Filters are metal sheets placed in the x-ray beam between the window and the patient that are used to attenuate the low-energy (soft) x-ray photons from the spectrum. Filtering is the removal of these low energy x-rays from the beam spectrum which would otherwise not contribute to image quality but would add to patient dose and scatter. If unfiltered these low-energy x-ray photons are generally absorbed by superficial structures of the body and contribute to the entrance surface dose (ESD). As they are absorbed by the superficial structures they contribute minimally to image formation. Using a filter reduces the ESD and to a lesser extent effective dose for the patient 1.

There are two types of filtration 2:

  • inherent filtration from components in the x-ray tube, i.e. window, housing, cooling oil (equivalent to 0.5-1.0 mm Al)
  • added filtration from interchangeable metal sheets (Al, Cu, etc)

Total filtration is the combined effect of inherent and added filtration, with US guidelines stating a minimum total filtration of 2.5 mm of aluminium is required for x-ray tubes operating above 70 kVp 3.  

The added filtration component is customised (filter thickness, type of metal) for individual examinations and procedures (e.g fluoroscopy) and takes advantage of specific metals filtration characteristics (e.g. absorption edges) to improve image quality and contrast 4

The units of filtration are expressed in mm of Aluminium equivalence (mm Al eq). 

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Article information

rID: 29737
Section: Physics
Tag: stub
Synonyms or Alternate Spellings:
  • Filter
  • Filtration
  • Hardening

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