Scatter to primary ratio

Last revised by Andrew Murphy on 31 Oct 2021

The scatter to primary ratio is a ratio of the scattered radiation to the primary unscattered radiation transmitted by the object being imaged.

Hence, the scatter to primary ratio provides an indication of the degree of unwanted scattered radiation arising from a particular imaging study.

the scatter to primary ratio increases with:

  • field size
  • patient thickness
  • increasing applied voltage (kV)

Measures to decrease the scatter ratio:

  • decreasing field size 
  • decreasing  kV 
  • decreased subject to film distance 
  • using a grid
  • air gap

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