Species: Canine   |   Classification: Miscellaneous


  • X-ray exposure charts are individual to the x-ray machine, intensifying screen, film and developing combination.
  • An exposure chart for one x-ray machine cannot be transferred to another machine even although this may be identical.
  • Exposure charts are important to:
    • Provide consistent results.
    • Reduce number of exposures (important for radiation safety and cost).
    • Allows direct comparison between films taken on different dates in the same animal.
  • When compiling an exposure chart as many variables as possible should be kept constant:
  • Several charts may be needed for different film/screen combinations and different species.

Variable factors

Variable kV
  • This is used if:
    • Machine allows variation in kVp of 1-2.
    • Due to the variation in dog size and shape selecting an exposure based on dogs weight may be inaccurate.
    • Breed variability in conformation can be overcome by basing exposure on tissue thickness.

    Measure tissue thickness with calipers.
  • Keep mAs constant and as high as possible and alter kV based on tissue thickness.
  • A grid should be used if tissue depth is >10 cm and may be useful in smaller obese animals.

    Use of a grid results in higher exposures which in low-powered machines may result in longer exposure times ’ movement blur.


  • If using a grid the exposure will need to be increased.
  • Multiply grid factor by mAs to obtain new mAs.

Compiling exposure chart

Trial exposure
  • Select patient:
    • Medium sized, moderately muscled dog approximately 25 kg.
    • Clean short haircoat.
  • Take trial exposure selecting approximate kV and mAs.
  • Retake same radiograph at twice and four times mAs setting (leaving kV unchanged).
  • Develop films.
  • Select most aesthetically pleasing film.
  • If no film suitable alter kV and mAs and repeat procedure.
  • Once suitable radiograph obtained record settings.
  • Record result on chart indicating anatomical location of radiograph and size of dog, ie weight or thickness of body tissue.
  • The same process must be repeated:
    • For all body locations.
    • With and without a grid.
    • For non-screen techniques.

Estimating exposure from exposure chart

  • If films underexposed, ie cannot see bones increase kVp by 10-15.
  • If can see anatomy but poor contrast ie film looks gray with no white bones reduce kVp by 10-15% and keep mAs constant.
  • For areas with high inherent contrast, eg thorax use high kVp and low mAs.
  • For abdomen aim for maximum mAs and low kVp.
  • Increase kVp by 5-10 kVp for contrast studies.
  • For each increase of abdomen depth of 1 cm add:
    • 2 kV up to 80 kV.
    • 3 kV for 80-100 kV.
    • 4 kV above 100 kV.
  • Keep film focal distance (FFD) constant where possible.
  • [(new FFD x new FFD)/(old FFD x old FFD)] = (new mAs)/(old mAs).
  • ie doubling FFD requires a fourfold increase in mAs.