Species: Canine | Classification: Miscellaneous
- 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 ).
- 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 .
- The heat dissipation can be assisted by two different mechanisms:
- Simple x-ray machine:
- 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.
- 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.
- 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
- 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.
- Enables illumination of the site of the primary x-ray beam:
- Avoids excess scatter.
- Allows two views to be radiographed on a single film ’ improved economy.
- Centering over area of interest is more accurate.
- Delineates the primary so that if manual restraint is necessary the handler can avoid the primary beam.
Electronic exposure button
- Two stage button:
- Stage 1 (prep): heats the cathode and rotates anode is applicable.
- Stage 2 (exposure): applies potential acros 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 pentrating 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
- 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.
- Rotating anode (heat lost by radiation)
- Usually full wave rectified - 2-pulse.
- May be capacitor discharged.
- High and variable mA facilitating shorter exposure times.
- Higher output allows grid Radiography: scattered radiation and grid use 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!
- 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.
- 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.