A pixel is a picture element (pix, abbreviation of pictures + element). Tomographic images are composed of several pixels; the pixel size is determined by the used field of view and the number of elements in the display image matrix. The corresponding size of the pixel may be smaller than the actual spatial resolution.
Pixels do not have a fixed size; their diameters are generally measured in micrometers (microns). Although the pixel is not a unit of measurement itself, pixels are often used to measure the resolution (or sharpness) of images. As a hypothetical example, a 600 x 1000 pixel image has 4 times the pixel density and is thus 4 times sharper than a 300 x 500 pixel image, assuming the two images have the same physical size.

An atom is the basic particle of matter, the smallest (~10^-8cm) particle of a chemical element. Atoms consist of a nucleus with neutrons, positive charged protons and orbiting negative charged electrons. The chemical properties of elements are defined largely by the number of protons in the nucleus. The number of electrons is similar to the number of protons. An atom is ionized when the number of electrons is not equal to the number of protons; the resulting electrical charge depends on the difference between the number of protons and the number of electrons.

See Rutherford-Bohr Atom Model, Ionization.

A barcode is an information code with a linear sequence of machine-readable elements. The elements are made up of rectangular black bars and white spaces between the bars. In document imaging, barcodes are typically used to encode index data and/or separate documents.

There are two kinds of beta decay: beta minus and beta plus decay. The differentiation depends on the charge of the emitted particle.
At the beta plus decay in the nucleus a proton changes to a neutron and emits a positron and a neutrino. The atom is after the decay a different element, but with the same number of particles in the nucleus.
At the beta minus decay in the nucleus a neutron changes to a proton and emits an electron and an antineutrino. As with the beta plus decay the atom changes to a different element but with the same number of particles in the nucleus.
Sometimes the electron capture is mentioned as a third kind of beta decay.
Beta decay is used for example in positron-electron tomography or in iodine-131 therapy.

See also Electron Capture.

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.