Diagnostic Radiology III

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Diagnostic Radiology III

• X-ray Generators

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Introduction

• Principal function of x-ray generator is to
  provide current at high voltage to the x-ray tube
• Electrical power available to hospital/clinic
  provides up to about 480 V need 20,000 to
  150,000 V
• Transformers are principal components of
  generators
• Convert low voltage into high voltage through
  process of electromagnetic induction

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Transformers

• Perform task of transforming alternating input
  voltage into alternating output voltage using
  principles of electromagnetic induction
• Generic transformer has two distinct,
  electrically insulated wires wrapped about a
  common iron core

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Law of Transformers

• 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

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Transformers (cont.)

• Transformer can increase, decrease, or isolate
  voltage
• Depends on ratio of the number of turns in the
  two coils
• NS gt NP step-up transformer, increases
  secondary voltage
• NS lt NP step-down transformer, decreases
  secondary voltage
• NS NP isolation transformer, secondary
  voltage equal to primary voltage

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Power

• Power is the rate of energy production or
  expenditure per unit time (1 Watt 1 J/s)
• For electrical devices, P IV
• For an ideal transformer
• Decrease in current must accompany increase in
  voltage, and vice versa

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Autotransformers

• Consists of a single coil of wire wrapped around
  an iron core
• Law of Transformers still applies
• Operates on principle of self-induction rather
  than mutual induction
• Smaller increases or decreases in secondary
  voltage than normal transformers
• Does not electrically isolate primary from
  secondary circuit

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Diodes

• Electrical devices with two terminals that allow
  current flow in one direction only
• Example of a diode is the x-ray tube itself
• Solid-state diode contains a crystal of a
  semiconductor material
• Crystal doped with trace amounts of impurity
  elements
• Conductivity increased when voltage applied in
  one direction but reduced to very low level when
  voltage applied in opposite polarity

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Triodes

• A vacuum tube diode with a third electrode placed
  close to the cathode (a grid)
• Electrons en route from cathode to anode must
  pass through the grid
• Small negative voltage applied to grid exerts
  large force on electrons from cathode, enabling
  on/off switching or current control

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Operator Console

• 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
• Peak kilovoltage (kVp) determines x-ray beam
  quality (penetrability), which plays a role in
  subject contrast
• Tube current (mA) determines the x-ray flux
• Selection of focal spot size usually determined
  by mA setting
• Some generators support preprogrammed techniques

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Voltage ripple

• Voltage ripple of a DC waveform is defined as the
  difference between the peak voltage and the
  minimum voltage, divided by the peak voltage and
  multiplied by 100

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Timing the x-ray exposure

• Digital timers have largely replaced electronic
  timers (high reproducibility and microsecond
  accuracy)
• Mechanical switches only used in single-phase,
  low-power generators
• High-voltage triode switches used in 3-phase and
  constant potential circuits
• High-frequency inverter uses electronic switching
  on the primary side of the high-voltage
  transformer

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Phototimers

• Measure the actual amount of radiation incident
  on the image receptor
• Terminate the x-ray production when the proper
  amount is obtained
• Provides a consistent exposure to the image
  receptor by compensating for thickness and other
  variations in attenuation in a particular patient
  and from patient to patient

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Falling-Load Generator

• Works in concert with the phototiming (AEC)
  subsystem
• Delivers the maximum possible mA for the selected
  kVp by considering the instantaneous heat load
  characteristics of the x-ray tube
• Continuously reduces the power as the exposure
  continues
• Delivers the desired amount of radiation to the
  image receptor in the shortest possible exposure
  time

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