X-Ray Machine Operation The X-Ray Circuit

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X-Ray Machine OperationThe X-Ray Circuit

• DMI 50B
• Kyle Thornton

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An Overview Of The X-Ray Unit

• Diagnostic x-ray machines generally operate at 25
  - 150 kVp and 100 - 1200 mA
• Most general purpose rooms contain a radiographic
  unit and a fluoroscopic w/image intensification
  capabilities
• X-ray tables must be of the radiolucent material
  possible
• Most table tops are floating
• All contain a cassette tray

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The Three Major Parts Of An X-Ray Unit

• X-Ray Tube
• Operating Console
• High Voltage Generator

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Operating Console

• Components
• On/off switch
• mAs selection
• kVp selection
• Exposure switch
• Table/wall unit activator
• Automatic exposure control - if available
• Adjustment of line compensation - older units

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The Operating Console
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Overview Of X-Ray Machine Circuitry

• Two Major Subcircuits
• Filament Circuit
• Allows the radiographer to adjust filament
  current
• Regulates electron emission
• Regulates tube current
• Regulates subsequent x-ray production
• High Voltage Circuit
• Steps up incoming voltage for x-ray production

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High Voltage Box
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Other Important Components

• Timing circuit
• Monitor and compensate for fluctuations in
  incoming line voltage
• High voltage rectifiers
• Ammeters and voltmeters

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X-Ray Generator Components

• The components and circuitry that supply power to
  the X-ray tube
• Source of electrical power
• Line voltage compensation
• High voltage circuit
• Timer circuit
• Filament circuit
• Generator components can be found in the
  operating console and transformer assembly

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Source Of Electrical Power

• Supplied from local power in AC
• Commercial electric power commonly supplied in
  three-phase, 60 Hz. AC
• A simple generator supplies single-phase, 60 Hz.
  AC
• In a three-phase AC the voltage never drops to
  zero

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Line Voltage Compensation

• Line voltage is the voltage supplied by the power
  company
• This voltage is subject to fluctuations
• A compensator is used in the x-ray machine to
  monitor incoming voltage and keep it at a
  constant value
• On older units this can be done manually
• Newer units accomplish this automatically

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The High Voltage Circuit

• Converts incoming voltage to kilovoltage
• Allows for kVp selection
• Accomplished via the autotransformer and high
  voltage step-up transformer
• After increase voltage is then rectified
• Contains a timing circuit
• Determines when the exposure will be terminated
• Production of x-rays is controlled by regulating
  the voltage across the cathode and anode

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High Voltage Circuit
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The Autotransformer

• The first major circuit component encountered by
  incoming line voltage
• Has only one core with a single winding
• Has a variable turns ratio
• Can be used as a step-up or step-down transformer
• The autotransformers turns ratio is adjusted by
  major or minor kVp selection

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The Autotransformer
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kVp Selection
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High Voltage Step-Up Transformer

• The output voltage of the autotransformer becomes
  the input voltage of the high voltage transformer
• This transformer brings line voltage to
  kilovoltage levels
• The turns ratio is generally 500-600
• The turns ratio is fixed - cannot be varied
• A voltmeter is connected to the primary side
• This value can be read prior to the exposure

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Transformer Box
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Ammeter

• Located on secondary side of high voltage
  transformer
• This provides an average value of the tube
  current
• This cannot be read prior to exposure

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Rectifiers

• To operate efficiently, AC must be changed to DC
  prior to moving through the tube
• This process is known as rectification
• Accomplished via rectifiers
• Solid state rectifiers are generally used in
  modern x-ray equipment
• The resulting wave form depends on the number of
  rectifiers used
• Rectifiers are also used to measure current
  through the mA meter

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Half-wave v. Full wave Rectification

• Half wave requires 0 - 2 rectifiers
• Uses only the positive half of the AC cycle
• The negative half is omitted
• Produces 60 DC pulses
• Full-wave requires 4 rectifiers
• Makes the most efficient use of both halves of
  the AC cycle
• The negative half is inverted to the positive
  half of the cycle
• 120 DC pulses are produced

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Half-wave Rectification
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Full-wave Rectification
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Timing Circuitry

• Terminates the exposure at a preset time
• Types of timers
• Mechanical timers
• Synchronous timers
• Impulse timers
• Electronic timers
• mAs timers
• Automatic timers

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Types Of Timers

• Mechanical timers
• Least complex, least used
• Similar to an egg timer
• Accurate to app. 1/4 sec. , .25 s. or 250 ms.
• Synchronous timers
• Uses a synchronous motor
• Turns in synch with incoming AC current
• Shortest time possible is 1/60 sec.
• Can only use multiples of 60

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Types Of Timers

• Impulse Timers
• Works on the principles of voltage pulses
• Shortest possible time is 1/120 sec.
• Accurate to ms.
• Electronic Timers
• Provide very short exposure times
• Capable of exposures as short as 1 ms.
• Can be used for rapid serial exposures

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Types Of Timers

• mAs Timers
• Uses the current passing through the tube
• Designed to provide the highest tube current at
  the shortest exposure time
• Automatic exposure controls
• Terminates the exposure when a certain amount of
  radiation reaches film
• Uses an ionization chamber, photomultiplier tube
  or solid state detector

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Photomultiplier Tube
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Checking Timer Accuracy

• Spinning top test
• A special type of top is x-rayed while it is
  spinning
• The top has a small hole in it
• A certain time will have a corresponding number
  of dots on the film
• Calculating the number of dots
• 120 pulses/sec.in full-wave rectified circuit
• Multiply time X 120

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The Spinning Top
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Spinning Top For Three-Phase Equipment

• Must use a motorized spinning top
• Produce an arc image
• Solid state radiation detectors are generally
  used to check timers more accurately

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The Filament Circuit

• Produces electrons by heating the cathode to
  incandescense
• Voltages of 5-10 are needed to produce this
  heating capacity
• A step down transformer is necessary
• The primary voltage is supplied by the
  autotransformer
• The resulting output voltage is less
• This voltage is then supplied to the filament of
  the cathode

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Filament Circuit
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Single v. Three Phase

• Half and full-wave rectification result in a
  pulsed x-ray beam
• The output is single-phase power
• This causes voltage to swing from zero to maximum
  potential 120 times/sec for full-wave
  rectification
• During single phase the x-rays emitted are too
  low energy to be of use during the omitted
  negative half of the cycle

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Three-Phase Power

• Multiple voltage waveforms are initiated
• This happens with full-wave rectification only
• Three coils of wire are in a magnetic field
• Each current is slightly out of step with the
  other
• The resulting waveform never drops to zero

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Advantages Of Three-Phase Power

• Minimize voltage fluctuation
• Provide a more constant voltage waveform
• More efficient means of x-ray production
• Shorter exposure times are available
• Higher mA stations are available
• Higher energy x-rays are available
• Results in somewhat of a lower patient dose
• Results in a higher initial cost

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Six and Twelve Pulse Generators

• Three Phase Six Pulse
• 3 60 Hz AC voltage waveforms, 120 degrees out of
  step with each other
• Requires 3 phase power
• Uses 3 phase transformer with 6 - 12 rectifiers
• Three Phase Twelve Pulse
• 3 60 Hz AC voltage waveforms, 120 degrees out of
  step with each other
• Requires 3 phase power
• Uses 3 phase transformer with slight variation in
  design from 3-phase, six-pulse
• Has 12 rectifiers

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High Frequency Generators

• Full-wave rectified power at 60 Hz is converted
  to a higher frequency (MHz)
• Uses a DC chopper
• The resulting beam has almost zero voltage ripple
• The generators are much smaller
• Image quality is improved with a lower patient
  dose
• Mobile x-ray machines were the first to use this
  technology

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High-Frequency Unit
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Voltage Ripple

• The amount of voltage peak and trough between
  waves
• Single phase has 100 voltage ripple
• 3-phase, 6-pulse has 13
• 3-phase, 12-pulse has 4
• High-frequency has lt 1

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Voltage Ripple Waveforms