X-ray filter

An X-ray filter is a device to block or filter out some or all wavelengths in the X-ray spectrum.

X-ray filters are used to selectively attenuate (block) low-energy X-rays during medical x-ray imaging (radiography). Low energy x-rays (less than 30 keV) contribute little to the image as they are so heavily absorbed by the patient's soft tissues, particularly the skin. This absorption add to the risk of stochastic (e.g. cancer) or non stochastic radiation effects (e.g. tissue reactions). X-ray filtration may be inherent (due to the x-ray tube and housing material itself) or added (additional sheets of filter material). The total filtration must be at least 2.5 mm Aluminium (Al) equivalent, although there is an increasing trend to use greater filtration. Manufacturers of modern fluoroscopy equipment utilise a system of adding a variable thickness of copper (Cu) filtration according to patient thickness. This typically ranges from 0.1 to 0.9 mm Cu.

X-ray filters are also used in X-ray crystallography, where crystalline lattice spacings can be determined using Bragg diffraction. The filters allow only a single X-ray wavelength to penetrate through to a target crystal, allowing the resulting scattering to determine the diffraction distance.
Various elemental effects

Suitable for X-ray crystallography:

Zirconium - Absorbs Bremsstrahlung & K-Beta.
Iron - Absorbs the entire spectra.
Molybdenum - Absorbs Bremsstrahlung - Leaving K-Beta & K-Alpha.
Aluminium - 'Pinches' Bremsstrahlung* & Removes 3rd Generation peaks.
Silver - Same as Aluminium, But to greater extent.
Indium - Same as Iron, But to lesser extent.
Copper - Same as Aluminium, Leaving only 1st Generation Peaks.

Suitable for Radiography:

Molybdenum - Used in Mammography
Rhodium - Used in Mammography with Rhodium anodes
Aluminium - Used in general radiography x-ray tubes
Copper - Used in general radiography - especially in paediatric applications.
Silver - Used in Mammography with tungsten anode
Tantalum - Used in fluoroscopy applications with tungsten anodes
Niobium - Used in radiography and dental radiography with tungsten anodes
Erbium - Used in radiography with tungsten anodes

Notes:

- Bremsstrahlung pinching is due to the atomic mass. The denser the atom, the higher the X-Ray Absorption. Only the higher energy X-Rays pass through the filter, appearing as if the Bremsstrahlung continuum had been pinched.
- In this case, Mo appears to leave K-Alpha and K-Beta alone while absorbing the Bremsstrahlung. This is due to Mo absorbing all of the spectra's energy, but in doing so produces the same characteristic peaks as generated by the target.