INTRODUCTION
Radiography is a branch of veterinary science with many potential hazards. X-rays have an adverse effect on living tissue caused by cellular chemical reactions. X-rays are latent and cumulative, and most danger arises from repeated exposure to small amounts of radiation. The tests described here are of such importance in general practice that they often are incorporated into the practical examination. Not all of the tests are directly related to occupational dangers, but procedures that help to prevent the production of poor quality, non-diagnostic radiographs can only be beneficial to both the staff and patients involved.

1. TESTING FOR SCATTERED RADIATION
   Scattered radiation is also known as secondary radiation. Scatter occurs when x-ray photons interact with the tissues, lose some of their energy and emerge from the patient in random direction as photons of lower energy. Scattered radiation travels in all directions and may rebound off the walls, table or floor. Scatter is not only undesirable due to the fogging effect that it has on the radiographic image, but also because of the radiation risk to personnel. In order to assess levels of scattered radiation, leave a metal instrument such as a pair of scissors on an unexposed x-ray cassette placed in the controlled area for radiography. Develop the film after one week. An unexposed film will normally appear transparent. If the outline of the scissors is visible, scatter is at an unacceptable level.
   
   The Radiation Protection Supervisor is duty-bound to improve the situation before further radiographs are performed. Steps that may be taken include:
   • Servicing of the xray machine; this should be undertaken every 6 months or when a fault prevails.
   • The use of grids when radiographs of tissue deeper than 10cm are taken. The Potter-Bucky moving grid incorporated into the table top is by far the most expensive but also the most efficient.
   • Compression of large areas of soft tissue.
   • Decrease the penetrating power (KV) where possible.
   • Collimate the light beam diaphragm to as small an area as possible; if the x-ray machine is still fitted with a cone this should be updated.
   • The use of lead-backed cassettes and a lead-backed table top.

   Personnel involved in radiographic procedures should be issued with a personal dose-meter enabling external assessment of ionising radiation levels.

2. TESTING FOR FOGGING BY THE SAFE-LIGHT
   Poor quality radiographs mean that repeated exposures need to be made; thus subjecting patient and personnel to further doses of ionising radiation. Developing faults are frustrating and unnecessary. Fogging is not only caused by scatter; it can also be caused by the darkroom safelight. To check the efficiency of the safelight, lay a metal instrument such as a pair of scissors on a cassette containing unexposed film. Place this in the darkroom on the work-bench with the safelight switched on. Leave for 2 minutes before processing the film. If the scissors are visible on the exposed film, fogging has occurred. The following steps should be taken if safelight fogging is present:
   • Check that the bulb wattage is no greater than 25W; replace the bulb if this is the case.
   • Ensure that the correct colour filter is used for the type of film in use; brown or amber for blue light sensitive film and red for green light sensitive film.
   • Check for cracks in the filter and replace if damaged.
   • Check that the safelight is not too close to the film handing area.
3. ASSESSMENT FOR UNDER-DEVELOPMENT
   Underdevelopment is the most common developing fault, although with many practices now using automatic processors this problem is less frequent. The "finger test" is a simple method employed to test for underdevelopment. Place your finger between the exposed x-ray film and the viewer, choosing an area not occupied by the view of the patient. The film should be completely black, and if the finger is visible through the film - underdevelopment is evident. Corrective measures for underdevelopment include:
   • Ensure that the developer is used at the correct temperature; 20°C for manual developing, 28°C for automatic processing.
   • Regular replenishment of developer according to the manufacturers instructions; ensure that the concentration of the solution is correct.
   • Use a developer recommended by the manufacturer of the processor.
   • Mix the developer well prior to use when manually processing.
4. LIGHT BEAM DIAPHRAGM ALIGNMENT TEST
   The light beam diaphragm is the part of the x-ray machine which is attached to the x-ray tube head in order to produce light over the area covered by the primary beam. A shadow cross shows the position of the central ray and enables accurate collimation by adjustment of the size of the light area. The centre of the cross is situated in the centre of the area of interest with the beam collimated as tightly as possible.
   
   To check the light beam diaphragm alignment, mark the corners of the light image with radiopaque markers. Take an exposure and develop the film. Now compare the position of the markers on the cassette with those on the exposed image. Any difference in position indicates a fault with the LBD. An approved engineer should be called to service the machine and right the fault; special calibration tests will be necessary and these should only be undertaken by an expert.
5. FILM - SCREEN CONTACT ASSESSMENT
   X-ray cassettes usually contain 2 intensifying screens, the purpose of which are to strengthen the effect of x-rays on the film. They consist of a plastic or carbon fibre base covered with a white reflective surface and a coating of luminescent phosphors. Calcium tungstate phosphors emit blue light when irradiated, while rare earth phosphors emit blue or green light. (It is important to use the correct film-screen combination). The light emitted greatly reduces the exposure necessary and is beneficial therefore in reducing both scattered radiation and movement blur. The film is placed within the cassette between the screens and close film-screen contact is provided by a thick felt pressure pad sited between the rear screen and the back plate of the cassette. Poor film-screen contact causes blurring of the radiographic image in the affected areas.
   
   To test film-screen contact, scatter an x-ray plate with small metal objects such as paper clips. Ensure that these are placed close to the cassette edges in addition to centrally. Expose and develop the film. Assessment of quality may now be made; the pressure pad should be replaced if any part of the exposed film shows blurring. You may be confronted with chicken wire in the practical examination; this serves the same purpose as the paper clips.