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 'Half Value Layer' 
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Half Value Layer
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.
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Radiation Shielding
Radiation shielding is the process of limiting the penetration of radiation into the environment, by blocking with a barrier made of impermeable material. This protective barrier is usually formed of a material with high density, for example lead that absorbs the radiation.
Radiation sources are self-shielded with absorbing material incorporated into the equipment, adjacent to the source to reduce stray radiation to the surrounding area below dose limits.
Rooms with x-ray or other radiation equipment are additionally shielded with lead-lined walls to reduce the radiation exposure to humans within the facility. The amount of shielding required to protect against different kinds of radiation depends on how much energy they have. The shielding calculations are based on the half value layer of the primary radiation beam. Sufficient half value layers of shielding are calculated to reduce the radiation exposure outside the room to reasonable levels.
Personal shielding requirements depending on the type of radiation:
point Alpha rays are shielded by a thin piece of paper, or even the outer layer of human skin. Unlike skin, living tissue inside the body, offers no protection against inhaled or ingested alpha radiation.
point Beta particles, depending on their energy can penetrate the skin. Shielding and covering, for example with heavy clothing, is necessary to be personally protected against beta-emitters.
point Gamma rays and x-rays penetrate the body and other matter. Dense shielding material, such as lead, is necessary for protection. The higher the radiation energy, the thicker the lead must be. Lead aprons protect parts of the body against stray radiation.
See also Radiation Safety.

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Radiation protectionOpen this link in a new window
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Effective Energy
The effective energy (related to bremsstrahlung radiation from an x-ray machine) is the monoenergetic photon energy which produces the same first half value layer in a given material as the x-ray beam.
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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.
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