Equis ISSN 2398-2977

Radiography: x-ray generation

Contributor(s): Tim Donovan, Patricia Rose, Jeffrey Watkins, Jonie Watkins, Chris Whitton

X-rays

  • X-rays are electromagnetic radiation.
  • 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 deccelerated.
  • The x-ray tube head requires:
    • A source of electrons.
    • A means of accelerating them.
    • A target to convert incident energy from electrons   →   x-rays.
  • The tube head consists of 2 electrodes in a vacuum (see diagram   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 disk 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 a lead-lined metal casing.
  • There is a small window in the casing which allows a narrow beam of x-rays(primary beam)to escape.
    Aluminum 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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