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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. The units of filtration are expressed in mm of aluminum equivalence (mm Al eq).
There are two types of filtration 2:
inherent filtration from components in the x-ray tube, i.e. window, housing, cooling oil, anode target itself (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 aluminum is required for x-ray tubes operating above 70 kVp 3.
The added filtration component is customized (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.
Beryllium is commonly used in mammography (which use low-energy photons) as it provides very little filtration. Other types of x-ray generally use aluminum, copper or tin.
Filtration reduces x-ray intensity, but not the maximum energy of the x-ray beam spectrum. The change in the shape of the beam spectrum with filtration is referred to as beam hardening. This is due to the loss of lower energy photons from a polychromatic beam. The average x-ray energy is increased and becomes more penetrating 6.