RADIATION

Radiation is everywhere. Life would not exist on Earth without radiation from the sun. Additionally, many important technological activities are based on radiation, such as radio and telecommunications. Another type of radiation is used for producing X-ray images in industrial and medical applications. Radiation is also emitted as a side effect from various technological activities. Some types of radiation are known to be harmful to human beings and need to be carefully managed. Other types are not believed to be dangerous, but are a source of worry among the general public. An example is possible radiation risks from power lines, cellular phones, and cellular base stations, which since the 1980s have received considerable media attention.

Protection of humans and the environment from the harmful effects of radiation is called radiation protection. The field of radiation protection evaluates scientific knowledge of adverse health effects from radiation and influences legislation and regulations for protection. The field is complex and involves intricate ethical problems. Lauriston S. Taylor, one of the pioneers of radiation protection during the early 1900s, once said, "Radiation protection is not only a matter for science. It is a problem of philosophy, morality and the utmost wisdom" (1980, p. 854).

It is important to distinguish between ionizing and nonionizing radiation. The biological effects of the two types of radiation are very different, as are therefore the methods of protection. Radiation is ionizing if the energy of the radiation suffices to remove an electron from an atom to create an ion. Conversely, if the energy does not suffice to create ions it is called nonionizing.

Nonionizing Radiation

The most important types of nonionizing radiation are electromagnetic and consist of electric and magnetic waves propagating at the speed of light. Electromagnetic radiation comes from both natural and technological sources and has different properties depending on the frequency of the electromagnetic waves. Low-frequency electromagnetic fields and radio waves come from electric appliances, power lines, radio and television broadcasting, and natural sources such as thunderstorms. Microwaves are used in microwave ovens, radar, and telecommunications. Infrared radiation, visible light, and ultraviolet radiation are emitted from the sun, artificial light, and other technical applications. Electromagnetic radiation with frequencies above visible light has enough energy to change chemical bonds and cause ionizations. Ultraviolet radiation lies on the borderline between nonionizing and ionizing radiation, but is usually considered nonionizing.

The biological effects of nonionizing electromagnetic radiation depend on the frequency and the intensity of the radiation. Low-frequency electromagnetic fields and radio waves pass through human bodies without any apparent effects, but can induce electrical currents and stimulate human nerve cells at high intensities. Microwaves cannot penetrate far into human bodies, but high intensities can cause heating of tissue and burn injuries to the skin. Infrared radiation and visible light can produce surface heating and cause harm to the eye in high intensities. Ultraviolet radiation cannot penetrate the skin, but is known to cause skin cancers.

Claims that low-frequency electromagnetic fields and microwaves can cause cancer are controversial. These types of radiation have insufficient energy to damage the DNA directly, and no other mechanism is known through which they could cause cancer. The prevailing scientific view is that these types of radiation are unlikely to cause cancer. Other effects, such as reduced fertility, memory loss, and fatigue, have been reported, but there is no consistent evidence for these kinds of adverse health effects.

International and national recommendations on exposure limits for nonionizing radiation are based on guidelines from the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The ICNIRP is a nongovernmental organization officially recognized by the World Health Organization (WHO), the International Labour Organization (ILO), and the European Union (EU). The ICNIRP recommends exposure limits for different types of nonionizing radiation. The exposure limits are set with a margin of safety to the level at which health effects occur. The ICNIRP guidelines are based on scientifically verified health effects of nonionizing radiation. Potential, but not proven, hazards are not used as a basis for the limits.

The most important ethical issue regarding nonionizing radiation concerns how to deal with potential health hazards that are scientifically controversial. Examples include the possible risks of radiation from power lines and cellular base stations. Typical exposure levels in these cases are substantially lower than the exposure limits recommended by the ICNIRP, but they do introduce new exposures into society and, in the case of cell phones, such exposures are centered around sensitive parts of the human body. Thus, some countries have, in addition to the recommended exposure limits, adopted precautionary strategies for managing possible hazards from nonionizing radiation. These strategies include the use of prudent avoidance and the precautionary principle.

Prudent avoidance can be defined as a general reduction of needless exposure. This means taking simple, easily achievable, low-cost measures, even in the absence of a demonstrable health hazard. Prudent refers to expenditures and does not include any requirement for assessment of the potential health benefits of adopted measures. In practice, this means that the location of new facilities can be influenced by prudent considerations, but need not be modified, because this would involve higher costs. Prudent avoidance can also take the form of voluntary measures, for example, to recommend that manufacturers of mobile phones minimize radiation exposure to the head.

The precautionary principle is not a single, well-defined principle, but the basic idea is that measures against a possible hazard ought to be taken even if evidence for the existence of the hazard does not suffice to be treated as a scientific fact. It is usually thought that the application of the precautionary principle should be science-based and should reference plausible explanations for possible mechanisms for hazards. A common further requirement is that precautionary measures should be temporary and subject to review when further knowledge is gathered. Because scientific evidence and plausible mechanisms are missing for possible risks of low levels of nonionizing electromagnetic radiation, it has been argued that the precautionary principle is inappropriate for these types of radiation.

Adopting precautionary approaches are not unproblematic. What level of precaution should be taken, and what should be the basis for the decision? The WHO has argued that precautionary approaches regarding non-ionizing electromagnetic radiation should be adopted with care, and under the condition that scientific assessments of risk and science-based exposure limits are not undermined by arbitrary precautionary approaches.

Ionizing Radiation

Radiation is ionizing if it has enough energy to ionize atoms and molecules. There are two types of ionizing radiation: high-frequency electromagnetic radiation and particle radiation. Examples of ionizing electromagnetic radiation include gamma rays and X rays. Most particle radiation is ionizing. Common types of particle radiation are alpha (helium nuclei), beta (electrons), neutron, and proton radiation.

Ionizing radiation originates from both nonhuman and human sources. Nonhuman or natural sources of ionizing radiation are cosmic rays and naturally occurring radioactive substances in Earth's crust, the human body, air, water, and food. The level of natural exposure varies around the globe, and cosmic radiation is more intense at higher altitudes. The total exposure from all natural sources is called natural background radiation. The natural background radiation is by far the greatest contributor to human exposure to ionizing radiation.

Some human activities can enhance the exposure from natural sources. Examples include radon gas from the soil that concentrates in buildings, mining, and the combustion of fossil fuels that contain radioactive substances. Aircraft passengers and crew are subject to higher levels of cosmic radiation at flight altitudes. Environmental contamination by radioactive residues come from atmospheric nuclear weapons tests (performed between 1945 and 1980), the Chernobyl accident (1986), and the operation of nuclear power plants. These activities contribute only a small fraction of the global average exposure to ionizing radiation.

The largest human-made exposures to ionizing radiation stem from medical procedures. Medical exposures include diagnostic exposures (such as X-ray examinations) and therapeutic exposures (as in tumor treatment). Occupational exposure to ionizing radiation affects workers in industry, medicine, and research. The level of occupational exposure is generally similar to that of the average natural exposure. A few percent of workers are exposed to radiation levels several times greater than the average natural exposure. A comparison between the average exposures from different sources of ionizing radiation is listed in Table 1.

The biological effects of ionizing radiation are generally well known. Ionizing radiation can cause cell death and acute harm to organs if sufficient numbers of cells are damaged. Another type of damage occurs in cells that are modified. This may lead to inheritable genetic changes and the development of cancer, which may manifest itself decades after exposure. Acute effects

occur if the radiation dose is substantial (as in accidents), while it is believed that cancer and hereditary effects may be caused by the modification of a single cell. As the dose increases, the probability of these effects also increases.

The effects and penetration of ionizing radiation depend on the type of radiation. Exposure from ionizing radiation is therefore quantified by the effective dose, which is a measure that takes the type of radiation into account. The unit for the effective dose is the sievert (Sv). One sievert is a very large dose, and it is common to express the effective dose in millisieverts instead (1 mSv = 0.001 Sv). Sometimes the unit rem is used instead (1 rem = 0.01 Sv).

Epidemiological data argue for a linear relation between the dose and the cancer risk from ionizing radiation for intermediate dose levels. A linear dose–effect relation means that an increase in dose implies a corresponding increase in effect. Because of statistical limitations, the dose–effect relation cannot be determined for low doses. Therefore, the risks of low-dose ionizing radiation must be estimated based on knowledge of biological mechanisms that cause or inhibit cancer and inheritable defects. The dose–effect relation for low doses is important, because the exposure to the public or in normal work situations are in ranges where the risk is uncertain (below 50 mSv).

It is especially important to know if there is a threshold for the dose–effect relation for ionizing radiation. If there is no threshold, there is a (small) risk associated with even very low exposure levels. The prevailing scientific consensus, represented by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), is that a threshold is unlikely and that a linear dose–effect relationship for small doses is consistent with current knowledge about the mechanisms by which ionizing radiation causes harmful effects. This view is challenged by those who believe that there is a threshold (and thus no risk) for very low doses of ionizing radiation. Some even argue for a positive effect called hormesis at very low levels.

Setting Standards

Radiation protection from ionizing radiation is generally the same all over the world, because of the profound influence of the International Commission on Radiological Protection (ICRP), a nongovernmental organization whose recommendations are used by both national radiation protection authorities and international organizations as a basis for more detailed guidelines. The ICRP works under the assumption that the risk of cancer and hereditary effects from low doses of ionizing radiation is without a threshold and that the dose–effect relation is linear—the so-called linear, no threshold assumption. This approach to the risks of low-dose ionizing radiation can be seen as precautionary, although the assumption is supported by scientific knowledge.

The 1990 ICRP recommendations are based on a system of three principles: justification, optimization, and dose limitation. The justification principle states that no additional dose should be tolerated unless there is an associated benefit to the exposed individuals or to society that outweighs the detriment. Though the principle may seem obvious, its application gives rise to complex ethical issues. The concepts of benefit and detriment are difficult to define, and calculations are often associated with great uncertainties and errors. Other ethical issues include how the benefit for society can be weighed against the detriment to individuals, issues of free and informed consent, and who should make the decisions (for example, stakeholders or experts).

According to the optimization principle, total exposure should be kept as low as reasonably achievable (or ALARA), with economic and social factors taken into account. (Based on the acronym, this principle is sometimes called the ALARA principle.) What is reasonable depends on economic considerations, which means that doses need not be lowered further if the economic cost would be too high. The principle is thus a trade-off between economics and protection. Cost–benefit analysis has often been applied for optimization of protection, although the ICRP stresses that it is only one possible method.

The optimization principle does not consider the distribution of doses among individuals. A strict application of the principle may thus, at least in theory, lead to a situation in which a few individuals are exposed to substantially higher doses than others. The optimization principle can be seen as utilitarian or consequentialist, focusing on total rather than individual effects.

The dose-limitation principle requires that individual doses not exceed unacceptable levels. This principle can be seen as deontological, because it implies a duty to protect individuals from undue harm. In many cases, the optimization principle and the dose-limitation principle coincide, but there can be cases in which the two principles conflict. In the ICRP system such conflicts are resolved by first applying the dose-limitation principle and after that the optimization principle, deontology before utility.

Under the common assumption that cancer and hereditary effects do not have a threshold, a dose limit (above zero) cannot yield a completely safe level. The dose limits should, according to the ICRP, be regarded as the boundary to unacceptable doses, and protection should essentially be due to the optimization principle. As a dose limit cannot yield a wholly safe dose, a decision on a dose limit will always involve value judgments and ethical considerations. What is acceptable or not is a complex ethical issue, and judgments are not necessarily the same in all contexts.

The dose limits recommended by the ICRP are 1 mSv per year for the public and 20 mSv per year for occupational exposure. A special question regarding dose limits is why it is acceptable for workers to be exposed to higher risks than the public. This is an ethically problematic issue, not just for radiation protection. Arguments that have been used are that the limit for the general public concerns exposure for the whole life and not just the working life, and that the public includes children and other more susceptible individuals. Workers may also be informed of their exposure levels and thus voluntarily accept them, whereas the public has no alternative.

An important concept in radiation protection from ionizing radiation is the collective dose. The collective dose is defined as the mean dose for each individual in an exposed population multiplied by the number of individuals. There has been considerable controversy over what influence the value of the collective dose should have. Considerable collective doses can arise from exposure to large populations even if the dose to each individual is very low. This may be the case in global contamination from radioactive substances (such as in atmospheric nuclear weapons tests) or in contamination that stretches very far into the future. If the risk of cancer from ionizing radiation is proportional to the dose and without a threshold, it follows that the expected number of cancer cases is proportional to the collective dose. In spite of this, it has been argued that small individual doses should not pose a problem even if the collective dose is great.

Arguments to the effect that "risks ought to be disregarded if they are sufficiently small" are called de minimis arguments. Common arguments for calling risks de minimis are that they are trivial compared to other risks humans accept, that they are trivial in comparison to natural risks, or that they have to be disregarded in order to avoid the allocation of unreasonably large economic resources to investigate or manage them. It has often been claimed that risks with a probability on the order of magnitude of one in a million or smaller are de minimis. Nevertheless, such a general de minimis level is ethically problematic because it would allow many small risks that in combination may yield a large risk for an individual. Furthermore, many small risks to many people may also yield a large total effect. For example, exposing each of ten million persons to an independent risk of death of one per million yields ten expected fatalities. Also, the mathematical "law of large numbers" yields that the actual outcome will be around ten fatalities.

Another ethical problem in radiation protection arises from the long-term management of radioactive waste. Radioactive materials may be dangerous for hundreds of thousands of years, and mistakes made now may affect future generations. This problem is not exclusive to radioactive waste, because many other technological activities have consequences reaching far into future; examples include emissions that may lead to global climate change and damage to the ozone layer. The discussion regarding radioactive waste is nevertheless important, because many countries have not made final decisions for long-term management of the radioactive waste from nuclear reactors and/or nuclear weapons. The problem of distant future effects poses intriguing ethical problems. What is the moral status of future, nonexisting individuals and what duties do persons today have toward them? The International Atomic Energy Agency (IAEA) is of the opinion that radioactive waste should be managed in such a way that predicted impacts on the health of future generations will not be greater than today and that no undue burden is imposed on future generations.

PER WIKMAN

SEE ALSO Chernobyl;Hormesis;International Commission on Radiological Protection;Regulatory Toxicology.

BIBLIOGRAPHY

International Atomic Energy Agency (IAEA). (1996). Radiation Protection and the Safety of Radiation Sources. Vienna: Author. The foundation document of the IAEA system of protection.

International Commission on Non-Ionizing Radiation Protection (ICNIRP). (1998). "Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (Up to 300 GHz)." Health Physics 74(4): 494–522. Describes the rationale for the ICNIRP guidelines for limiting exposure to electromagnetic fields.

International Commission on Non-Ionizing Radiation Protection (ICNIRP). (2001). "Review of the Epidemiologic Literature on EMF and Health." Environmental Health Perspectives 109(suppl. 6): 911–933. An authoritative overview of the knowledge base for the risks from electromagnetic fields.

International Commission on Non-Ionizing Radiation Protection (ICNIRP). (2002). "General Approach to Protection against Non-Ionizing Radiation." Health Physics 82(4): 540–548. The foundation document of the ICNIRP system of protection.

International Commission on Radiological Protection (ICRP). (1992). 1990 Recommendations of the International Commission on Radiological Protection. Oxford: Pergamon Press. The foundation document of the ICRP 1990 system of protection that has had a profound influence on radiation protection internationally.

Lindell, Bo. (1985). Concepts of Collective Dose in Radiological Protection: A Review for the Committee on Radiation Protection and Public Health of the OECD Nuclear Energy Agency. Paris: Organisation for Economic Co-operation and Development, Nuclear Energy Agency. A detailed guide to the concept of collective dose, and related ethical issues.

Silini, Giovanni. (1992). "Ethical Issues in Radiation Protection: The 1992 Sievert Lecture." Health Physics 63(2): 139–148. Reviews ethical issues that have been considered in the development of radiation protection from ionizing radiation.

Sowby, David, and Jack Valentin. (2003). "Forty Years On: How Radiological Protection Has Evolved Internationally." Journal of Radiological Protection 23(2): 157–171. An overview of international organizations in radiation protection and the development of the ICRP recommendations since 1950.

Taylor, Lauriston S. (1980). "Some Nonscientific Influences on Radiation Protection Standards and Practice: The 1980 Sievert Lecture." Health Physics 39(6): 851–874. A classic text on more philosophical aspects of radiation protection by one of the first members of ICRP.

United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). (2000). Sources and Effects of Ionizing Radiation: UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes. 2 vols. New York: United Nations. An authoritative overview of the knowledge base for the risks from ionizing radiation.

Wikman, Per. (2004). "Trivial Risks and the New Radiation Protection System." Journal of Radiological Protection 24(1): 3–11. Examines ethical and philosophical aspects of a novel way of thinking in radiation protection.

Radiation

BIBLIOGRAPHY

This article is devoted to examining the branch of psychology that is concerned with the behavior of living organisms after their exposure to ionizing radiations. The subject is a relatively new one in the field of psychology and is allied most closely to topics subsumed under the headings “physiological psychology” and “radiobiology.” At various times the subject has fallen under the headings “radiation psychology” and “radiopsychology” or, most often, under various forms of the heading “effects of radiation upon behavior.”

The field has been of practical interest to the medical sciences since the discovery of X rays in 1895, although behavioral scientists did not enter the field formally for another three decades. Since the inception of radiation research, the biomedical literature has reflected concern with behavioral data in terms of their application to diagnosis, abnormal growth patterns, heredity, and lethality. From the beginning, biomedical scientists found that behavioral effects of radiation provided substantive data to aid in the understanding of the structure and function of organ systems. Perhaps the most recent application of this field to applied sciences has taken place since the advent of the space age, when those concerned with the effects of unusual environmental conditions upon man have focused attention on the radiation hazards in space (Hanrahan & Bushnell 1960, pp. 155-188).

The influence of radiation no longer remains the exclusive interest of radiation scientists. Nuclear weapons have vastly widened the scope of attention, and the layman has long recognized that he lives in an age when radiation is a highly publicized fact of life. The employment of atomic energy for peaceful uses has led to the introduction of nuclear power plants into community life. It has now become relevant to acquaint the nonspecialist with the legal implications of radiation for labor and management. It is to the general interest of all to understand the relationship of radiation to all living matter so that appreciation may be developed for the opportunities and the complications now presented to us. Power plants will create more and cheaper energy sources; but power plants will create health hazards and contamination. New concepts of waste disposal will have to be developed, and these must be integrated with the thinking of city planners, oceanographers, sociologists, economists, and others who are concerned with the rapidly changing modern world. Those interested in population change will find the radiation scientists have much to say about processes of aging and heredity. It will be of interest to the geopolitically minded that radiation psychologists are investigating the social dynamics of groups who have been, or who may be, exposed to radiation. To those involved in influencing public opinion, the implications of this field are self-evident in a world of ideological conflict.

The problems of studying the effects of radiation are numerous and complex (see Bibliography). The extent of this complexity is well described by Van Cleave (1963, p. 356), who stresses that most new discoveries “lead to new horizons of the unknown.”

Major variables to be studied. The psychologist interested in radiation is concerned with relating his behavioral data to their underlying physiological processes. This requires that the following variables must be assessed from both the psychological and biological point of view, under comparable environmental conditions:

The stimuli. There are two classes of ionizing radiations, material and electromagnetic. The former include neutrons, alpha and beta particles, deu-terons, and protons; the latter include X rays and gamma rays. All, by definition, are capable of changing cell structure by dislodgement of electrons from atoms. (The references provide extensive descriptions of the characteristics of these stimuli.) The psychologist studying radiation must concern himself with them individually and in combinations. Thus, he is faced with examining the effects of many energy levels, e.g., from 70 kilovolts to several million electron volts for X rays and gamma rays, from 1,000,000 to 14,000,000 electron volts for fast neutrons, and so on, for each of these radiations.

Method of exposure. Radiation is known to affect the target differentially, depending on how it is delivered. For example (Bond et al. 1965), doses of a few hundred rads to the whole body will cause death within days, while doses of several thousands of rads are delivered to focal points of the body in radiation therapy without lethal effect. The effects of exposure, therefore, need to be considered in terms of whether the dose is delivered to the whole body or to a part of it. Both types of exposure must be related to frequency and duration of exposure and to rates of delivery of the dose over various distances from source to target. Any shielding conditions in the exposure environment must likewise be examined.

Behavior to be studied. All behavior is the object of the radiation psychologist’s investigations. The affective, cognitive, sensory, and motor aspects of an individual organism’s behavior, as well as social interaction of the radiated individuals, are considered.

Subjects to be employed. All living species are potential objects of investigation. Special consideration must be given to such factors as age at time of exposure, age at time of behavioral study, sex, and inherited and acquired characteristics related to the behavior to be studied. Certain animals become subjects of choice because of their known biological characteristics, the presence of which may aid in the clarification of a specific question related to a radiation effect.

Instrumentation and procedures. The equipment and methods employed may be those standardized by experimental psychology, or they may be especially devised to test for unique aspects of behavior elicited by radiation. Experimenters have frequently used normal control subjects for comparison with experimental populations, but they have just as often used radiated subjects as their own controls for preradiation and postradiation capabilities.

Nature of studies undertaken. An examination of the experimental findings in the behavioral field reveals attempts in many parts of the world to explore the aforementioned variables. The many recent and comprehensive reviews of the subject permit the reader to follow the work to a degree impossible to duplicate in this brief report (Stahl 1959; I960; Lebedinskii et al. 1958; Russell 1954; Rugh 1962; 1963; Furchtgott 1956; 1963; 1964; Van Cleave 1963; International Symposium …1964; WHO 1957; Northwestern Univ. 1962; Lebedinskii & Nakhil’nitskaia 1963; Bond et al. 1965; Kimeldorf & Hunt 1965). All of these sources call attention to the relevant allied science reviews and provide extensive bibliographies.

Kaplan (1962, pp. 153-160) has provided an additional review, in which he calls attention to the techniques and procedures employed in the field. Although this paper is limited to an analysis of the rat maze experimentation, it conveys im-plications about the problems associated with techniques involving all species, with all behavioral subject matter. The summaries of all the referenced compilations yield the following types of information:

(1) A wide assortment of living organisms have been employed as subjects. Men, subhuman primates, dogs, cats, rodents, fowl, fish, and many invertebrate forms have variously been exposed to X, gamma, alpha, beta, and neutron radiation, as well as to mixtures of these. Dose levels have been given from fractions of a roentgen (r) to over 100,000 r. Rates of radiation delivery to both the whole body and to focal points of the body vary from experiment to experiment. Some studies have been devoted to single and others to additive radiation exposures. Studies have utilized both lethal and sublethal doses.

(2) Learning, general activity, emotionality, mating behavior, classical and instrumental conditioning, and visual, auditory, vestibular, cutaneous, olfactory, and kinesthetic functions have been explored. The employment of mazes, classical and operant conditioning devices, primate general test apparatus, and open field boxes are among the many testing instruments employed.

(3) Differences in procedures and apparatus and failure of behavioral scientists to standardize methods and equipment have led to slow advancement and difficulty in generalizing from available findings.

Effects of radiation on behavior. While a vast amount of knowledge has been accumulated by all the disciplines concerned with radiation, there remain unresolved problems. Nonetheless, even with controversial and contradictory evidence casting some doubt, it is relatively safe to state that some points have been established:

Developing organisms. The following data seem to be indicative of some of the radiation effects on developing organisms.

(1) Deficiencies in maze learning occur in rats irradiated on any gestation day and during the neonatal period. These deficiencies can be elicited by a dose as low as 25 r. The extent of the deficit is a function of the time and amount of radiation given and the age at time of testing, the type of maze employed, and the sex of the animal. In general, maze-learning deficiency is more apparent for sublethal doses (100-200 r) delivered in utero around the fourteenth day. Testing of animals for acquisition of maze-learning skill when they are around one year of age reveals greater deficit than appears in rats tested at an earlier age. The female is more adversely affected than the male; a Lashley III type maze, which has been shown to be especially sensitive for studying brain damage (Lashley 1929), is the instrument used to obtain the findings.

(2) Alterations in activity and locomotion result from prenatal and neonatal exposure with doses as low as 25 r. Radiation in utero seems to enhance these behaviors, while neonatal exposure inhibits them.

(3) Prenatally irradiated subjects exhibit greater “fearfulness,” a measure of emotionality, than do normals. A 50 r dose seems to be sufficient to elicit this behavior.

(4) Although there are reports of a few cases of personality aberrations and mental deficiencies in Japanese irradiated in utero, as a result of the bombing of Hiroshima and Nagasaki, there is insufficient data available to draw conclusions regarding this population (Miller 1956).

Adult organisms. For adult organisms some of the effects of radiation are:

(1) It has been generally accepted that immediately following doses in sublethal and even lethal ranges, higher-order learning processes and their retention are not impaired (Northwestern Univ. 1962; Furchtgott 1963). For example, Kaplan and his associates (see Pickering, Langham, and Ram-bach 1956) have shown that after exposure to massive doses of gamma radiation, monkeys were able to learn to discriminate between stimuli to which they had never before been exposed. These same animals manifested memory for discriminations they had learned to make before they were irradiated. Exceptions have been reported (Sharp & Keller 1965) which indicate that monkeys exposed to doses which cause death within hours will show impairment of preradiation-learned performances which involve memory of time perception. If the irradiated subject survives beyond a thirty-day period the dose is then not viewed as lethal. For animals receiving nonlethal doses, there is evidence after months and years of changes in performance which are attributable to radiation exposure (Davis 1965). Among the changes noted are alterations in motivation, extent of distracti-bility, and narrowing of attention, all factors related to higher-order learning and retention.

(2) Drive is adversely affected by radiation exposure, in direct association with the reduction of food and water intake and the decrement in general activity.

(3) Radiation has been shown to be effective as an unconditioned stimulus. Conditioned avoidance responses can be elicited with as little as lOr, providing a means for testing conditioned responses to sound, light, taste, and object quality.

(4) Sublethal doses up to l,100r have resulted in increases in self-involved, and a diminution in socially oriented, behavior of survivors. This finding was consistent during a period of seven years of observation of subjects.

(5) Low doses of X rays have been shown to be visible. Also, it appears that visual functions are depressed by radiation.

(6) Cutaneous, gustatory, olfactory, and intero-ceptive mechanisms are found to have reversible and unstable alterations of response to radiation. Auditory responses, also, reflect these characteristics, with conflicting reports of both increases and decreases of threshold.

(7) Very low doses have been shown by some Russians to alter conditioned reflex behavior (Stahl 1959; I960; Lebedinskii et al. 1958).

Neurophysiological aspects. Since radiation psychology falls quite logically under the broader fields of physiological psychology and radiobiology, attention should be directed to interrelationships between neurophysiological processes affected by radiation and related behavioral adjustments. The body of biological data is extensive and may be studied in the previously mentioned source books. Some of the most relevant findings can be seen to relate closely to behavioral results:

(1) The relative radiosensitivity and resistance of various cells and tissues have been identified, with strong evidence that the greatest sensitivity is in the lymphocytes and granulocytes and the least sensitivity is in the muscle, bone, and nerve cells.

(2) Radiation impairs the ability of the cells to maintain metabolic equilibrium and ultimately provides evidence for cell deviancy both morphologically and functionally.

(3) For developing organisms, the anatomical changes are maximal if radiation occurs during the period of major organogenesis. There is also considerable morphological data to support the conclusion that radiation delivered at any time to a developing organism has a deleterious effect upon neurophysiological function.

(4) It is particularly relevant to mention the many neurological signs observed in organisms after neonatal radiation: tremor, leg dragging, head deviations, uncoordinated gait, elevated pelvic posture, and chronically abducted limbs.

(5) Adult neural tissue has for the most part been established as being markedly radioresistant, with doses in the l,000r level failing to create impairment. Nonetheless, a growing body of data from European and Russian laboratories gives functional evidence of neural impairment after minute doses.

(6) Innumerable references are available that reflect radiation effects upon the interaction of the nervous system with endocrine, cardiovascular, and gastrointestinal functions. All of these factors are strongly related to the behavioral findings noted.

It is too early in the development of the field of radiation to predict the direction it will take. However, evidence does indicate that radiation can be a useful tool in the study of embryonic development (Hicks 1958). Suggestions are being made for the exploration of the use of radiation as a diagnostic tool in clinical psychology and psychiatry, through employment of radioisotopes in chemotherapy and physical therapy. Its usage in studying electronically developed brain models points to a new behavioral domain of investigation allied to computer technology. The need for exploring the sociological implications of a radiated society demands development of behavioral science techniques for identifying radiation factors that interact with other group attributes.

Perhaps the most important need in the field today is to standardize procedures when findings appear to be consistent. The reduction of controversial data to a minimum will permit generalization within psychology and between the other disciplines in this field. The fact that so many disciplines are linked to the radiation field demands this type of liaison and interdisciplinary cooperation if the massive task of integrating the knowledge of radiation already obtained in the atomic era is to be accomplished.

Sylvan J. Kaplan

[Other relevant material may be found inNervous System; Nuclear War; Space, Outer.]

BIBLIOGRAPHY

Bond, Victor P.; Fliedner, Theodore M.; and Archam-beau, J. O. 1965 Mammalian Radiation Lethality: A Disturbance in Cellular Kinetics. New York: Academic Press.

Davis, Roger T. 1965 The Radiation Syndrome. Volume 2, pages 495–524 in Allan M. Schrier, Harry F. Harlow, and Fred Stollnitz (editors), Behavior of Non-human Primates: Modern Research Trends. New York: Academic Press.

Ellinger, Friedrich P. (1935) 1947 The Biological Fundamentals of Radiation Therapy. London: Cleaver-Hume. → First published in German.

Furchtgott, Ernest 1956 Behavioral Effects of Ionizing Radiation. Psychological Bulletin 53:321-334.

Furchtgott, Ernest 1963 Behavioral Effects of Ionizing Radiations: 1955-1961. Psychological Bulletin 60: 157-199.

Furchtgott, Ernest 1964 Postscript to “Behavioral Effects of Ionizing Radiations: 1955-1961.” Psychological Bulletin 61:236-238.

Glasser, Otto et al. (1944)1961 Physical Foundations of Radiology. 3d ed., rev. & enl. New York: Hoeber.

Gray, L. H. 1949 Comparative Studies of the Biological Effects of X-rays, Neutrons, and Other Ionizing Radiations. Pages 114–137 in N. Howard-Jones (editor), Applied Biopfit/sics; Survey of Physical Methods Used in Medicine: A Symposium. New York: Chemical Publishing Company.

Hanrahan, James S.; and Bushnell, David 1960 Space Biology: The Human Factors in Space Flight. New York: Basic Books.

Hicks, S. P. 1958 Radiation as an Experimental Tool in Mammalian Developmental Neurology. Physiological Reviews 38:337-356.

Hollaender, Alexander (editor) 1954–1956 Radiation Biology. 3 vols. New York: McGraw-Hill. → Volume 1: High Energy Radiation. Volume 2: Ultraviolet and Related Radiations. Volume 3: Visible and Near-visible Light.

International Symposium On The Response Of The Nervous System To Ionizing Radiation, Second, Los Angeles, 7963 1964 Response of the Nervous System to Ionizing Radiation. Edited by Thomas J. Haley and Ray S. Snider. Boston: Little.

Kaplan, Sylvan J. 1962 Radiation Research in Psychology: An Analysis of Techniques in Maze Experimentation. Psychological Bulletin 99:153-160.

Kimeldorf, Donald J.; and Hunt, Edward L. 1965 Ionizing Radiation: Neural Function and Behavior. New York: Academic Press.

Lapp, Ralph E.; and Andrews, Howard L. (1948) 1963 Nuclear Radiation Physics. 3d ed. Englewood Cliffs, N.J.: Prentice-Hall.

Lashley, Karl S. 1929 Bram Mechanisms and Intelligence: A Quantitative Study of Injuries to the Brain. Univ. of Chicago Press.

Lea, Douglas E. (1947)1955 Actions of Radiations on Living Cells. 2d ed. Cambridge Univ. Press.

Lebedinskii, Andrei V.; and Nakhil’nitskaia, Z. N. 1963 Effects of Ionizing Radiation on the Nervous System. Amsterdam and New York: Elsevier.

Lebedinskii, Andrei V. et al. 1958 On the Biological Effect of Small Doses of Ionizing Radiation. Pages 17–28 in International Conference on the Peaceful Uses of Atomic Energy, Second, Geneva, Proceedings. Volume 22: Biological Effects of Radiation. Geneva: United Nations.

Miller, R. W. 1956 Delayed Effects Occurring Within the First Decade After Exposure of Young Individuals to the Hiroshima Atomic Bomb. Pediatrics 18:1-18.

Northwestern University, Evanston, Illinois; and Medical School, Chicago 1962 Response of the Nervous System to Ionizing Radiation: Proceedings of an International Symposium. New York: Academic Press.

Pickering, John E.; Langham, Wright H.; and Rambach, Walter A. 1956 The Effects From Massive Doses of High Dose Rate Gamma Radiation on Monkeys. USAF, Aerospace Medical Center (ATC), Report No. 60-57. Brooks Air Force Base, Texas: School of Aviation Medicine.

Rugh, R. 1962 Ionizing Radiations and Congenital Anomalies of the Nervous System. Military Medicine 127:883-907.

Rugh, R. 1963 The Impact of Ionizing Radiations on the Embryo and Fetus. American Journal of Roentgenology. Radium Therapy and Nuclear Medicine 89:182-190.

Russell, Liane B. 1954 The Effects of Radiation on Mammalian Prenatal Development. Volume 1, pages 861–918 in Alexander Hollaender (editor), Radiation Biology. New York: McGraw-Hill.

Sharp, Joseph C; and Keller, Barbara K. 1965 A Comparison Between the Effects of Exposure to a Mixed Fission Spectrum Delivered in a Single “Pulse” and X-rays Delivered at a Slower Rate Upon the Conditioned Avoidance Behavior of Primates. Technical Report No. 4. Washington: Walter Reed Army Institute of Research, Division of Neuropsychiatry.

Stahl, Walter R. 1959 A Review of Soviet Research on the Central Nervous System Effects of Ionizing Radiations. Journal of Nervous and Mental Disease 129: 511-529.

Stahl, Walter R. 1960 Recent Soviet Work on Reactions of the Central Nervous System to Ionizing Radiation. Journal of Nervous and Mental Disease 131: 213-233.

Van Cleave, Charles D. 1963 Irradiation and the Nervous System. New York: Rowman & Littlefield.

World Health Organization 1957 The Effect of Radiation on Human Heredity. Geneva: The Organization.

Radiation

Radiation takes many forms, including both electromagnetic waves and sub-nuclear particles. The electromagnetic spectrum consists of light waves ranging in length from very short (10⁻¹⁶ meters, or 3.937 × 10⁻¹⁵ inches) to very long (10⁸ meters, or 621,400 miles). The product of the velocity of electromagnetic waves and their wavelength is a constant equal to the velocity of light, 3 × 10⁸ meters per second (m/s); therefore, as the length of waves increases, the frequency decreases. Thus, if the waves were 1 meter(3.3 feet) long, the frequency would be 3 × 10⁸ hertz (Hz) or 300,000,000/s (meaning 300,000,000 light waves would pass by each second). The electromagnetic spectrum consists of light waves ranging in length from very short γ (gamma) rays through x rays, ultraviolet (UV) rays, the spectrum of visible light, infrared (IR) rays, and microwaves, to very long radio and television waves.

Forms of Radiation

The shortest electromagnetic waves are classified as γ rays. One of the forms of energy emanating from natural sources of radioactivity here on Earth and also from energy sources in space, γ rays can be thought of as very short x rays. Discovered by the German physicist Wilhelm Conrad Röntgen in 1895, the remarkable penetrating effect of rays and x rays results from their very short wavelength (from about 10⁻¹² to 10⁻⁸ meters, or 3.28 × 10⁻¹¹ to 3.28 × 10⁻⁷ feet). The waves are so small that they can pass through many substances with little interaction. X rays pass through skin and organs with little effect but are diffracted somewhat when they pass through denser materials such as bone; the resulting pattern enables technicians to make xray images of bones and of the contents of packages in airport scanners.

The energy of electromagnetic radiation is directly proportional to the frequency. Since both x rays and γ rays have very high frequencies, they carry large amounts of energy, and high intensities of x rays and γ rays can damage many materials (including living tissue). The rays may be focused by special lenses and used to kill cancer cells or organisms that might cause disease or hasten spoilage in food.

Bonds between atoms in chemical compounds vibrate at characteristic frequencies. Some molecules possess bonds capable of absorbing electromagnetic

energy, causing the bonds to bend, stretch, or vibrate and sometimes break. Certain bonds in particular (e.g., that between carbon and oxygen atoms) capture energy at specific frequencies of IR radiation, allowing technicians to use instruments called spectrophotometers to detect the presence of these bonds in chemical compounds. UV, visible, and IR spectroscopy are tools that permit chemists to readily identify and characterize small amounts of chemical substances.

Most animals, including humans, have visual receptors that detect light in the visible spectrum ranging from 400 nanometers (15.75 × 10⁻⁶ for blue light to 700(27.56 × 10⁻⁶ inches) for red light. (A nanometer equals 10⁻⁹ meters.) Just below the visible spectrum lies UV light, ranging in wavelength from about 10 to 400 nanometers (3.937 × 10⁻⁷ to 157.5 × 10⁻⁷ inches). UV light is more energetic than visible light; UV radiation in the sunlight can damage molecules in the skin and is the cause of sunburn. Green plants carry out photosynthesis by using chlorophyll molecules that readily capture light energy in the visible spectrum.

Microwave radiation consists of electromagnetic waves somewhat longer than infrared waves (from about 10⁻³ meters, or 0.3937 inches, to 1 meter, or 39.37 inches, long) and having a lower frequency, ranging from about 1,000 to 300,000 megahertz (MHz). Waves in this range are readily absorbed by bonds in water molecules. Microwave ovens take advantage of the fact that foods usually contain large amounts of water, and dishes do not. The waves of IR or microwave radiation are usually too long to pass through the small holes in the doors of microwave ovens; thus, one can use a microwave oven to heat food without heating the dish or being harmed by the radiation. Magnetron tubes generate radiation that can be used for radar or for the microwave transmission of electronic signals.

An immense amount of radiation passes undetected through the environment. Our surroundings contain large amounts of radio waves, generally from one to thousands of meters long. X rays and rays also pass through us and the space around us with little effect. From time to time, fears have been raised concerning the electromagnetic radiation emanating from power lines, cathode ray terminals such as television sets and computer monitors, and the earphones of personal transistor radios or CD players, but there is little actual evidence of injury or illness from low intensity radiation. However, workers have been injured or killed by high intensity microwave radiation, and technicians working with radioactive materials must take special precautions.

Much radiation arrives on Earth from the Sun, and some of the energy of this radiation exists in the form of UV light. UV light waves can damage skin and would be much more hazardous were it not for the layer of ozone that exists in Earth's upper atmosphere. In a process known as the Chapman cycle, UV radiation splits oxygen molecules (O₂) in the stratosphere to form free oxygen atoms (O). Some of these atoms combine with oxygen molecules to form ozone molecules (O₃). The O₃ molecules are especially sensitive to UV radiation; the absorption of UV photons converts the ozone back into oxygen atoms and oxygen molecules:

UV
O₂ → 2 O
O + O₂ → O₃
UV
O₃ → O₂ + O          (1)

In recent years the amount of O₃ in the stratosphere over the South Pole has decreased periodically, resulting in an ozone "hole" in the atmosphere. The decrease is most pronounced during the summer months of the Southern Hemisphere. If the amount of ozone continues to decrease, more UV light will reach the surface of Earth, probably causing some skin damage and increasing the incidence of cancer. Chlorine atoms react with and destroy ozone:

C1 + O₃ → C1O + O₂          (2)

Increasing atmospheric amounts of chlorine atoms or free radicals probably result in the destruction of ozone; the source of the chlorine atoms is thought to be synthetic substances known as chlorofluorocarbons (CFCs) . Some CFCs may be released from air-conditioning equipment or aerosol spray cans, and some may result from the production of plastic foams. Several international agreements, including the Montréal Protocol of 1987 and the Copenhagen amendment of 1992, have been established to limit the production of CFCs.

Radioactivity

Although many forms of electromagnetic radiation exist, special consideration is often given to radiation from unstable (radioactive) atomic nuclei. This radiation is usually one of three types, α - and β -particles or γ rays, but some nuclear reactions may also result in the emission of neutrons.α -particles are relatively large and highly charged particles identical with the nuclei of helium atoms. Each has a mass of four atomic mass units (AMU) and a charge of +2. Radioactive ores containing sources of α -particles often produce helium gas as a result of the capture of electrons by the

α -particles. β -particles are high-speed electrons, having a mass of about 1/1,800 amu and a charge of −1. γ rays are similar to x rays.

X rays, γ rays, and neutron beams are considered ionizing radiation. Ionizing radiation may break molecules into pieces, creating ionic free radicals that can be very damaging to tissue. Contaminated food or dust containing radionuclides that emit α - and β -particles may be very dangerous if the sources of radiation are ingested. Strontium-90 (⁹⁰Sr) present in fallout from nuclear weapons testing may be absorbed from soil, incorporated in plant tissues, eaten by cows, and eventually find its way into milk. Strontium is chemically similar to calcium. The ⁹⁰Sr may then be absorbed from the digestive tract and deposited in bone, where α -particles released by radionuclide decay damage the blood-producing reactions in bone marrow.

Several methods are used to detect radiation; the earliest of these, also discovered by Röntgen, is exposure of photographic film. Since x rays can pass through solid materials, they expose photographic film sealed in lightproof envelopes. Workers in industrial settings today often wear film badges that contain a sheet of photographic film inside a plastic container fitted with aluminum and lead shields. In order to determine the amount and type of radiation exposure, the badges are collected periodically and the film developed. Darkening of the film indicates exposure. Workers may also wear dosimeters, pencil-like tubes that are examined daily for exposure. For large-scale operations or as survey monitors at factory gates, Geiger-Mueller counters are utilized. These devices detect radiation by using a tube consisting of a metal can containing a charged wire. The tube is sealed with a thin plastic or mica window. Radiation penetrating the can or window ionizes molecules of gas inside the tube, and the ions allow an electrical discharge, which can be detected and registered by an electrical circuit. Civil defense and military personnel often carry Geiger counters to survey large areas for contamination by radioactivity.

Uses of Radiation

Radiation is a versatile tool for the diagnosis and treatment of disease, as well as a means of industrial testing and treating foods to avoid spoilage. Many common metabolic substances can be labeled by replacing atoms such as carbon or hydrogen with radioactive atoms such as ¹⁴C or ³H. The resulting molecules are absorbed by the body and react in the same way as nonradioactive molecules, but the decay of the radioisotope releases tiny amounts of radioactivity that can be detected with sensitive instruments. Some compounds are absorbed more rapidly by diseased tissue (a cancerous organ, for example, might rapidly absorb glucose from blood), and the use of substances such as radioactive iodine can help diagnose tumors of the thyroid gland without invasive surgery. Some diseases such as cancer can be treated by administering a preparation containing radionuclides within molecules that are taken up by an organ and release their radioactivity within the diseased tissue.

Cells that are growing and dividing rapidly are the most sensitive to radiation. In the human body, these include gonadal tissue, hair follicles, the immune system, bone marrow, intestinal epithelium , and cancer cells. Cancer may be treated by external beam radiation, using γ -type radiation to deliver energy to abnormal cells, in the hope of killing them. Normal tissues are protected by lead shielding, and also by rotating the radiation source, passing the beam of rays through a larger range of tissue, and avoiding the intense irradiation of nontumorous tissues. In some cases, malignant cells can be treated with brachytherapy, the implantation of tiny metallic seeds containing radioisotopes that emit small amounts of radiation, killing the cancer from the inside. Radioisotopes decay at a known rate; often a nuclide that decays rapidly may be chosen, allowing the patient to be radiation-free upon discharge.

Gamma radiation is widely used in manufacturing to make images of welds in pipes. A recent application of radiation in the food industry involves the use of radiation (usually either γ rays or high-energy beams of electrons) to irradiate food, killing organisms that cause spoilage. The irradiation of food presents several advantages: foodstuffs may last weeks longer with little refrigeration; the use of dangerous chemical preservatives can be avoided; and foods may be prepared, wrapped, and preserved with less contact by human workers, lessening the chances of spreading disease-producing organisms. In recent years, especially virulent and damaging strains of Escherichia coli have caused outbreaks of illness among persons who consumed contaminated hamburger, and some cases of salmonella poisoning have been associated with the consumption of poultry products. Food irradiation may help to make such foods much safer.

Our environment contains many sources of radiation, such as cosmic rays that constantly bombard Earth. The atmosphere filters out some cosmic rays, so exposure is greater at higher altitudes than at sea level. Radiation sources also include smoke detectors, luminous watch dials, television and computer monitors, and medical x rays. We are exposed daily to electromagnetic radiation in the form of radio waves as well as α - and β -particles and γ rays emanating from radioactive carbon, hydrogen, and potassium, which are part of all living things. A small amount of radiation is probably harmless and may, in fact, be helpful. Radon in our homes is a potential cause of cancer. Although radon is a colorless, odorless gas, it decays to more chemically reactive and radioactive products that may bind in lung tissue. Like many dangerous gases, radon is much more hazardous in the presence of particulate matter such as the tiny particles present in cigarette smoke.

see also Photosynthesis; Radioactivity; RÖntgen, Wilhelm.

Dan M. Sullivan

Bibliography

Christie, Maureen (2001). Ozone Layer. A Philosophy of Science Perspective. New York: Cambridge University Press.

Dessler, Andrew (2000). The Chemistry and Physics of Stratospheric Ozone. San Diego: Academic Press.

Miller, E. Willard, and Miller, Ruby M. (1990). Environmental Hazards: Radioactive Materials and Wastes: A Reference Handbook. Santa Barbara, CA: ABC-CLIO.

Molins, R. A., ed. (2001). Food Irradiation: Principles and Application. New York: Wiley.

Stanitski, Conrad L.; Eubanks, Lucy P.; Middlecamp, Catherine H.; and Pienta, Norman J. (2003). Chemistry in Context: Applying Chemistry to Society, 4th edition. Boston: McGraw-Hill.

Radiation

Definition

Radiation and radioisotopes are extensively used medications to allow physicians to image internal structures and processes in vivo (in the living body) with a minimum of invasion to the patient. Higher doses of radiation are also used as means to kill cancerous cells.

Radiation is actually a term that includes a variety of different physical phenomena. However, in essence, all these phenomena can be divided into two classes: phenomena connected with nuclear radioactive processes are one class, the so-called radioactive radiation (RR); electromagnetic radiation (EMR) may be considered as the second class.

Both classes of radiation are used in diagnoses and treatment of neurological disorders.

Description

There are three kinds of radiation useful to medical personnel: alpha, beta, and gamma radiation. Alpha radiation is a flow of alpha particles, beta radiation is a flow of electrons, and gamma radiation is electromagnetic radiation.

Radioisotopes, containing unstable combinations of protons and neutrons, are created by neutron activation. This involves the capture of a neutron by the nucleus of an atom, resulting in an excess of neutrons (neutron rich). Proton-rich radioisotopes are manufactured in cyclotrons. During radioactive decay, the nucleus of a radioisotope seeks energetic stability by emitting particles (alpha, beta, or positron) and photons (including gamma rays).

Radiation—produced by radioisotopes—allows accurate imaging of internal organs and structures. Radioactive tracers are formed from the bonding of short-lived radioisotopes with chemical compounds that, when in the body, allow the targeting of specific body regions or physiologic processes. Emitted gamma rays (photons) can be detected by gamma cameras and computer enhancement of the resulting images and allows quick and relatively noninvasive (compared to surgery) assessments of trauma or physiological impairments.

Because the density of tissues is unequal, x rays (a high frequency and energetic form of electromagnetic radiation) pass through tissues in an unequal manner. The beam passed through the body layer is recorded on special film to produce an image of internal structures. However, conventional x rays produce only a two-dimensional picture of the body structure under investigation.

Tomography (from the Greek tomos, meaning "to slice") is a method developed to allow the detailed construction of images of the target object. Initially using the x rays to scan layers of the area in question, with computer assisted tomography a computer then analyzes data of all layers to construct a 3D image of the object.

Computed tomography (also known as CT, CT scan ) and computerized axial tomography (CAT) scans use x rays to produce images of anatomical structures.

Single proton (or photon) emission computed tomography (SPECT) produces three-dimensional images of an organ or body system. SPECT detects the presence and course of a radioactive substance that is injected, ingested, or inhaled. In neurology, a SPECT scan can allow physicians to examine and observe the cerebral circulation . SPECT produces images of the target region by detecting the presence and location of a radioactive isotope. The photon emissions of the radioactive compound containing the isotope can be detected in a manner that is similar to the detection of x rays in computed tomography (CT). At the end of the SPECT scan, the stored information can be integrated to produce a computer-generated composite image.

Positron emission tomography (PET) scans utilize isotopes produced in a cyclotron. Positron-emitting radionuclides are injected and allowed to accumulate in the target tissue or organ. As the radionuclide decays, it emits a positron that collides with nearby electrons to result in the emission of two identifiable gamma photons. PET scans use rings of detectors that surround the patient to track the movements and concentrations of radioactive tracers. PET scans have attracted the interest of physicians because of their potential use in research into metabolic changes associated with mental diseases such as schizophrenia and depression . PET scans are used in the diagnosis and characterizations of certain cancers and heart disease, as well as clinical studies of the brain. PET uses radio-labeled tracers, including deoxyglucose, which is chemically similar to glucose and is used to assess metabolic rate in tissues and to image tumors, and dopa, within the brain.

Electromagnetic radiation

In contrast to imaging produced through the emission and collection of nuclear radiation (e.g., x rays, CT scans), magnetic resonance imaging (MRI) scanners rely on the emission and detection of electromagnetic radiation.

Electromagnetic radiation results from oscillations of components of electric and magnetic fields. In the simplest cases, these oscillations occur with definite frequency (the unit of frequency measurement is 1 Hertz (Hz), which is one oscillation per second). Arising in some point (under the action of the radiation source), electromagnetic radiation travels with the velocity that is equal to the velocity of the light, and this velocity is equal for all frequencies. Another quantity, wavelength, is often used for the description of electromagnetic radiation (this quantity is similar to the distance between two neighbor crests of waves spreading on a water surface, which appear after dropping a stone on the surface). Because the product of the wavelength and frequency must equal the velocity of light, the greater the wave frequency, the less its wavelength.

MRI scanners rely on the principles of atomic nuclear-spin resonance. Using strong magnetic fields and radio waves, MRIs collect and correlate deflections caused by atoms into images. MRIs allow physicians to see internal structures with great detail and also allow earlier and more accurate diagnosis of disorders.

MRI technology was developed from nuclear magnetic resonance (NMR) technology. Groups of nuclei brought into resonance, that is, nuclei absorbing and emitting photons of similar electromagnetic radiation such as radio waves, make subtle yet distinguishable changes when the resonance is forced to change by altering the energy of impacting photons. The speed and extent of the resonance changes permit a non-destructive (because of the use of low-energy photons) determination of anatomical structures.

MRI images do not utilize potentially harmful ionizing radiation generated by three-dimensional x-ray CT scans, but rely on the atomic properties (nuclear resonance) of protons in tissues when they are scanned with radio frequency radiation. The protons in the tissues, which resonate at slightly different frequencies, produce a signal that a computer uses to tell one tissue from another. MRI provides detailed three-dimensional soft tissue images.

These methods are used successfully for brain investigations.

Radiation therapy (radiotherapy)

Radiotherapy requires the use of radioisotopes and higher doses of radiation that are used diagnostically to treat some cancers (including brain cancer) and other medical conditions that require destruction of harmful cells.

Radiation therapy is delivered via external radiation or via internal radiation therapy (the implantation/injection of radioactive substances).

Cancer, tumors, and other rapidly dividing cells are usually sensitive to damage by radiation. The goal of radiation therapy is to deliver the minimally sufficient dosage to kill cancerous cells or to keep them from dividing. Cancer cells divide and grow at rates more rapid than normal cells and so are particularly susceptible to radiation. Accordingly, some cancerous growths can be restricted or eliminated by radioisotope irradiation. The most common forms of external radiation therapy use gamma and x rays. During the last half of the twentieth century, the radioisotope cobalt-60 was the frequently used source of radiation used in such treatments. More modern methods of irradiation include the production of x rays from linear accelerators.

Iodine-131, phosphorus-32 are commonly used in radiotherapy. More radical uses of radioisotopes include the use of boron-10 to specifically attack tumor cells. Boron-10 concentrates in tumor cells and is then subjected to neutron beams that result in highly energetic alpha particles that are lethal to the tumor tissue.

Precautions

Radiation therapy is not without risk to healthy tissue and to persons on the health care team, and precautions (shielding and limiting exposure) are taken to minimize exposure to other areas of the patient's body and to personnel on the treatment team.

Therapeutic radiologists, radiation oncologists, and a number of technical specialists use radiation and other methods to treat patients who have cancer or other tumors.

Care is taken in the selection of the appropriate radioactive isotope. Ideally, the radioactive compound loses its radioactive potency rapidly (this is expressed as the half-life of a compound). For example, gamma-emitting compounds used in SPECT scans can have a half-life of just a few hours. This is beneficial for the patients, as it limits the contact time with the potentially damaging radioisotope.

The selection of radioisotopes for medical use is governed by several important considerations involving dosage and half-life. Radioisotopes must be administered in sufficient dosages so that emitted radiation is present in sufficient quantity to be measured. Ideally the radioisotope has a short enough half-life that, at the delivered dosage, there is insignificant residual radiation following the desired length of exposure.

New areas of radiation therapy that may prove more effective in treating brain tumors (and other forms of cancers) include three-dimensional conformal radiation therapy (a process where multiple beans are shaped to match the contour of the tumor) and stereotactic radiosurgery (used to irradiate certain brain tumors and obstructions of the cerebral circulation). Gamma knives use focused beams (with the patient often wearing a special helmet to help focus the beams), while cyberknifes use hundreds of precise pinpoint beams emanating from a source of irradiation that moves around the patient's head.

Resources

BOOKS

Saha, Gopal B. Fundamentals of Nuclear Pharmacy. New York: Springer-Verlag, 1999.

WEBSITES

Society of Nuclear Medicine. "What Is Nuclear Medicine?" May 12, 2004 (May 27, 2004). <http://www.snm.org/nuclear/index.html>.

Alexander Ioffe

Radiation

Electromagnetic radiation

Particulate radiation

Radiation and health

The word “radiation” comes from the Latin for “ray of light” and is used in a general sense to cover all forms of energy that travel through space as “rays.” Radiation may consist of a spray of subatomic particles, like miniature bullets from a machine gun, or of electromagnetic waves, which include light, radio waves, x rays, and several other types.

“Radiation” is also sometimes used to describe the transfer of heat from a hot object to a cooler one that it is not touching; a hot object is said to radiate heat. You can feel radiant heat on your face when standing near a red-hot furnace, even if there is no movement of hot air between the furnace and you. What you are feeling is infrared radiation, a form of electromagnetic energy that makes molecules move faster, and therefore behave hotter, when it strikes them.

When many people think of radiation, they think of the radiations that come from radioactive materials. These radiations, some of which are particles and some of which are electromagnetic waves, are harmful because they are of such high energy that they damage materials through which they pass. This is in contrast to light, for example, which has no lasting effect on, say, a pane of glass through which it passes.

The higher energies of radiation are called ionizing radiations because when they tear apart atoms they leave behind a trail of ions, or atoms that have had some of their electrons removed. Ionizing radiations include x rays, alpha particles, beta particles, and gamma rays.

Many kinds of lower-energy radiations are quite common and are harmless in reasonable amounts.

They include all colors of visible light, ultraviolet and infrared light, microwaves and radio waves, including radar, TV and FM, short wave and AM. All of these radiations are electromagnetic radiations.

Electromagnetic radiation

Electromagnetic energy travels in the form of waves, moving in straight lines at a speed of 3←× 10⁸meters per second, or 186,400 mi (299,918 km) per second. That speed is usually referred to as the speed of light in a vacuum, because light is the most familiar kind of electromagnetic radiation and because light slows down a little bit when it enters a transparent substance such as glass, water, or air. The speed of light in a vacuum, the velocity of electromagnetic waves, is a fundamental constant of nature.

Electromagnetic radiation can have a variety of energies. Because it travels in the form of waves, the energies are often expressed in terms of wavelengths. The higher the energy of a wave, the shorter its wavelength. The wavelengths of known electromagnetic radiation range from less than 10^–10 centimeter for the highest energies up to millions of centimeters (tens of miles) for the lowest energies.

The energy of a wave can also be expressed by stating its frequency: the number of vibrations or cycles per second. Scientists call one cycle per second a hertz (1 Hz). Electromagnetic radiations range in frequency from a few Hz for the lowest energies up to more than 10²⁰ Hz for the highest.

Particulate radiation

Sprays or streams of invisibly small particles are often referred to as particulate radiation because they carry energy along with them as they fly through space. They may be produced deliberately in machines such as particle accelerators, or they may be emitted spontaneously from radioactive materials. Alpha particles and beta particles are emitted by radioactive materials, while beams of electrons, protons, mesons, neutrons, ions, and even whole atoms and molecules can be produced in accelerators, nuclear reactors, and other kinds of laboratory apparatus.

The only particulate radiations that might be encountered outside of a laboratory are the alpha and beta particles that are emitted by radioactive materials. These are charged subatomic particles: the alpha particle has an electric charge of +2 and the beta particle has a charge of +1 or –1. Because of their electric charges, these particles attract or repel electrons in the atoms of any material through which they pass, thereby ionizing those atoms. If enough of these ionized atoms happen to be parts of essential molecules in a human body, the body’s chemistry can be altered, with unhealthful consequences such as illness, cancer, or death.

Radiation and health

Even small doses of ionizing radiation can harm health. However, large doses of any form of radiation, ionizing or not, can be dangerous. Too much sunlight, for example, can be blinding or can cause burns or skin cancer. Lasers can deliver such intense beams of light that they can burn through metal. High levels of microwaves in ovens can cook meats and vegetables.

See also Electromagnetic field; Ionizing radiation; Nuclear medicine; Radioactive pollution; Radioactive waste.

Robert L. Wolke