Download X-Ray Generators

Resident Physics Lectures
 Christensen, Chapter 3
X-Ray
Generators
George David
Associate 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
Magnetic Field
Current Flow
Transformer Coil Designations
Incoming
AC Power
 primary
 coil to which power is
applied
 secondary
 coil which feels magnetic
field of primary coil
Primary
Coil
Secondary
Coil(s)
Transformer Coils
 When secondary coil feels changing
(increasing or decreasing) magnetic
field of primary coil
 power is induced in secondary coil
 no physical connection
Incoming
AC Power
Primary
Coil
Secondary
Coil(s)
Turns Ratio Definition
 number of windings of secondary coil divided by
number of windings of primary coil
 850 / 1200 for transformer below
NP = 1200
NS = 850
Transformer Theory
 Transformers alter both voltage & current of
AC waveforms
 Voltage in secondary can be > or < voltage in
primary
Input AC
Voltage &
Current
Output AC
Voltage &
Current
Transformer Law
 Voltage 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
NS VS
----- = ----NP VP
240
VAC
?V
NP = 1200
NS = 850
850
?
------ = ----1200 240
If VPRIM = 240 Volts then VSEC = 170 Volts
Transformer Types
 Step down Transformer
 # primary coils > # secondary coils
 primary voltage > secondary voltage
Step
up Transformer
• # primary coils < # secondary coils
• primary voltage < secondary voltage
Autotransformer
Taps
 Only one winding
 incoming AC voltage
connected across coils
 primary
 Output voltage
Input
proportional to # coils
between taps
NP
NS
 secondary
Primary
Secondary
Autotransformer
 Voltage law for
autotransformers
same as for
transformers
 Secondary voltage
adjustable by moving Input
to a different tap
NP
 changes # secondary coils NS
NS VS
----- = ----NP VP
Primary
NS
Autotransformer
Input
100
80
50
20
Primary
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 cathode
 no source of free electrons at anode
Diodes are One Way Streets for
Electric Current
 Allows current to flow cathode to anode
 Blocks current from flowing anode to cathode
C
A
C
A
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
 control console
 kVp adjust
 mA adjust
or mAs
adjust
 time adjust
 transformer
 high voltage (step up)
 filament

low voltage (step down)
 electronics cabinet
 support circuitry
X-ray Circuit
Timer
Circuit
Autotransformer
mA
selector
Rectifier
Circuit
+
Line
High
Voltage
Transformer
Filament
Transformer
Timer
Circuit
Rectifier
Circuit
Autotransformer
mA
selector
+
Line
High
Voltage
Transformer
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
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
Incoming
Power
Line
Generator
Circuit
Breaker
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
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
Halfwave Rectifier Circuit
+
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)
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 &
Tube
applies to x-ray tube
Output of High Tension Transformer
Applied to X-ray 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
Ripple
 variation of kilovoltage from maximum
 usually expressed as percentage of
maximum kV
Ripple
Ripple Example
80 kVp
72 kVp
Ripple = 80 - 72 = 8 kVp
OR
8 / 80 = .1 = 10%
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
Three Phase Output
Constant Potential or
High Frequency Output
Timer
Circuit
Autotransformer
mA
regulator
Rectifier
Circuit
+
Line
High
Voltage
Transformer
Filament
Transformer
Timer
•Starts & stops exposure
•Turns transformer primary (low voltage) on & off
Exposure Timing
 Manual
 Operator sets time
 Automatic (Phototimed)
 Equipment measures exposure
 Terminates exposure when
designated exposure is
measured
Phototiming Geometry
 entrance type
 detector in front of film
 detector must be
essentially invisible
 exit type
 detector behind film
 obsolete except for film
mammography

detector visible because
of high contrast image
Grid
Recptor
Entrance type
Sensor
Exit type
Sensor
Ionization Chambers
 thin parallel aluminum
plates are electrodes
 voltage applied between
plates
 radiation ionizes air
 ions collected 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
Primary
switching
Timer
Circuit
Rectifier
Circuit
Autotransformer
mA
regulator
+
Line
High
Voltage
Transformer
Filament
Transformer
•Switching (timing) almost always done using
low voltage
•High voltage switching used only in high end
applications
•Angio
•Cardiac Cath
Primary Switch Types
 solid state
 silicon-controlled
rectifiers (SCR’s
or thyristers)
 turned on by
voltage pulse
 Can only be
turned off by
interrupting
current through it
High
Voltage
Transformer
Secondary switching
Autotransformer
mA
regulator
+
Line
Filament
Transformer
 High voltage switched, not
filament
 Requirement
 fast
 smooth
Rectifier
Circuit
Timer
Circuit
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)
Power Storage Generators
 Application
 Remote locations
 Inadequate power from power line
 Outlet inaccessible
 Types
 Battery-powered generators
 Capacitor discharge generators
Battery-Powered Generators
 Batteries used for
 x-ray
 transport
 Independent of power line during
exposure
 Disadvantages
 Batteries must be charged
 Batteries maintenance
 Heavy
 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
transformer
 Conventional generators use power line’s
sine wave
 very efficient
 transformer & generator very small
 some transformers integral with x-ray tube
head
Medium Frequency
Generator Operation
 incoming AC converted to DC
AC
DC
Medium Frequency
Generator Operation
 Pulsating DC smooth to constant voltage
Pulsating DC
Constant DC
Medium Frequency
Generator Operation
 smooth DC regulated to voltage level proper for
primary
Constant DC
Exposure-level DC
Medium Frequency
Generator Operation
 DC converted (chopped) to AC
Exposure-level DC
Chopped AC
Medium Frequency
Generator Operation
 AC sent to transformer
primary
 normal transforming &
rectification
 high voltage smoothing
Medium Frequency Generators
 Advantages
 conducive to computer control
 immune to power line fluctuations
 low ripple
 small size of electronics & transformer

small enough to spin on CT gantry
 Today’s trend in generators