Different filter types are used in medical imaging to improve image quality.
Filter materials such as aluminum, iron or copper, located in the primary x-ray beam between tube and collimator, filter out lower energetic x-rays (for example bow-tie filter). Filters partially absorb or attenuate the x-ray beam to prevent radiation overexposure. The sum of filtration consists of inherent and added filters. This filtration affects the beam energy and ability to penetrate materials.
Mathematical filter procedures are used for the convolution of the attenuation profiles and reconstruction of computed tomography (CT) images.

(FWHM) The full width at half maximum is a parameter to characterize the width of a peak on a graph. In nuclear medicine, the FWHM is used to determinate the energy resolution of gamma camera systems.

Gamma rays are a form of nuclear radiation that consists of photons emitted by radioactive elements from the nucleus. This high energetic light emission is also produced from subatomic particle interaction, such as electron positron annihilation. Gamma radiation, similar to x-radiation can injure and destroy tissue, especially cell nuclei.
Gamma rays have in general very high frequencies, short wavelengths, are electrically neutral and penetrate matter. The interaction of gamma rays with matter depends on the nature of the absorber as well as the energy of the gamma rays; these interactions determine also the type and amount of shielding needed for radiation protection.

See also Radiation Safety, Lead Equivalence, Lead Apron, Leaded Glove, Glove-Box, Radioactive Decay Law and Radiation Worker.

(Gy) Gray is the SI unit of absorbed radiation dose. 1 Gray is equal to the energy absorption of 1 Joule per kilogram. The absorbed dose due to any type of radiation is measured in Gray. 1 Gray is equal to 100 rads in the older terminology.

An intensifying screen is used to intensify the x-ray effect during radiation exposure of the x-ray film. Approximately 5% of the x-ray photons will be absorbed by the film only. Intensifying screens consist of a sheet of inorganic salts that emits fluorescent light when stroked by x-rays. The fluorescent input and output screens of the image intensifier are very similar to intensifying screens.
Calcium tungstate and rare earths are two common salts (also called phosphors) used for intensifying screens. For example, a calcium tungstate (CaWo4) screen can absorb around 40% of the x-ray photons and convert the radiation into light photons. A basic feature of this screen types is related to the position of the k-edge on the energy axis. Tungsten (W) is a heavy element has a k-edge at 69.5 keV, while that for rare earth elements is in around 50 keV.
The fraction of x-rays absorbed by a screen is depending on the speed. Factors affecting the speed of a screen: Mammography cassettes contain usually one intensifying screen, but most others use two screens per film cassette. The intensifying screen as part of a film screen system has been an important component in radiology to reduce the radiation dose of the patient. Today, the conventional film cassette is being replaced by an imaging plate used in digital systems.

See also Actinides, Cinefluorography and Added Filtration.