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 strength of filtration is commonly expressed in mm of aluminium half-value layer (HVL) in the filtered beam (this is a different measure than the thickness of the filter itself, even if it is made of aluminium) ⁷.

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 (resulting in around 0.5-1.0 mm Al HVL)

• 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 HVL 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. 

Aluminium and copper are the most commonly used filter materials for general purpose x-ray systems. Molybdenum and rhodium filters are commonly used in mammography, where low energy beams are required ⁷.

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.