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.

(FDA) An agency of the US federal government established by Congress in 1912 and presently part of the US Department of Health and Human Services. The FDA gives classifications of medical devices according to potential risks and controls the safety of marketed drugs.
For more details please visit the FDA's 'Our Mission and Mandate'.

See also Class I II III Devices and Phase 1 2 3 4 Drug Trials.

The half value layer (also half value thickness) is the amount of absorbing material, which is needed to reduce the x-ray intensity by one half.
The half value layer provides important information about the energy characteristics of the radiation. A low (or thin) half value layer for a given x-ray beam indicates that the x-ray beam contains more low energy and less penetrating radiation. An x-ray beam containing high energy or highly penetrating radiation has a high (or thick) half value thickness.

[Also: Half-Life Time, Radioactive Half-Life] The half-life is the time in which half the atoms (always a fraction, not a number) of a given radionuclide disintegrate from the amount of atoms present when measurement starts. From 200 atoms of a radionuclide with a half-life of one minute will 100 atoms disintegrate in the first minute, 50 in the second minute, etc. The half-life is a characteristic property of radioactive isotopes. The effective half-life includes all processes of elimination, including radioactive decay.
Different half-life terms:
- Physical Radioactive Half-Life
- Biological Radioactive Half-Life
- Effective Radioactive Half-Life.

See also Decay Constant, Decay.

(kg) The base SI unit of mass of the metric system.
Definition: 1 kilogram is defined as the mass of the standard kilogram, a platinum-iridium bar in the custody of the International Bureau of Weights and Measures (BIPM) near Paris, France.
A traditional unit of mass or weight is also the pound (in general use, e.g. in the United States and Great Britain), with the symbol lb (derived from the Latin word libra).

1 kg = 2.204627 pound (lb. av., lbs.)
1 pound (lb. av., lbs.) = 0.453 kg.

Smaller units are, e.g.
1 000 gram (g) = 1 kg
1 000 milligram (mg) = 1 g
1 000 microgram (µg) = 1 mg