The X-Ray Circuit

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The X-Ray Circuit
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Primary Circuit
1) Main Switch Location Between AC source and
primary of step-up transformer Purpose
Completes external circuit to x-ray machine
2) Fuses Protects machine from overloaded
circuit
3) Line Voltage Compensator Location Within
primary circuit attached to primary of
autotransformer Purpose Maintains constant
voltage in primary circuit
4) Autotransformer Location Between the AC
source and primary of the step-up
transformer Purpose Allows control of kVp by
varying voltage to primary of step-up
transformer Principle of operation
Self-induction
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The X-Ray Circuit
LOCATION FOR LINE VOLTAGE COMPENSATOR
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Primary Circuit
5) Pre-Reading kilovoltmeter Location Between
autotransformer and primary of step-up
trans. Purpose Indirectly measures kVp
selected/adjustment of line v. Principle of
operation Connected to circuit in parallel
works on motor principle
6) Exposure Switch Location Exposure switch is
between autotransformer primary of step-up
transformer Purpose Manually closes circuit
between autotransformer step-up
transformer Connected in series Special feature
Deadman switch
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Primary Circuit
7) Exposure Timer Location Between
autotransformer primary of step-up
trans. Purpose Terminates exposure at proper
time by opening circuit between autotransformer
step-up transformer
Types of Exposure Timers 1) Mechanical
Timer 2) Electronic Timer 3) mAs Meter 4)
Automatic Exposure Control (AEC) 5) Back-up Timer
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Electronic Timer
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AEC Exposure Timers
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Phototimer Operation
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AEC Exposure Timers
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Back-up Timer
Purpose Stops exposure in case AEC fails -
Prevents overexposure to patient and tube
overloads - May be set automatically by machine
or manually on some equipment
Setting Manual Back-up Time - Divide mAs/mA -
Time must be at least 1.5 times expected exposure
time or 150 of required mAs value for manual
setting - mAs is limited to 600 mAs for
exposures over 50 kVp
Example An AEC calls for 200 mA at .5 S
exposure, what back-up timer setting should be
used? .5 X 1.5 .75 S back-up time If setting
back-up timer using mAs 200 mA X .5 S 100 mAs X
1.5 150 mAs back-up mAs
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Primary Circuit
8) Filament Circuit Supplies heating current to
the filament. - Supplies 3 5 amps at 6 10
volts - This process is controlled by mA button
This circuit also consists of mA Selector
Location Connected in series between the
autotransformer and step-down transformer Purpose
Regulates amperage to filament circuit that
ultimately controls tube current. - May use
rheostat (variable resistance), choke coil
(self-inductance) or high frequency circuit or
saturable reactor (application of DC to iron
core to primary, creating impedance)
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mA Selector in Filament Circuit
mA selector
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8) Filament Circuit (continued) Also
contains Filament Stabilizer Corrects for
variation in line voltage Space Charge
Compensator Maintains filament current for
different kVp selections. Filament Ammeter
Measures filament current.
9) Primary Windings Step-up transformer
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Secondary Circuit
1) Secondary Coil of Transformer Principle of
operation Mutual induction Step-up transformer
Steps up voltage to tube, drives electrons
from cathode to anode Step-down transformer
Steps voltage down and steps up amperage to
filament of tube
2) mA Meter Measures average tube
current Principle of operation Motor
principle Location Connected in series to the
secondary of step-up transformer (includes
connection to ground to protect operator from
being electrocuted) mAs meter is used for very
short exposures
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3) Rectifier Purpose converts AC to DC to
prevent reverse bias Location Between
secondary of step-up transformer and x-ray tube
4) Cables to x-ray tube Conducts high voltage
between rectifier and x-ray tube
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The X-Ray Cables
Shock Hazard Minimized in Three Ways 1)
Insulation 2) Wire sheath that is grounded 3)
Secondary of high voltage transformer is grounded
at its midpoint to minimize amount of
insulation needed
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The X-Ray Cables
Consist of 3 Conductors Cathode end of cable
All 3 conductors attach to filament (attach to
the 2 filament wires) Other end of wire connects
to secondary of transformer and filament
circuit Anode end of cable One wire attaches
to anode At the other end of the cable, all 3
conductors in the cable attach to a single
conductor that attaches to the secondary of
the transformer
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The Secondary Circuit
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The X-Ray Circuit
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The Control Panel
Varies by machine, but may include some of the
components below.
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Three Phase Generator Circuits
Consist of 3 single phase currents running 120
out of phase with each other.
3 ? may be rectified to provide with 6 pulses
using 6 rectifiers, 6 pulses with 12 rectifiers
or 12 pulses with 12 rectifiers (3 ?, 6 p 13
ripple, 3 ? 12 p 3 ripple
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Three Phase Generator Circuits

• To work properly must have 3 primary secondary
  windings in
• transformer (one for each current)
• Must have 3 autotransformers (one for each
  current)
• Primary windings must be in delta configuration
• Secondary may be arranged in delta or star (wye)
  configuration

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Advantages and Disadvantages of 3? Vs 1? Power
Generation

• Disadvantages
• 1) Possible power surges
• Current never reaches 0 potential
• Circuit cannot be opened or closed at zero
  potential
• 2) Less image contrast
• Due to higher effective kVp generated

• Advantages
• 1) Higher tube rating with short exposures
• More mA can be applied during short
  exposure time
• 2) Nearly constant potential (less ripple)
• - 13 for 3 ?, 6 pulse
• - 4 for 3 ?, 12 pulse
• 3) Higher effective kVp 1? 3?
• x mAs 2/3 (6 pulse)
• 4) Higher mAs x mAs 1/2 (12
  pulse)
• x kVp - 12

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Conversion Factors When Changing From 1? to 3?
1? 3? x mAs 2/3 (6 pulse)
x mAs 1/2 (12 pulse) x kVp - 12
Example If 30 mAs is required for a single phase
exposure, how much mAs will be required for the
same density on the image with a 3 phase, 6 pulse
generator? 30 X 2/3 20 mAs
Example If 100 kVp were used on an x-ray machine
with single phase generation, how much should be
used on a three phase machine for the same
density? 100 X .12 12 100 12 88
kVp
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High Frequency Generation
Changes 60 Hz to high frequency current for even
less ripple!
Operational Steps of the High Frequency
Generator 1) 1? or 3? AC current is supplied to
machine 2) A DC chopper converts the AC wave to
a high frequency DC wave that is less
subject to line voltage fluctuations 3) An
inverter converts the DC waveform to a high
frequency AC wave that can be used by the
transformer 4) Voltage from the secondary side
of the transformer is then changed to DC
for application to the tube, rectified and
smoothed
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Operation of a High Frequency Generator
High Freq Inverter
DC Chopper
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Advantages of High Frequency Generators

• Smaller size
• Shorter exposure times available
• High kVp and mA can be used with short exposure
  times
• Very little ripple (1 ripple)
• Less variation in line voltage
• kVp can be more easily calibrated and
  controlled
• Real-time monitoring of kV, mA exposure time
• Error detection circuitry

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Power Rating of X-ray Generators and Circuits
Rated in kW (typically 30 80 kW) for x-ray
machines
1 Watt Energy expenditure of 1 joule For DC P
IV P (Watts) Power I Current intensity V
Voltage
Since high frequency generators produce a nearly
constant electrical waveform the same formula for
DC can be applied P mA X kV
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Generator Problems
A high frequency generator uses 100 mA at 80 kV
for an exposure. How much energy was consumed to
produce this exposure?
What is the maximum power rating for an x-ray
machine when the maximum mA for 100 kV is 300 mA.
Find the maximum power rating if the maximum
exposure factors for a particular x-ray machine
are 800 mA at 70 kVp.
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Falling Load Generators
A generator that automatically starts the
exposure at the highest mA for a selected kVp
curve and drops it during the exposure based on
maximum heat loading capacity of the tube.
1) A microprocessor automatically drops mA in
small steps based on the selected kVp
curve. 2) Tube operates at near maximum rating to
produce optimal mAs at each point on kVp
curve.
Two Types of Technique Selection 1) One-knob
selection - R.T. sets kVp (microprocessor sets
mAs) 2) Two-knob selection R.T. sets both kVp
mAs (microprocessor controls exposure time
by the mA it selects)
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Falling Load Generators
Advantages 1) Reduction of exposure time when
using high mA 2) Simplifies technique selection
by R.T. 3) Takes advantage of maximum tube
loading capacity
Disadvantages 1) Takes control away from R.T. in
choosing technical factors
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Mobile X-Ray Units

• 1) Battery Powered Mobile Units
• Uses nine, 12 Volt DC batteries connected in
  series Powers
• mobile unit and x-ray tube (recharged by 110 V
  AC)
• Circuit Operation
• a. Inverter changes DC from battery to 1kHz AC
• for transformer use
• b) Step-up transformer increases voltage
• c) Rectification system changes AC to DC for
  tube
• operation
• d) Microprocessor control of kVp and mAs for
  improved
• accuracy
• Uses rotating anode
• Nominal focal spot size of .75 m.m. (varies by
  manufacturer)
• Allows selection of kVp mAs (no exp. time
  selection)

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Mobile X-Ray Units

• 2) Capacitor Discharge Unit
• Older type of mobile unit
• Operates by charging a capacitor immediately
  prior to exposure to operate x-ray tube (does
  not drive unit)

Operation a) kVp/mAs values are chosen b) A
charger button is pressed immediately prior to
exposure to charge the capacitor c) Exposure
switch is depressed to start exposure d) Exposure
is terminated via a grid-controlled (triode)
x-ray tube
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Capacitor Discharge Diagram
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Grid-Controlled X-ray Tubes

• Act as switches to start and stop exposure
• Grid is negatively charged focusing cup
  insulated from filament
• 1 2 kVp is applied to cup to break mA current
  in tube

Operation

• Exposure is started by removing the negative
  charge from the
• grid
• Exposure is terminated by restoring the negative
  charge

Advantage Allows precise control of short
exposures.
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Grid-controlled X-Ray Tube
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Wavetail Cutoff With Capacitor Discharge Units

• Process of stopping capacitor discharge at a
  pre-set point on a discharge curve.