Download X-Ray Generators

Resident Physics Lectures (Year 1)
 Christensen, Chapter 3
X-Ray
Generators
George David, MS FAAPM, FACR
Associate Professor of Radiology
Requirements to Produce X-Rays
 Filament Voltage
 High Voltage
anode
+
high
voltage
source
filament
filament
voltage
source
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
 Filament heated before exposure
 High voltage switched on and off
anode
+
high
voltage
source
filament
filament
voltage
source
Voltage
 Voltage from US power company

Home


120 / 240 V
Industrial


480 V
240 V
 Voltage required

Home





Most stuff: 120 V
AC / Dryer: 240 V
Door bell: 15 V
Computer 5 V
X-Ray


Filament: 8-12 V
High voltage: 40-150 kV (40,000 – 150,000)
Transformers
 Magical devices that allow voltage to
be changed to any desired value
Current Flow and Magnetic Fields
 Magnetic field surrounds conductor carrying electric
current
 Magnetic field concentrated by coiling conductor
Magnetic Field
N
Magnetic Field
S
Current Flow
Transformer Construction
 Transformers have 2 coils of wire
 coils not in electrical contact with each another
 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
 primary
 coil to which power is applied
 secondary
 coil which feels magnetic field
of primary coil
Secondary
Coil(s)
Transformer Coils
 When secondary coil feels changing (increasing or
decreasing) magnetic field of primary coil
 power induced in secondary coil
 no physical connection
Incoming
AC Power
Primary
Coil
Secondary
Coil(s)
Turns Ratio Definition
TR=NS / NP
 number of windings of secondary coil divided by
number of windings of primary coil
 40 / 20 for transformer below
NP = 20
NS = 40
Transformer Theory
 transformers do not work with direct (unchanging) current
(DC)
 Current induced in secondary coil only when primary coil
current / magnetic field is changing
Input DC
Voltage &
Current
No Output
Voltage
Voltage
Time
Time
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
Turns Ratio = ---------------# Prim. Coils
NS VS
----- = ----NP VP
Sec. Voltage
= ------------------Prim. Voltage
Transformer Types
 Step down Transformer
 # primary coils > # secondary coils
 primary voltage > secondary voltage
 Step
up Transformer
• # primary coils < # secondary coils
• primary voltage < secondary voltage
How Does a Transformer
Magically Increase Voltage
without Some Source of Power?
Transformer Law (cont.)
 Current ratio is inverse of voltage ratio
# Sec. Coils
Sec. Voltage Prim. Current
----------------- = --------------- = ----------------# Prim. Coils Prim. Voltage Sec. Current
NS
VS
IP
----- = ----- = ----NP
VP
IS
VPRIM X IPRIM = VSEC X ISEC
Transformers
Power = Voltage X Current
 Electrical power not changed
 Current exchanged for voltage
 Voltage goes up – current goes down
 Voltage goes down – current goes up
Power
Power is rate of energy usage
Power defined as Voltage X Current
Units
 Voltage => Volts
 Current => Amps
 Power => Watts
•Voltage => Kilovolts
•Current => milliamps
•Power => Watts
Power
 Power = Voltage X Current
 Transformer primary power = secondary power
 transformer neither creates nor consumes power
PowerPRIM = PowerSEC
VPRIM X IPRIM = VSEC X ISEC
Transformer Ratio
 Ratio = Output voltage / Input voltage
 Most transformers have fixed ratios
 X-Ray requires variable ratios
 Accommodate selection of different
kVs
Autotransformer
Taps
 Only one winding
 Incoming AC voltage
connected across coils
 primary
Input
 Output voltage proportional
NP
NS
to # coils between taps
 secondary
Primary
Secondary
Autotransformer
 Voltage law for
autotransformers same
as for transformers
 Variable ratio
transformer
 Secondary voltage
adjustable by moving to a Input
NP
different tap
 changes # secondary
coils NS
NS VS
----- = ----NP VP
Primary
NS
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
Line
Autotransformer
mA
regulator
Timer
Circuit
Rectifier
Circuit
Fixed
ratio
+
Variable
ratio
High
Voltage
Transformer
Filament
Transformer
•Auto transformer
• Variable ratio transformer allowing operator to specify kVp
•High voltage transformer
• Boosts output of autotransformer by fixed ratio
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 radiation
 Terminates exposure when
designated radiation is
measured
Phototiming
 Detector in front of
receptor
 Must be essentially
invisible
Radiation
Sensor
Grid
Image
Receptor
Phototiming Fields
 1, 2, or 3
 fields may be selected individually or in
combination
 proper positioning critical
Timer
Circuit
Rectifier
Circuit
Autotransformer
mA
regulator
+
Line
High
Voltage
Transformer
Filament
Transformer
Timer
•Develops DC high voltage for x-ray tube
High Voltage Transformer
 Grounded metal box filled with oil
 electrical insulator
 Function
 Develops proper high voltage for tube
 Also contains rectifier circuit
Rectification
 allows current flow in one direction only
 Transformers only work with alternating current (AC)
 Rectifier changes alternating current output of high
voltage transformer to direct current
 DC voltage applied to tube
Timer
Circuit
Rectifier
Circuit
Autotransformer
mA
regulator
+
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
Changes AC voltage to smaller voltage
required by filament (8-12 volts typical)
Power Storage Generators
 Use batteries
 Application
 Remote locations
 Inadequate power from power
line or power line not accessible
 Outlet inaccessible
Battery-Powered Generators
 Batteries used for
 x-ray
 transport
 Independent of
power line during
exposure
 Disadvantages
 Require charging
 Heavy