• 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 decelerated.
  • 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  ).


  • 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 focused into a beam by a negatively charged molybdenum or nickelfocusing 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 is spread over a larger area - 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 stationary anode 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  .
    • Oil surrounding the glass envelope further dissipates heat.
  • Rotating anode machine:
    • The target area is the bevelled rim of a metal disc.
    • The disc rotates rapidly (up to 9000 revs/minute) 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.
    • Oil surrounding the glass envelope then further dissipates heat.
    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.


  • 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 x-rays, theprimary beam, to escape.

    Aluminum filters

  • Absorb the lower energy or "soft" x-rays from the spectrum produced by the anode.
  • These have insufficient energy to be diagnostically useful but increase radiation dose to patient.

General construction of x-ray machines

Line-voltage compensator

  • Allows calibration of x-ray machine to compensate for the variation in electrical supply due to different locations in the building.

Mains-voltage compensator

  • If mains electricity output fluctuates different exposures will be achieved with the same x-ray machine settings.
  • Fluctuations in mains voltage are compensated for by a transformer within the x-ray machine circuit.

Light beam diaphragm

  • Enables illumination of the site of the primary x-ray beam light corresponds to x-ray beam due to lead shutters:
    • Light corresponds to x-ray beam due to lead shutters.
    • Avoids excess scatter.
    • Allows two views to be radiographed on a single film   →   improved economy.
    • Centring over area of interest is more accurate, with cross lines marked on window of collimator.
    • Delineates the primary beam so that if manual restraint is necessary the handler can avoid exposure to this.

Electronic exposure button

  • Two stage button:
    • Stage 1 (prep): heats the cathode and rotates anode if applicable.
    • Stage 2 (exposure): applies potential across tubehead to release x-rays.
  • At least 2 m of coiled cable should be available to allow radiographer to stand as far away from tubehead as possible.

Kilovoltage (kV) control

  • Alters the potential difference applied across the tube head during exposure.
  • Alters the speed and energy with which electrons hit the target and hence the penetrating power of the subsequent x-ray beam.
  • In some machines it is linked to mA so that if high mA is selected, kV must be reduced.

Milliamperage (mA) control

  • Controls the heating of the filament and hence the number of electrons released by the cathode.
  • This directly affects the quantity of x-rays produced.


  • The time for which the exposure is applied affects the number of x-rays produced.
  • The quantity is usually measured as a combination of amperage and time, ie mAs.
  • The longer the exposure the more chance there is of a patient moving so it is preferable to use the highest mA permissible with a given kV and reduce the exposure time accordingly.
  • Older machines had clockwork timers but new machines have electronic timers which are quieter and more accurate.

Types of x-ray machines

Portable machines
  • Stationary anode (heat lost by convection and conduction).
  • Self or half wave rectified.
  • Often fixed mA
  • Occasionally fixed kV.
  • Run from domestic supply (13 amp).
  • Cheaper than mobile/ 3-phase machines to buy and maintain.
  • Can be dismantled and used for domicillary examinations.

Mobile machines

  • Rotating anode (heat lost by radiation)
  • Usually full wave rectified - 2-pulse.
  • May be capacitor discharge.
  • High and variable mA facilitating shorter exposure times.
  • Higher output allows grid   Radiography: scattered radiation and grid  to be used more readily.
  • More expensive to buy and maintain than portable machines.
  • Limited to use within the practice unless van or trailer used!

3-Phase machines

  • Rotating anode.
  • Full wave rectified - 6 pulse.
  • High and variable mA and kV.
  • Very high exposures and short exposure times possible.
  • Expensive to buy and service.
  • Fixed installation   →   dedicated room needed.

Medium frequency (high frequency) machines

  • An invertor increases the frequency of the electrical supply so that with vastly increased number of pulses the ripple factor is negligible and the generator is equivalent to a constant potential unit.
  • Used in some mobile machines which may use a battery supply and some fixed machines which run off a 13 amp supply.