X-RAY GENERATOR COMPONENTS

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• X-RAY GENERATOR COMPONENTS

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Electromagnetic Induction and Voltage
Transformation

• The principal function of the x-ray generator is
  to provide current at a high voltage to the x-ray
  tube.
• Electrical power available to a hospital or
  clinic provides up to about 480 V.
• Much lower than the 20,000 to 150,000 V needed
  for x-ray production.

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• Transformers are principal components of x-ray
  generators
• They convert low voltage into high voltage
  through a process called electromagnetic
  induction.

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• Electromagnetic induction is an effect that
  occurs with changing magnetic fields and
  alternating (AC) electrical current.
• Note that for constant-potential direct current
  (DC), like that produced by a battery, magnetic
  induction does not occur.

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• Electromagnetic induction is reciprocal between
  electric and magnetic fields.
• An electrical current (e.g., electrons flowing
  through a wire) produces a magnetic field, whose
  magnitude (strength) and polarity (direction) are
  proportional to the magnitude and direction of
  the current .
• With an alternating current, such as the standard
  60 cycles per second (Hz) AC in North America and
  50 Hz AC in most other areas of the world, the
  induced magnetic field increases and decreases
  with the current.

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Transformers

• Transformers perform the task of transforming
  an alternating input voltage into an alternating
  output voltage, using the principles of
  electromagnetic induction.
• The generic transformer has two distinct,
  electrically insulated wires wrapped about a
  common iron core.
• Input AC power (voltage and current) produces
  oscillating electrical and magnetic fields.

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• One insulated wire wrapping (the primary
  winding) carries the input load (primary voltage
  and current).
• The other insulated wire wrapping (secondary
  winding) carries the induced (output) load
  (secondary voltage and current).

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• The primary and secondary windings are
  electrically (but not magnetically) isolated by
  insulated wires.

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• The induced magnetic field strength changes
  amplitude and direction with the primary AC
  voltage waveform and freely passes through
  electrical insulation to permeate the transformer
  iron core, which serves as a conduit and
  containment for the oscillacing magnetic field.
• The secondary windings are bathed in the
  oscillating magnetic field, and an alternating
  voltage is induced in the secondary windings as a
  result.

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• The Law of Transformers states that the ratio of
  the number of coil turns in the primary winding
  to the number of coil turns in the secondary
  winding is equal to the ratio of the primary
  voltage to the secondary voltage
• where Np is the number of rums of the primary
  coil, Ns is the number of turns of the secondary
  coil, Vp is the amplitude of the alternating
  input voltage on the primary side of the
  transformer, and Vs is the amplitude of the
  alternating output voltage on the secondary side.

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• A transformer can increase, decrease, or isolate
  voltage, depending on the ratio of the numbers of
  turns in the two coils.
• For Ns gt Np, a step-up transformer increases
  the secondary voltage

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• For Ns lt Np, a step-down transformer decreases
  the secondary voltage

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• And for Ns Np, an Isolation transformer
  produces a secondary voltage equal to the primary
  voltage (see later discussion).

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• A key point to remember is that an alternating
  current is needed for a transformer to function.

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• Power is the rate of energy production or
  expenditure per unit time.
• The SI unit of power is the watt (W), which is
  defined as 1 joule (J) of energy per second.
• For electrical devices, power is equal to the
  product of voltage and current

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• Because the power output is equal to the power
  input (for an ideal transformer), the product of
  voltage and current in the primary circuit is
  equal to that in the secondary circuit

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• Therefore, a decrease in current must accompany
  an increase in voltage, and vice versa.
• Power losses due to inefficient coupling cause
  both the voltage and current on the secondary
  side of the transformer to be less than predicted
  by these equations.

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• The high-voltage section of an x-ray generator
  contains a step-up transformer, typically with a
  primary-to-secondary turns ratio of 1500 to
  11,000.
• Within this range, a cube voltage of 100 kVp
  requires an input peak voltage of 200 V to 100 V,
  respectively.

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• The center of the secondary winding is usually
  connected to ground potential (center tapped to
  ground).
• Ground potential is the electrical potential of
  the earth.
• Center tapping to ground does not affect the
  maximum potential difference applied between the
  anode and cathode of the x-ray tube, but it
  limits the maximum voltage at any point in the
  circuit relative to ground to one half of the
  peak voltage applied to the tube.

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• Therefore, the maximum voltage at any point in
  the circuit for a center-tapped transformer of
  150 kVp is -75 kVp or 75 kVp, relative to
  ground.
• This reduces electrical insulation requirements
  and improves electrical safety.
• In some x-ray tube designs (e.g., mammography),
  the anode is maintained at the same potential as
  the body of the insert, which is maintained at
  ground potential.
• Even though this places the cathode at peak
  negative voltage with respect to ground, the low
  kVp (less than 50 kVp) used in mammography does
  not present a big electrical insulation problem.

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Autotransformers

• A simple autotransformer consists of a single
  coil of wire wrapped around an iron core.
• It has a fixed number of turns, two lines on the
  input side and two lines on the output side.

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• When an alternating voltage is applied to the
  pair of input lines, an alternating voltage is
  produced across the pair of output lines.
• The Law of Transformers applies to the
  autotransformer, just as it does to the standard
  transformer.
• The output voltage from the autotransformer is
  equal to the input voltage multiplied by the
  ratio of secondary to primary turns.
• The primary and secondary turns are the number of
  coil turns between the caps of the input and
  output lines, respectively.

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• The autotransformer operates on the principle of
  self-induction, whereas the standard transformer
  operates on the principle of mutual induction.
• The standard transformer permits much larger
  increases or decreases in voltage, and it
  electrically isolates the primary from the
  secondary circuit, unlike the autotransformer.

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• A switching autotransformer has a number of taps
  on the input and output sides, to permit small
  incremental increases or decreases in the output
  voltage.

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• The switched autotransformer is used ro adjust
  the kVp produced by an x-ray generator.
• Standard alternating current is provided to the
  input side of the autotransformer, and the output
  voltage of the autotransformer is provided to the
  primary side of the high-voltage transformer.
• Although variable resistor circuits can be used
  to modulate voltage, autotransformers are more
  efficient in terms of power consumption and
  therefore preferred.

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Operator Console

• At the operator console, the operator selects
• The kVp,
• The mA (proportional to the number of x-rays in
  the beam at a given kVp),
• The exposure time, and
• The focal spot size.

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• The peak kilo-voltage (kVp) determines the x-ray
  beam quality (penetrability), which plays a role
  in the subject contrast.
• The x-ray tube current (mA) determines the x-ray
  flux (photons per square centimeter) emitted by
  the x-ray rube at a given kVp.
• The product of tube current (mA) and exposure
  time (seconds) is expressed as milliampere-seconds
  (mAs).

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• Some generators used in radiography allow the
  selection of three-knob technique (individual
  selection of kVp, mA, and exposure time), whereas
  others only allow two-knob technique
  (individual selection of kVp and mAs).
• The selection of focal spot size (i.e., large or
  small) is usually determined by the mA setting
• low mA selections allow the small focus to be
  used, and
• higher mA settings require the use of rhe large
  focus due ro anode heating concerns.

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• On some x-ray generators, preprogrammed
  techniques can be selected for various
  examinations (i.e., chest kidneys, ureter, and
  bladder cervical spine).
• All console circuits have relatively low voltage
  and current levels that minimize electrical
  hazards.

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• X-RAY GENERATOR CIRCUIT DESIGNS

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• Several x-ray generator circuit designs are in
  common use, including single-phase, three-phase,
  constant potential, and medium/high-frequency
  inverter generators.
• All use step-up transformers to generate high
  voltage, step-down transformers to energize the
  filament, and rectifier circuits to ensure proper
  electrical polarity at the x-ray tube.

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Rectifier Circuit

• A rectifier is an electrical apparatus that
  changes alternating current into direct current.
• It is composed of one or more diodes.

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• In the x-ray generator, rectifier circuits divert
  the flow of electrons in the high-voltage circuit
  so that a direct current is established from the
  cathode to the anode in the x-ray tube, despite
  the alternating current and voltage produced by
  the transformer.
• Conversion to direct current is important.
• If an alternating voltage were applied directly
  to the x-ray tube, electron back-propagation
  could occur during the portion of he cycle when
  he cathode is positive with respect to the anode.
  
• If the anode is very hot, electrons can be
  released by thermionic emission, and such
  electron bombardment could rapidly destroy the
  filament of the x-ray rube.

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• To avoid back-propagation, the placement of a
  diode of correct orientation in the high-voltage
  circuit allows electron flow during only one half
  of the AC cycle (when the anode polarity is
  positive and cathode polarity is negative) and
  halts the current when the polarity is reversed.

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• As a result, a single-pulse waveform is
  produced from the full AC cycle, and this is
  called a half-wave rectified system.

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• Full-wave rectified systems use several diodes (a
  minimum of four in a bridge rectifier) arranged
  in a specific orientation to allow the flow of
  electrons from the cathode to the anode of the
  x-ray tube throughout the AC cycle (see Fig.
  5-26B).

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• During the first half-cycle, electrons are routed
  by two conducting diodes through the bridge
  rectifier in the high-voltage circuit and from
  the cathode to the anode in the x-ray tube.
• During the second half-cycle, the voltage
  polarity of the circuit is reversed electrons
  flow in the opposite direction and are routed by
  the other two diodes in the bridge rectifier,
  again from the cathode to the anode in the x-ray
  tube.
• The polarity across the x-ray tube is thus
  maintained with the cathode negative and anode
  positive throughout the cycle.

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• X-rays are produced in two pulses per cycle,