ISSN 2398-2942      

Radiography: X-ray generation


X-ray generation

  • 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 vaccuum (see diagram Radiation physics x-ray tube head construction ).


  • 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.


  • 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 vaccuum.

Glass envelope

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


  • 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

This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login

Types of X-ray machines

This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login

Further Reading


Refereed papers

Related Images

Want more related items, why not
contact us

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!


To show you are not a Bot please can you enter the number showing adjacent to this field