The terms used to describe radiation are confusing and complex. To make matters worse, two different sets of terms are commonly used to describe radiation related quantities. The United States has persisted in its use of older terms whereas much of the rest of the world has adopted newer terms call Standard International (SI) units. Despite this difficulty, mastery of a basic radiation vocabulary is necessary in order to understand some important fundamental concepts regarding radiation and in order to effectively communicate with other medical personnel who routinely work with radiation.
Radiation is a general term describing the outward propagation of energy in any one of a wide variety of forms (Fig. 2.1). A broad distinction is made between "ionizing" radiation, which has sufficiently high energy to strip electrons from atoms, and the less energetic "nonionizing" radiation.
Ionizing radiation is subdivided into non-particulate and particulate radiation. Non-particulate ionizing radiation (photons) consists of x-rays and gamma rays. Higher energy x-rays and gamma rays penetrate the body, are detectable outside the body and therefore are useful for imaging. X-rays and gamma rays are, by definition, distinguished by the way in which they are formed and are Radioguided Surgery, edited by Eric D. Whitman and Douglas Reintgen. © 1999 Landes Bioscience
indistinguishable once they have been emitted. X-rays result from transitions in the energy state of an electron. Gamma rays and the particulate forms of ionizing radiation are created when the nucleus undergoes a transition from a high energy state to a lower energy state. Gamma rays generally are of higher energy than x-rays, but there is considerable overlap. The magnitude of the energy content of a photon is expressed in electron volts (eV). Visible light photons (one form of nonionizing radiation) have energies of a few eV. X-rays and gamma rays used in diagnostic imaging have energies in the range of tens to hundreds of kilo-electron-volts (1000 eV = 1 kilo-electron volt or 1 keV).
Charged particulate radiation (e.g., alpha-rays and beta-rays) does not penetrate the body well and causes much greater biological harm than similar amounts of x-rays and gamma-rays. Charged particles can be used for therapy but not for imaging. Uncharged particles (e.g., neutrons and neutrinos) do penetrate the body, but are not easily detectable and neutrons have unfavorable dosimetry.
The words "radiation" and "radioactive" are often confused. An atom that is unstable spontaneously gives off radiation and is therefore radioactive. In contrast, an x-ray machine is not radioactive since it cannot spontaneously give off radiation (without an external power source). Patients who have had a chest radiograph do not spontaneously emit radiation. In contrast, patients who have been injected with a radioactive material will continuously emit radiation for a period of time.
The amount of radiation that patients emit from diagnostic nuclear medicine studies is quite small and not a significant problem (see the section on radiation safety below). The amount of radiation emitted decreases with time due to physical decay of the radionuclide (defined by a physical half-life) and elimination from the body (defined by a biological half-life). The physical half-life (t1/2p) depends on the radionuclide (radioactive atom or radioisotope). The most commonly used radionuclide, Technetium-99m (Tc-99m), has a t1/2p of 6 hours. This means that the amount of radiation decreases by a factor of two every 6 hours from physical decay alone. The biological half-life (t1/2b) depends upon the chemical compound to which the radionuclide is bound. The combination of the radionuclide and carrier molecule is termed a "radiopharmaceutical". In the United States, the most commonly used radiopharmaceutical for sentinel node lymphoscintigraphy is filtered Tc-99m sulfur colloid. For this radiopharmaceutical the rate at which the radiation decreases is primarily determined by physical decay since its t1/2b is very long.1,2
The amount (activity) of a radionuclide is expressed in terms of the number of atoms that are decaying (disintegrations) per unit time (second). This term is used because the number of counts detected by a radiation meter is generally directly related to the amount of radionuclide. For reasons discussed below, it is difficult to simply relate the amount of a radionuclide to the dose that a worker or patient might receive. In the United States, the unit for the measurement of the amount of activity is the curie (3.7 x 1010 disintegrations/second). The curie approximately equals the number of atoms disintegrating per second in a gram of radium. The SI unit for the amount (activity) of a radionuclide is the becquerel (1.0 disintegration/second). Curie amounts of radionuclides are not used in diagnostic studies. For most diagnostic studies, millicurie (mCi; 1000 mCi = 1 curie) amounts of a radionuclide are injected. For lymphoscintigraphy, microcurie (|Ci; 1000 |Ci = 1 mCi = 37 megabecquerels [MBq]) amounts are transported to the lymph nodes.
As a first approximation, the potential harmful effects of radiation are related to the amount of energy that was deposited from the radiation per gram of tissue. The unit used for absorbed dose is the rad (1 erg/ gram of tissue). The corresponding SI unit is the gray (1 gray = 100 rads). Another important determinant of the possible effects of radiation is the type of radiation. Some types of radiation are more damaging than others for the same amount of energy deposited because the energy is deposited more densely. For example, alpha rays can cause up to 20 times as much damage as gamma rays. The unit that takes into account the biological effectiveness of the radiation is called the rem. The SI unit for the rem is the sievert (1 sievert [Sv] = 100 rems). One rad of alpha rays might equal 20 rems but one rad of gamma rays equals one rem. A rem also equals 1000 millirems (mrem). Other determinants of the possible effects of radiation include the portion of the body exposed and the period of time over which the exposure took place.
The average radiation exposure to members of the public from natural sources of radiation (radionuclide in the soil and air, our food and bodies) is about 300 mrem per year. The occupational exposure limit is 5,000 rems per year to the body. The radiation dose to patients from lymphoscintigraphy would be less than 100 mrem.
The dose to workers from radionuclides is related to the amount of activity, the type of radiation emitted, the distance and the time spent near the source of radiation. Lead aprons are not as effective at decreasing the dose from most radionuclides as they are from x-rays because of the relatively higher photon energy of the gamma rays. Approximately 75% of the gamma rays from Tc-99m will be stopped by a standard 0.5 mm-thick lead apron compared to 95% of diagnostic x-rays.3 The dose to patients is also related to the amount of activity and the type of radiation emitted, however patients have little control over the distance and the time spent near the source of radiation. The physical and biological half lives and the biodistribution of the radiopharmaceutical determine how long the source of radiation is in the patient.
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