Radiographic exposure settings hints and tips

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Radiographic exposure settings hints and tips

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

The following are a selection of hints and tips that can help you achieve this.

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).

  • Accurate positioning improves diagnostic accuracy.

Centring and collimation

The beam should be centred over the primary region of interest and the collimation should always be adjusted to include the region of interest only, wherever possible.

  • Tighter collimation reduces radiation scatter (scatter can reduce image quality).

Exposure factors

Many factors can affect the quality of an x-ray exposure therefore it is vital to remember the basics of image acquisition.

mAs

This is a product of the mA set and the time of exposure. Many generators will display mAs as a single combined value, but a higher specification system may enable the user to alter these individually.

mA

  • The amount of electrical current applied to the cathode.
  • It affects amount of radiation produced

Time in Seconds (s)

  • Length of time that the current is applied to the cathode and radiation is produced.
  • The shorter the time(s), the better. Reduced acquisition time reduces likelihood of motion artefact.

Kilovoltage (kV)

  • The voltage applied across the x-ray tube which affects the energy of the x-ray produced.
  • The kV impacts the contrast seen in the final image; higher kV radiation will penetrate tissue better but can easily result in poor visualisation of soft tissues (the higher energy x-ray essentially passes through the soft tissues too easily).
  • Excessively high kV will reduce the contrast seen within an image, the excessive kV results in high energy x-rays which pass straight through the patient and interact with the plate or detector.

Focal spot/Film distance (FFD)

Also known as FRD - Focal to Receptor Distance

  • This can impact magnification and exposure.
  • If FFD varies you need to alter the exposure factors to compensate if you are trying to achieve the same image quality.
  • g. a 20cm increase in FFD would require 2 x the mAs to result in the same exposure in the same patient.

Object to film distance (OFD)

Also known as ORD - Object to Receptor Distance

  • The distance between the patient and the film/receptor.
  • Increasing this will cause image blurring by increasing geometric unsharpness

hand

hand

In these images the wall represents the plate/receptor and the hand the patient. If you compare these two photos taken with a hand near and far from the wall, you can see how much sharper the shadow edges are when the hand is close to the wall. The reduced OFD results in a clearer shadow or image. Also note the size difference of the shadows. Although it is possible to accurately estimate the size of anatomy from a radiograph, you always need an object of a known size included in the primary beam to use for scaling purposes.

 

Too Low

Too High

mAs

Image too ‘light’ (under-exposed, grainy appearance)

Image too ‘dark’ (over-exposed)

kV

Image with higher contrast

Image with low contrast (too uniformly grey)

FFD

Image may be over-exposed (if other factors not changed)

Image may be excessively magnified

Image under-exposed (if other settings not changed)

OFD

Cannot be too low

Image will be magnified with edge blurring


It is important to remember that increasing either kV or mAs will result in a ‘darker’ image, therefore it is important to get both kV and mAs correct.

Often people increase the kV to compensate for an overall lack of radiation (usually resulting from use of an underpowered generator), where increasing the mAs can actually result in much better-quality image.

Remember the two reasons for an underexposed image:

  1. Not enough radiation overall – increase the mAs, get the generator to produce more photons.
  2. Not enough energy in the radiation produced resulting in minimal passing through the patient and reaching the plate – increase the kV, give the radiation more penetrating power.

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