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Radiography: x-ray generation

Contributor(s): Vetstream Ltd, David Vella

X-ray generation

  • X-rays are electromagnetic radiation   Radiography: basic physics  .
  • Their usefulness stems from a number of properties: 
    • Travel in straight lines. 
    • Can pass through a vacuum. 
    • Travel at constant speed. 
    • Variably absorbed by body tissue. 
    • Affect photographic film to produce a latent image   Radiography: processing  . 
    • Cause certain substances to fluoresce (emit visible light).

Construction of x-ray tube head

  • X-rays are produced when electrons are rapidly decelerated.
  • The x-ray tube head requires: 
    • A source of electrons. 
    • A means of accelerating them. 
    • A target to convert incident energy from electrons    →   rays.
  • The tube head consists of 2 electrodes in a vacuum (see Radiation physics: x-ray tube head construction   Radiation physics: x-ray tube head construction  ).

Cathode 

  • Thecathodeis a coiled tungsten wire which releases a cloud of electrons when heated. 
  • The number of electrons produced is dependent on the temperature of the filament. 
  • The electron cloud is focussed into a beam by a negatively charged molybdenum or nickelfocussing cup
  • A high potential difference is applied across the tube head such that electrons are accelerated towards the anode.

Anode 

  • The electrons are attracted to the anode by virtue of its positive charge. 
  • Theanodecontains a tungsten target which the electrons strike at high speed. 
  • 99% of the energy is lost as heat but 1% is converted to x-rays. 
  • Target must be able to withstand high temperature without melting or vaporizing. 
  • The target should be as large as possible so that the heat can be lost more quickly - however a large target area produces a wide beam of x-rays which produces poor image quality. 
  • To compensate for this the target is set at an angle so that the incident electron beam strikes a wide area but the x-ray beam appears to originate from a smaller focal spot   Radiation physics: focal spot size  . 
  • The heat dissipation can be assisted by two different mechanisms: 
  • Simple x-ray machine
    • The target is set into a copper heat sink. 
    • Copper is a good conductor of heat and the heat is rapidly drawn away from the target to cooling fins which absorb heat    Radiation physics: stationary anode  .
  • Rotating anode machine
    • The target area is the bevelled rim of a metal disk. 
    • The disc rotates rapidly (up to 9000 revs/min) during exposure so that the incident electron stream is constantly striking a different area of the target    Radiation physics: rotating anode  . 
    • The heat generated is spread over a larger area allowing higher exposures to be made. 
    • Heat is dissipated through radiation into the vacuum.

Glass envelope

  • The cathode, anode and part of the copper stem are contained within a glass envelope which maintains the vacuum. 
  • The envelope is bathed in oil to act as a heat sink and electrical insulator.

Casing 

  • The whole unit is enclosed in an earthed, lead-lined metal casing. 
  • There is a small window in the casing which allows a narrow beam of electronsprimary beamto escape.

Aluminium filters

  • Filter out "soft" x-rays produced by the anode. 
  • These have insufficient energy to be diagnostically useful but increase radiation dose to patient.

General construction of x-ray machines

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Types of x-ray machines

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Further Reading

Publications

Refereed papers
  • Recent references fromPubMedpublished during the last 12 months.


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