A text annotation is a sequence of characters permanently added to images before storing.

Decimation is the reduction of data at the digitized signal. Fewer samples are used to represent the detected signal. The opposite process is called interpolation, more samples are added.

See also Digital Radiography, and Digital Subtraction Angiography.

X-rays contain a range of energies (polychromatic photons), the higher energies pass through the patient, the lower energies are absorbed or scattered by the body. Ideally, the x-ray beam should be monochromatic or composed of photons having the same energy. Strong filtration of the beam results in more uniformity. The more uniform the beam, the more accurate the attenuation values or CT numbers are for the scanned anatomical region.
There are two types of filtration utilized in CT:
Inherent tube filtration and filters made of aluminum or Teflon are utilized to shape the beam intensity by filtering out the undesirable x-rays with low energy. Filtration of the x-ray beam is usually done by the manufacturer prior to installation. The half value layer provides information about the energy characteristics of the x-ray beam. Too much filtration produces a loss of contrast in the x-ray image.
A mathematical filter such as a bone or soft tissue algorithm is included into the CT reconstruction process to enhance resolution of a particular anatomical region of interest.

Ionization is the process of converting an atom or molecule with no electrical charge into an ion by adding or removing charged particles such as electrons or other ions, through high temperatures, electrical discharges, radiation etc.

See also Ion.

The numbers of nuclei or electrons in different energy levels. At thermal equilibrium, the relative populations of the energy levels will be given by the Boltzmann distribution.

See also Boltzmann Distribution.