Basic Considerations of
Radiation Safety and Barrier Protection

Glossary of Radiation Terms Absorbed dose: The concentration of energy deposited locally in tissue and an important measure of the potential for biological effects. Absorbed dose is measured in units of gray (Gyt) or milligray (mGyt) where the subscript "t" specifies the dose as being in tissue. Alpha radiation: High-energy radiation that comes from radioactive atoms when broken apart. Attenuate: To become thin, weak or fine. Reduced or weakened, as in strength, value or virulence. To reduce in force, value, amount, or degree; weaken. Attenuation: The process by which the number of particles or photons entering a body of matter is reduced by absorption and scattering. Background radiation: Unavoidable radiation that occurs all around us. The typically quoted average individual exposure from background radiation is 360 millirems per year. Beta radiation: High-speed electrons that come from radioactive atoms when broken apart. Cumulative dose: The total dose resulting from repeated exposures of ionizing radiation to the same portion of the body, or to the whole body, over a period of time. Deterministic effect: Health effects of radiation; the severity of which varies with the dose and for which a threshold is believed to exist. Deterministic effects generally result from the receipt of a relatively high dose over a short period of time. (Formerly called non-stochastic effect.) Dosimeter: A small portable instrument such as a film badge, thermoluminescent, or pocket dosimeter for measuring and recording the total accumulated dose of ionizing radiation. Effective dose: The dose averaged over the entire body. Fluoroscopy: The momentary production and display of serial x-ray images for the purpose of observing real-time motion of internal anatomic structures. Fluoroscopy can deliver much larger doses of radiation than conventional x-rays. Gamma rays: Electromagnetic waves or photons emitted from the nucleus (center) of an atom that are of a very short wavelength and very high frequency, similar to x-rays. Genetic effects: Effects from some agent that are seen in the offspring of the individual who received the agent. The agent must be encountered pre-conception. Grid: A flat plate device that improves image contrast by selectively shielding the image intensifier from scattered x-rays. Gray (Gy): The new international system (SI) unit of radiation dose expressed in terms of absorbed energy per unit mass of tissue. The gray is the unit of absorbed dose and has replaced the rad. 1 gray = 1 Joule/kilogram, and also equals 100 rad. High voltage: Tube voltage controls x-ray energy. High voltage is expressed as kilovolt peak (kVp) and usually ranges from around 60 to approximately 125 kVp. The kVp affects the penetration and the intensity of x-rays. HVL: Half-value layer of x-ray beams (the thickness of a material required to attenuate a raw x-ray beam intensity by a factor of 2). Aluminum is usually the material used to measure HVL. Ionizing radiation: Radiation that can ionize and is especially dangerous to living tissues. Examples are gamma rays and neutrons. Joule: A unit of energy. kVp: Stands for kilovoltage peak and is the power and strength of the x-ray beam (quality of the x-rays). This is the highest voltage (measured in thousands of volts) that will be produced by the x-ray machine during an exposure. kVp controls the penetrating strength of an x-ray beam (beam quality). Whenever an exposure is made, the x-rays must be strong enough to adequately penetrate through the area of interest. The higher the kVp, the more likely the x-ray beam will be able to penetrate through thicker or denser material. Most x-rays used in medical imaging are between 50 and 120 kVp (50,000 to 120,000 volts). Higher kVp settings produce more scatter radiation. mSv: The scientific unit of measurement for radiation dose commonly referred to as the effective dose.We are exposed to radiation from natural sources all the time. The average person in the U.S. receives an effective dose of about 3 mSv per year from naturally occurring radioactive materials and cosmic radiation from outer space. These natural background doses vary throughout the country. Non-stochastic effect: Effects that can be related directly to the dose received. The effect is more severe with a higher dose; i.e., the burn gets worse as the dose increases. It typically has a threshold below which the effect will not occur. A skin burn from radiation is a non-stochastic effect. (This term has been replaced with deterministic effect.) Photon: A quantum or packet of energy emitted in the form of electromagnetic radiation. X-rays and gamma rays are examples of photons. Rad: Radiation absorbed dose - a measure of the quantity of energy absorbed from ionizing radiation. The rad has been replaced by the gray in the SI system of units (100 rad = 1 gray). Radiation: Energy given off by the nucleus of an atom in the form of particles or rays. Rem: Röntgen equivalent man - a measurement for how dangerous or potentially harmful radiation is to living tissue. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Scattered radiation: Radiation that, during its passage through a substance, has been changed in direction. It is a form of secondary radiation. When x-rays interact in a patient, many are scattered in random directions from the exposed volume of the patient. (These scattered x rays are the principal source of radiation exposure to personnel during fluoroscopy.) Sievert: The international system (SI) unit for dose equivalent equal to 1 joule/kilogram. The Sievert has replaced the rem. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. One Sievert is equal to 100 rem. Somatic effects: Effects from some agent, like radiation, that are seen in the individual who receives the agent. Stochastic effect: An effect that occurs on a random basis, with its effect being independent of the size of dose. The effect typically has no threshold and is based on probabilities, with the chances of seeing the effect increasing with dose. In the context of radiation protection, the main stochastic effect is cancer.

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