage &
Current
Output AC Voltage &
Current
Transformer LawVoltage Ratio = Turns Ratio
# Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------- # Prim. Coils Prim. Voltage
Transformer Law # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ----------------- # Prim. Coils Prim Voltage
NP = 1200 NS = 850
If VPRIM = 240 Volts then VSEC = 170 Volts
850 ?------ = -----1200 240
NS VS ----- = ----- NP VP
240VAC ? V
Transformer TypesStep down Transformer
# primary coils > # secondary coilsprimary voltage > secondary voltage
Step up Transformer• # primary coils < # secondary coils
• primary voltage < secondary voltage
Autotransformer
Only one windingincoming AC
voltage connected across coilsprimary
Output voltage proportional to # coils between tapssecondary
Primary
NP
Input NS
Secondary
Taps
AutotransformerVoltage law for
autotransformers same as for transformers
Secondary voltage adjustable by moving to a different tap changes # secondary coils NS
Primary
NP
Input NS
NS VS ----- = ----- NP VP
Autotransformer
Primary
100Input 50
Primary
100Input 50
20
80
Rectification
Changes alternating current output of high voltage transformer to direct current
allows current flow in one direction only x-ray tube is a rectifier because current will not
flow from anode to cathodeno source of free electrons at anode
Diodes are One Way Streets for Electric Current
Allows current to flow cathode to anodeBlocks current from flowing anode to cathode
C A C A
X-Ray Generator
Supplies electrical power to x-ray tube
high voltage between anode & cathodefilament voltage
Controls exposure timingTurns exposure on and offHigh voltage switched on and off
Filament heated before exposure
Generator Componentscontrol console
kVp adjustmA adjusttime adjust
transformerhigh voltage (step up)filament
low voltage (step down)
electronics cabinetsupport circuitry
or mAs adjust
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
Circuit BreakerGenerator connected to power line through
a circuit breakerLimits current from power line to
generator Allows generator to be disconnected from
power line
Incoming Power Line
Generator
CircuitBreaker
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
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
High Voltage CircuitSupplies high voltage for x-ray tubeStep-up transformer
primary from autotransformersecondary to rectifier circuitmA monitored at center grounded point of
secondary
Auto-transformer
RectifierCircuitmA
High Voltage Transformer
High Voltage Transformer
Grounded metal boxfilled with oil
electrical insulatorFunction
increases or decreases alternating voltage
Also contains rectifier circuit
changes alternating current into direct current
Halfwave 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
-
-
Fullwave RectifierFour diodes120 pulses/secondexposure times half of halfwave circuit
Secondary of High Voltage Transformer
Voltage applied to tube(also mA waveform)
Fullwave Rectifier
+
-X
X
First Half Cycle Second Half Cycle
Voltage applied to tube(also mA waveform)
X
X
+
-
Full-Wave RectificationRectifiers
Four diode “bridge” configuration used with single phase
both + & - half cycle of high tension transformer usedefficientcircuit reverses negative half cycle
& applies to x-ray tube
Applied to X-ray TubeOutput of High Tension Transformer
Tube
Pulsed Radiationsingle 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 GeneratorsRectifier circuit
Inverts negative voltagesends highest of 3 phases to x-ray tube
To X-Ray Tube
Input 3 Phase VoltageRectified
Three-Phase Generatorsmuch higher tube ratings than single
phasemore efficient than single phase
shorter exposureslower exposure
Three Phase OutputSingle Phase Power
Ripple
variation of kilovoltage from maximum
usually expressed as percentage of maximum kV
Ripple
Ripple Example
Ripple = 80 - 72 = 8 kVpOR
8 / 80 = .1 = 10%
80 kVp
72 kVp
Ripple Typical Values
single phasealways 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
LineAuto-trans-former
High Voltage
Transformer
Rectifier Circuit
TimerCircuit
+
•Starts & stops exposure•Turns transformer primary (low voltage) on & off
Timer
Filament Transformer
mA regulator
Manual
Operator sets time
Automatic (Phototimed)
Equipment measures
exposure
Terminates exposure
when designated exposure
is measured
Phototiming Geometryentrance type
detector in front of filmdetector must be
essentially invisibleexit type
detector behind filmobsolete except for
film mammography detector visible
because of high contrast image
Exit typeSensor
Grid
RecptorEntrance typeSensor
Ionization Chambersthin parallel aluminum
plates are electrodesvoltage applied
between platesradiation ionizes airions collected in air
between electrodescollected ions produce
electric current
+-
+Photon
-
Solid State Detectors
PN semiconductor junction generates current when struck by radiation
smallfast responselittle beam attenuation
Photon Electric Current
Phototiming Fields1, 2, or 3fields may be selected individually or in
combinationproper positioning critical
LineAuto-trans-former
High Voltage
Transformer
Rectifier Circuit
TimerCircuit
+
•Switching (timing) almost always done using low voltage•High voltage switching used only in high end applications
•Angio•Cardiac Cath
Filament Transformer
mA regulator
Primary switching
Primary Switch Typessolid statesilicon-
controlled rectifiers (SCR’s or thyristers)
turned on by voltage pulse
Can only be turned off by interrupting current through it
LineAuto-trans-former
High Voltage
Transformer
Rectifier Circuit
+
Filament Transformer
mA regulator
Secondary switching
TimerCircuit
High voltage switched, not filament
Requirementfastsmooth
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
Power Storage Generators
ApplicationRemote locationsInadequate power from power lineOutlet inaccessible
TypesBattery-powered generatorsCapacitor discharge generators
Battery-Powered Generators
Batteries used forx-raytransport
Independent of power line during exposure
DisadvantagesBatteries must be chargedBatteries maintenanceHeavy
Battery’s DC converted to AC for high voltage transformer to operate
Medium (or high) Frequency Generators
higher frequency square wave voltage sent to primary of high voltage transformerConventional generators use power line’s
sine wave
very efficienttransformer & generator very smallsome transformers integral with x-ray tube
head
Medium FrequencyGenerator Operation
incoming AC converted to DC
AC DC
Medium FrequencyGenerator Operation
Pulsating DC smooth to constant voltage
Constant DCPulsating DC
Medium FrequencyGenerator Operation
smooth DC regulated to voltage level proper for primary
Exposure-level DCConstant DC
Medium FrequencyGenerator Operation
DC converted (chopped) to AC
Chopped ACExposure-level DC
Medium FrequencyGenerator Operation
AC sent to transformer primary
normal transforming & rectification
high voltage smoothing
Medium Frequency Generators
Advantagesconducive to computer controlimmune to power line fluctuationslow ripplesmall size of electronics & transformer
small enough to spin on CT gantry
Today’s trend in generators