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  ).

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 .
• 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 .
• 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 .
  • 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.

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

Types of X-ray machines

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.