Radiographic exposure settings hints and tips

A brief refresher of the primary factors that will optimise the quality of your film or digital radiographs

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Radiographic exposure settings hints and tips

29 March 2013


It is essential that radiographs taken in practice should be of diagnostic quality to enable accurate evaluation for the presence of disease/pathology.

Patient positioning

Whether you are working with companion animals, horses, farm or exotic species, it is important to properly restrain the patient and position them for optimal exposure of the region of interest and radiographic projection (view).

Centring and collimation

The beam should be centred over the primary region of interest and the collimation should always be adjusted to the region of interest.

Exposure factors

Radiographic Exposure Settings (1)

Milliamperage (mA)

  • Amount of electrical current applied to cathode
  • Affects number of X-rays produced
  • Usually combined with time (s) to enable mAs to be altered as a single setting
  • Alters the overall brightness of the image (overall more ‘light’ or ‘dark’)
  • Higher mAs = a greater number of X-rays will be produced and pass through the patient to reach the cassette (and vice versa)

Time in seconds (s)

  • Length of time current applied to cathode
  • Affects number of X-rays produced
  • Usually combined with milliamperage (mA) to enable mAs to be altered as a single setting

Kilovoltage (kV)

  • Voltage applied across the generator
  • Affects energy of X-rays produced
  • Alters the visible contrast between tissues (image more black and white or uniformly grey)
  • Higher kV = the X-rays leaving the generator will have greater energy and so will be able to pass through the patient more easily to reach the cassette (and vice versa)

Focal spot/film distance (FFD)

  • Distance between the source of X-rays (focal spot in the generator) and the cassette/plate
  • Increase/decrease in FFD requires alteration in other exposure factors (increase in FFD requires increase in mAs +/- kV and vice versa)

Object-film distance (OFD)

  • Distance between the patient and the cassette/plate
  • Increase in OFD results in image blurring and magnification

Radiographic Exposure Settings (1)
Object/area of interest (hand) farther away from ‘film/cassette’ (wall) – shadow cast is magnified with blurred edges.

 

Radiographic Exposure Settings (1)
Object/area of interest (hand) near to the ‘film/casette’ (wall) – shadow cast is of nearly equal size with sharp edges.

  Too low Too high
mAs Image too ‘light’ (underexposed) Image too ‘dark’ (overexposed)
kV Image with high contrast (too black and white – soft tissue may be lost) Image with low contrast (too uniformly grey – difficult to distinguish individual structures)
FFD Image overexposed (if other settings unchanged) Image underexposed (if other settings unchanged)
OFD Cannot be too low! Image magnification and blurring of edges

Remember that increasing either mAs or kV will cause the film to be darker, therefore a correctly exposed radiograph relies on both an appropriate mAs and kV setting to be used.