UDC 616-053.2-085.849.015.3(048.8)

S. A. Kalnitsky, N. I. Kirko, T. A. Maksimova

Radiation Safety of Patients in Pediatric Radiology

Leningrad Research Institute of Radiation Hygiene, Ministry of Health of the RSFSR

Pediatric radiology services now constitute a widely developed system encompassing children’s hospitals, outpatient clinics, and many health-care institutions. Radiological examinations are performed when medically indicated in children of all ages, from the neonatal period through the upper school years (O. A. Sporov).

Work with children has a number of distinctive features. In particular, it is necessary to account for the heightened sensitivity of the growing organism to X-rays, as well as the child’s level of anatomical, physiological, neurological, and psychological development. At the same time, pediatric radiological examinations are characterized by a continuing increase both in their number and in the variety of examinations, including specialized procedures (G. A. Zedgenidze and T. A. Osipkova). According to foreign authors, for example, the number of diagnostic X-ray examinations in infants rises by 10–15% annually (Schuster and Schorn). This increases both the risk of somatic damage to the growing organism and the likelihood of disorders in future generations. The generally accepted view that the potential benefits of radiological examinations in children outweigh the possible radiation risks requires rigorous substantiation (Saenpeg and Kereiakes). The principle of reducing doses as far as possible while retaining the necessary information that justifies the risk should therefore be applied more widely in pediatrics, particularly since it underlies the most important radiation protection documents (Publication No. 26 of the International Commission on Radiological Protection and the 1979–1980 reports of the Scientific Committee on the Effects of Atomic Radiation). At the same time, the domestic literature contains insufficient information on children’s radiation exposure during radiological examinations, and the published material is outdated and contradictory (S. Ya. Doletsky et al.; P. V. Ramzaev et al.). All this underscores the importance of studying and seeking ways to improve radiation safety conditions for this patient population (A. I. Lapitsky and S. P. Pripechenkov).

Most studies, particularly those from earlier years, have examined the frequency and distribution of radiological examinations in children, using these data to estimate their levels of exposure. The aggregated findings of these studies indicate that 30% of children, or approximately 3% of the total population, undergo radiological examinations. As O. A. Sporov and A. N. Sukhomlina et al. note, most of these children (77–98%) are exposed in hospitals, and only a small proportion (23%) in outpatient clinics. Of the children examined, 68% receive one radiological procedure, 25% receive two, and 7% receive three. The latter finding is mainly associated with examinations for chronic diseases and congenital dislocation of the hip. On average, each child examined receives 1.4 radiological procedures per year. Very many children are exposed during their first years and even their first days of life. For example, in Zaporozhye in 1977, 37% of all children in their first year of life underwent radiological examinations, and children under 3 years of age accounted for 43% of cases (A. N. Sukhomlina et al.). In recent years, the growth in the number of radiological examinations has outpaced growth in the child population by more than 1½ times. The average frequency of radiological examinations for children under 12 years of age is 600 per 1,000 pe-

…cytochrome oxidase activity. The threshold concentration of isopropyl nitrite for rats is 0.3 mg/L, at which up to 5.4% methemoglobin is formed in the blood. Chronic exposure to isopropyl nitrite at concentrations of 0.275, 0.14, and 0.059 mg/L for 2, 3, and 4 months, respectively, caused growth retardation in rats, decreased hemoglobin, erythrocyte, and leukocyte levels in the blood, and impaired the detoxifying and glycogen-forming functions of the liver. Catarrhal-desquamative bronchitis and mild fatty degeneration of liver cells were observed. At a concentration of 0.005 mg/L, minor reversible changes in the detoxifying function of the liver and desquamative bronchitis were noted (Kh. K. Yunusova and V. I. Fedorova). Application of isopropyl nitrite to the skin of mice and rabbits causes dry tissue necrosis without signs of systemic toxicity. The maximum permissible concentration of isopropyl nitrite in air is 0.001 mg/L (SN 245-71).

The literature contains information on the biological effects of nitrites in humans. According to A. M. Garbuz, ingestion of 3 g of sodium nitrite causes dizziness, vomiting, and loss of consciousness. Cases of severe methemoglobinemia have been described in children who consumed well water with a high nitrite content. In a study, workers exposed to cutting fluids containing 3% NaNO₂ complained of headache, rapid fatigue, weakness, poor appetite and sleep, and aching pain in the limbs and joints (R. A. Nishchii, 1960). According to V. A. Madorskii and A. K. Voronina, a similar pattern of effects was observed in people working with a fluid consisting of a mixture of 0.3% NaNO₂ and 1% triethanolamine solution; the sodium nitrite concentration in the breathing zone was 0.0009–0.0015 mg/L. Headaches began 3–5 months after work commenced and were localized in the temporal region; workers complained of pain in the heart region and numbness of the fingers. Examination revealed vascular dystonia of the peripheral and cerebral vessels, vasospasms, reduced mental work capacity, weakening of nervous processes, signs of atrophy of the nasopharyngeal mucosa, and a slight decrease in hemoglobin, erythrocytes, and platelets. Workers exposed to sodium nitrite exhibited yellowing of the skin on the palms, hyperemia, cyanosis and swelling of the hands, peeling, fissures, vesicular eruptions, brittle nails, and marked capillary changes (R. A. Nishchii, 1964; V. A. Madorskii and A. K. Voronina; N. B. Metlina and S. S. Shefer).

Ingestion of potassium nitrite at a dose of 0.1 g caused facial flushing, increased pulse rate, decreased arterial blood pressure, and a sensation of noise in the head. Ingestion of 0.3–0.5 g led to gastrointestinal disturbances, profuse sweating, cyanosis of the skin, shortness of breath, dizziness, drowsiness, and transient visual disturbances (Sleight and Atallah). Ethyl nitrite can cause headache and increased pulse rate in humans and, in severe cases, loss of reflexes and collapse.

In people affected by isopentyl nitrite, skin flushing, a sensation of heat in the head, and pulsation of the carotid arteries occurred. The effects of low concentrations passed very quickly, whereas exposure to high concentrations caused a substantial fall in arterial blood pressure. Dizziness, nausea, motor disturbances, weakness, impaired hearing and vision, palpitations, difficulty breathing, and sugar in the urine may occur. Palpitations and ECG changes were noted in healthy men who inhaled isopentyl nitrite for 20–30 days (Myburgh).

Thus, the material presented attests to the pronounced biological effects of nitrates and nitrites on animals and humans. Despite the considerable volume of research, a number of questions concerning the toxicology of these compounds remain unresolved. Information on the kinetics of nitrate and nitrite metabolism and distribution is practically absent, as are data on their blastomogenic effects as a function of dose and route of entry into the body. There are no data on the combined effects of nitrates and nitrites on animals. For many compounds, there are no data on acute, subacute, and chronic toxicity, nor are there substantiated recommendations for maximum permissible concentrations in water and workplace air. All these questions are of scientific and practical interest and therefore require serious attention and further study.

References

Aleksandrov V. A. In: Carcinogenic N-Nitroso Compounds: Action, Formation, Detection. Tallinn, 1978, pp. 68–69.

Vasyukovich L. Ya., Krasovskii G. N. Gigiena i sanitariya, 1979, no. 7, pp. 8–11.

Garbuz A. M. In: 12th Scientific and Practical Conference of Young Hygienists and Sanitary Physicians: Proceedings. Moscow, 1969, pp. 141–142.

Demeshkevich N. G. et al. Gigiena truda, 1972, no. 10, pp. 36–39.

Zaugolnikov S. D., Predtechenskii M. B. Ibid., 1967, no. 9, pp. 49–50.

Kamoga V. P., Yaremenko Z. M. Klinicheskaya meditsina, 1966, no. 11, pp. 49–59.

Kapnadze Sh. Kh. Gigiena i sanitariya, 1961, no. 9, pp. 7–11.

Kolyadich M. N. et al. Ibid., 1972, no. 4, pp. 100–101.

Kondratyeva I. I., Shefer S. S. Ibid., 1972, no. 3, pp. 41–44.

Krasovskii G. N. et al. Ibid., 1982, no. 4, pp. 23–27.

Madorskii V. A., Voronina A. K. Gigiena truda, 1966, no. 4, pp. 56–57.

Metlina N. B., Shefer S. S. In: First All-Union Conference on the Early Diagnosis, Treatment, Assessment of Work Capacity, and Prevention of Occupational Diseases of Chemical Etiology: Proceedings. Moscow, 1971, pp. 106–108.

Nishchii R. A. Gigiena truda, 1960, no. 4, pp. 50–52.

Nishchii R. A. In: Issues in the Hygiene of Occupational Pathology and Toxicology. Sverdlovsk, 1964, pp. 423–429.

Petukhov N. I. et al. Gigiena i sanitariya, 1972, no. 3, pp. 14–18.

…people, rising to 770 by age 19, whereas for the adult population the rate is more than 1 procedure per person per year. Thus, the frequency of X-ray examinations is lower in children than in adults, owing to the smaller number of X-ray examinations performed in children, particularly screening examinations.

A review of the literature on the relationship between the frequency of X-ray examinations and patient age showed that the frequency increases with age. This increase is especially pronounced in older age groups, apparently because of mass photofluorographic screening. Several authors have established a relationship between the frequency of X-ray examinations and patient sex (P. V. Ramzaev et al.). The frequency was found to be higher in boys—an average of 900 procedures per 1,000 persons, compared with only 420 in girls.

The overall distribution of X-ray examinations in children is as follows: fluoroscopy, 32%; radiography, 35%; and photofluorography, 33% (P. V. Ramzaev et al.; A. N. Sukhomlina et al.). Thus, an unfavorable situation exists because fluoroscopy, which entails the greatest radiation exposure, accounts for a large proportion of X-ray examinations. Examinations of the chest account for the largest share of radiological procedures—63% (up to 78%); those of the musculoskeletal system account for 24% (up to 32%); the gastrointestinal tract, 7% (up to 15%); and special examinations, 6% (up to 12%) (P. V. Ramzaev et al.). It follows that the chest is examined radiologically most often. The frequency of chest fluoroscopy is 190 procedures per 1,000 persons, whereas that of chest radiography is 120 procedures. Thus, here too the examination method producing the highest dose is used. Although the proportion of fluoroscopy among all examinations has tended to decrease in recent years, in chest examinations it has in some cases increased (A. N. Sukhomlina et al.).

Within the musculoskeletal system, the teeth and the hip joint in cases of congenital hip dislocation are examined most frequently. The latter procedure entails substantial radiation exposure, particularly for girls, whose mean gonadal dose by age 17 is approximately 5 rad (T. M. Koroleva and Yu. S. Zakharenko). With an optimal examination technique, the skin dose can be below 2 mR and the gonadal dose approximately 0.1 mR for patients of either sex.

The low effectiveness of procedures in pediatric radiology is striking. In many cases, the rate of discordance between the diagnosis and the disease substantially exceeds 50%, and X-ray examinations often fail to reveal pathology. This applies in particular to the increasingly frequent examinations of the gastrointestinal tract, which entail considerable radiation exposure (A. N. Sukhomlina et al.).

A very important issue from the standpoint of radiation safety is the technical provision of X-ray examinations for children, especially the equipment used. In the past, there was a profoundly mistaken belief that low-power single-rectifier and rectifierless units were entirely suitable for this purpose. The greatly expanded use of specialized examinations in children, higher demands on radiograph quality, and investigations of children’s radiation exposure during diagnostic procedures suggest that powerful four-rectifier units with a dual-focus tube are best suited for examining children (G. A. Zedgenidze and T. A. Osipkova). At present, the domestic units most frequently used in pediatric radiology are the ARD-2-110, ARD-2-125, and RUM-10. However, they are obsolete and unsuitable for examining children. It would seem advisable to use the most advanced X-ray units in pediatrics, equipped with image intensifiers, television attachments, and computers. Both domestic units of the RUM-20, RUM-20-P, and RUM-20-P1 types and various foreign X-ray units meet these requirements. For the same reason, the full range of modern X-ray equipment should be directed first to pediatric radiology departments. The same applies to X-ray film. The most sensitive film, such as RM-6, RF-3, and RF-Kh-1, should be used for images of children, together with intensifying screens. For example, the use of cassettes with intensifying screens in radiography increases the image intensity on the film 10- to 15-fold. Radiography without intensifying screens, by contrast, requires a 10- to 15-fold increase in the X-ray dose to obtain images of satisfactory quality. These measures would make it possible to redistribute the dose in diagnostic radiology and, in particular, reduce it in pediatric radiology (E. V. Ivanov et al.). In addition, pediatric X-ray departments should receive priority and more frequent inspections by public health and departmental radiation safety services.

A review of the literature on dosimetric studies in children showed that such studies are insufficiently frequent compared with those in adults. The gonads are the most extensively studied organs. It is known that gonadal radiation doses during X-ray examinations are considerably higher in children than in adults. This is because the radiation fields used in diagnostic X-ray examinations of children are comparable in size to…

…by the dimensions of the child’s body. Indeed, analysis of the published data made it possible to determine average gonadal doses in children. These proved substantial, amounting per procedure to 105 mrad for fluoroscopy, 70 mrad for radiography, and 0.4 mrad for fluoroscopy (P. V. Ramzaev et al.; I. A. Bergmanis et al.; A. N. Krongauz et al.). The average annual gonadal dose per child across all types of examination was 48.4 mrad, higher than the adult dose of 33.6 mrad (V. A. Knizhnikov et al.). The collective gonadal doses to children calculated on this basis were also substantial: \(3.2 \times 10^6\) person-rad/year for those under 14 years of age, representing 38% of the total collective gonadal dose for the entire population—\(8.4 \times 10^6\) person-rad/year—and \(4.5 \times 10^6\) person-rad/year for those under 19 years of age, representing 54% of the total dose. Clearly, the large contribution of children to the total population dose, and ways of reducing it, warrant serious attention.

Age- and sex-specific dosimetric studies of children show that the published data do not permit a pattern of change in gonadal dose with increasing patient age to be established, because the authors used different methods and X-ray equipment settings, and the groups of children examined were not classified by age in a uniform manner (M. I. Kostenetsky and A. N. Sukhomlina). At the same time, sex differences in gonadal exposure have been established: the gonadal dose received by girls during radiological procedures is considerably higher than that received by boys. For example, during anteroposterior abdominal radiography, a girl aged 2–3 years receives an ovarian dose of 104 mR, whereas a boy of the same age, with gonadal shielding, receives a testicular dose of only 10 mR (Hartung).

The domestic literature provides no dosimetric data for other organs—the eyes, thyroid gland, and others—during various types of radiological examination. Foreign authors have studied radiation exposure of these organs in detail during chest examinations, panoramic radiography, and cardiac catheterization (Myers and Shoaf; Ardran et al.; Kaude and Srahn; Rowley). Also noteworthy are the data of Martin and Olson on thyroid exposure during cardiac catheterization and angiography. In their series, the mean thyroid dose was 7.7 R (0.6–23.5 R), with the highest doses occurring in younger children. Studies by Gustafsson and Mortensson provide similar evidence: the thyroid dose was 370 mrad per examination, compared with 260 mrad for children aged 1 to 13 years. Use of cinefluorography with the method in question reduced the doses to the thyroid and gonads to 2.3 and 0.1 R, respectively (Martin et al.). Accordingly, it has been proposed that examinations imposing a high radiation dose on these organs be eliminated from pediatric practice. Moreover, Schuster and Schorn recommend limiting the dose rate in pediatric radiology to \(\leq 5\)–10 μR/s and the dose per image to \(\leq 40\)–50 μR. Other authors reach similar conclusions concerning special examinations and xeroradiography, which involve substantial radiation exposure (G. A. Zedgenidze and T. A. Osipkova; P. V. Ramzaev et al.; E. V. Ivanov et al.). These procedures should be used only as a last resort, when the objectives cannot be achieved by other means. For the same reason, the number of repeat radiological examinations should be reduced. This is all the more necessary because the risk from irradiation in children is comparable to, or even greater than, the corresponding risk in adults (Gustafsson and Mortensson; Vogel et al.).

Considerable attention has been devoted in the literature to reducing radiation exposure and protecting pediatric patients. A particularly promising approach is optimization of the physical and technical conditions of examination, especially tube voltage, anode current, the filter used, exposure time, skin-to-focus distance, and irradiation field size. The optimal combination of these parameters is one that yields the best image quality with the lowest radiation exposure (Rosenstain).

Tube voltage is of great importance in selecting radiographic exposure settings. Increasing the voltage during examinations of children has been found to reduce radiation exposure (Laugmead and Wall). This is primarily due to a favorable change in the ratio of entrance to exit doses. In addition, increasing the voltage makes it possible to reduce the tube current, increase X-ray filtration, and increase the skin-to-focus distance. Based on the findings of numerous authors, the optimal voltage for radiography in children should be considered to lie in the range of 60–100 kV (R. V. Stavitsky et al.; Heinrich and Shuster).

The exit dose, on the side of the patient facing the screen, has been found to be a major determinant of the absorbed dose. At the same time, the optimal exit dose can be achieved with different tube settings; in particular, the higher the voltage, the more the anode current can be reduced. Increasing the voltage by 1% permits a 5% reduction in anode current while maintaining the same image brightness. The integral dose, in turn, …

decreases by 2%, and the dose at the body surface by 3%. Hörmann and coauthors conducted dosimetric studies during irradiation with automatic dose-rate control and adjustment of the exposure intensity and generator voltage. They found that patients’ radiation doses were lower than with arbitrarily selected parameters. The authors concluded that a control system is necessary for all pediatric radiology departments. To ensure that optimal X-ray equipment settings are used, a reference table of exposure conditions should be kept in the X-ray room.

Filtration of X-rays makes it possible to remove soft X-rays that have no diagnostic value. Retained in the patient’s body, they increase the absorbed dose. The use of a 2–3 mm aluminum filter at increased voltage reduces the exposure dose at the body surface threefold or more (F. F. Telichko). Heinrich and Shuster note that, when the spectra of the entrance and exit doses are compared, the low-energy portion of the spectrum up to 28 kV is almost completely absorbed by the patient and therefore does not contribute to the received signal, but makes a substantial contribution to the exposure dose. Because the dose rises sharply as energy decreases, the contribution of the low-energy portion of the spectrum to the exposure dose is particularly large. Attempts were made to filter out the low-energy portion of the spectrum directly with a 0.2 mm molybdenum plate. The spectral curves showed a noticeable shift toward higher energies. The principal finding was that a 0.1 mm molybdenum filter can reduce the skin dose two- to threefold over the range of 40–100 kV.

The skin-to-focus distance when examining children should be at least 80 cm. A greater skin-to-focus distance improves the ratio of entrance to exit dose and thereby reduces the absorbed dose.

The size of the radiation field in children is especially important because of their high sensitivity to ionizing radiation. The small size of a child in the first years of life means that the distances between individual organs are shorter, creating conditions in which examination of an organ becomes whole-body irradiation. This increases the integral radiation dose received by the child. The smaller the radiation field, the lower the patient’s radiation burden. With a field area of 400 cm², the integral absorbed dose is four times greater than with a field of 100 cm². At the same time, a small radiation field improves image quality while reducing the radiation burden, because less scattered radiation is produced in the patient’s body.

The radiation field can be reduced by narrowing the cross-section of the X-ray beam with a slit diaphragm equipped with a light localizer and by shielding parts of the body not under examination. This is accomplished chiefly with sheets of leaded rubber having a lead equivalence of approximately 0.3 mm. Studies by I. P. Maktaz and coauthors showed that further increases in shield thickness are ineffective.

The primary radiation beam accounts for most of the radiation burden; scattered radiation is less significant. Nevertheless, shielding is used to protect not only the areas of the body adjacent to the radiation field but also the patient’s remaining body parts from scattered radiation. Thus, in children, all body parts not under examination should be shielded, regardless of the size of the X-ray beam.

Special attention should be paid to shielding during radiography of the gonadal region, given the gonads’ high sensitivity to ionizing radiation and the possibility of hereditary effects. Shielding the gonads in girls is considerably more difficult than in boys because their gonads are located in the abdominal cavity. Most often they lie at the level of the upper edge of the iliac wings, sometimes reaching the third lumbar vertebra (T. A. Osipkova). It follows that the gonads of newborn girls can be shielded only by covering the entire lower half of the abdomen and, in subsequent years, the pelvic region. Shielding the gonads in boys is much simpler: for example, lead capsules can reduce the gonadal dose by 80–85% during examination of the abdominal cavity (Hartung).

The literature indicates that the collective and individual protective equipment available in X-ray rooms is not used sufficiently effectively (O. A. Sporov; P. V. Ramzaev and coauthors). Further development of specialized protective devices, particularly for the gonads, is needed. In this respect, the positive experience of German colleagues who have already developed such devices should be adopted (Hartung; Bronsch; Krepler and coauthors). The protection of adults and staff members holding children during examinations must also be borne in mind.

In addition to improvements in protective devices, the pediatric radiology service should be strengthened through systematic training of pediatric radiologists and increased competence in radiation safety (O. A. Sporov).

Thus, a review of the literature reveals both the inadequacy of research on patient radiation safety in pediatrics and the importance of studying this issue. Considerable attention must be devoted to it. Analysis of the literature also makes it possible to define the principal direction of work on patient radiation safety in pediatric radiology: studying radiation exposure in children using modern methods of dose measurement, particularly thermoluminescent dosimetry; investigating the various somatic effects of radiological examinations; and ultimately reducing these effects as much as possible. This can be achieved by rationalizing the use of radiological examinations that entail high radiation exposure while providing little information—notably xeroradiography—and by increasing the effectiveness of radiological examinations through better justification for their use, optimal examination settings, selection of modern X-ray equipment and high-sensitivity X-ray film, use of individual and collective protective measures, improved radiation safety training for personnel, particularly physicians of all specialties, and various organizational measures. All these steps will reduce the radiation risk to the growing body and preserve children’s health.

References

Bergmanis I. A., Gubatova D. Ya., Nemiro E. A. — Med. Radiologiya, 1971, no. 5, pp. 26–30.

Xeroradiography in Pediatrics. Doletsky S. Ya., Filippkin M. A., Khutoretsky M. B., et al. Moscow, 1978.

Zedgenidze G. A., Osipkova T. A. Emergency Radiodiagnosis in Children. Leningrad, 1980.

Ivanov E. V., Nemiro E. A., Kalnitsky S. A., et al. — Gig. i San., 1981, no. 11, pp. 66–68.

Koroleva T. M., Zakharenko Yu. S. — Med. Radiol., 1969, no. 12, pp. 37–42.

Kostenețky M. I., Sukhomlina A. N. — Gig. i San., 1981, no. 2, pp. 64–65.

Krongauz A. N., Khomutova M. G., Lepskaya E. S., et al. — Med. Radiol., 1974, no. 3, pp. 14–19.

Lapitsky A. I., Pripechenkov S. P. — In: Age-Related Characteristics of the Child’s Body in Health and Disease. Minsk, 1977, pp. 100–101.

Maktaz I. P., Pavlova T. G., Rabinovich F. P., et al. — Vestn. Rentgenologii, 1972, no. 4, pp. 64–67.

Osipkova T. A. The X-ray Technician in a Pediatric Health Care Facility. Leningrad, 1977.

Radiation Protection. Moscow, 1978.

Radiation-Hygienic Assessment of Radiation Exposure of the Population during X-ray Diagnostic Examinations. Leningrad, 1971.

Radiation-Hygienic Assessment of Radiation Exposure during X-ray Diagnostic Examinations. Leningrad, 1972.

Sporov O. A. — Vestn. Rentgenol., 1971, no. 5, pp. 25–30.

Stavitsky R. V., Golikov V. Ya., Smekhov M. E., et al. — In: Hygienic Assessment of Radiation Safety for Personnel, Patients, and the Population in the Use of Ionizing Radiation Sources for Diagnostic Purposes. Leningrad, 1977, pp. 52–53.

Sukhomlina A. N., Rozenshtein I. A., Kostenetsky M. I. — Gig. i San., 1981, no. 2, pp. 65–66.

Telichko F. F. Radiation Exposure during Radiological Examinations. Moscow, 1976.

Ardran G. M., Hamill J., Emrys-Roberts E. et al. — Brit. J. Radiol., 1970, v. 43, p. 391–394.

Bronsch T. — Radiol-Diagn., 1977, Bd 18, S. 849–859.

Gustafsson M., Mortensson W. — Brit. J. Radiol., 1976, v. 49, p. 686–689.

Hartung K. — Chir. Praxis, 1964, Bd 8, S. 613–620.

Heinrich M., Shuster W. — Ann. Radiol. Med. Nucl., 1976, v. 19, p. 57–66.

Kaude J., Srahn G. — Acta radiol. (Kbh.), 1974, v. 15, p. 454–464.

Krepler P., Vana N., Havanck C. — Pediat. Radiol., 1977, Bd 5, S. 231–234.

Laugmead W. A., Wall B. F. — Brit. J. Radiol., 1976, v. 49, p. 956–962.

Martin E. C., Olson A., Steeg C. N. et al. — Circulation, 1980, v. 62, Pt 2, p. 163.

Martin E. C., Olson A. — Brit. J. Radiol., 1980, v. 53, p. 100–106.

Myers D. R., Shoaf H. K. — Oral. Surg., 1978, v. 46, p. 588–593.

Rosentain M. — In: Radiation Protection. Oxford, 1980, v. 1, p. 61–64.

Rowley K. A. — Brit. J. Radiol., 1974, v. 47, p. 169–178.

Saenger E. L., Kereiakes J. G. — In: Nuclear Medicine in Clinical Pediatrics. New York, 1975, p. 209–229.

Vogel H., Löhr H., Wallbaum F. — Klin. Pädiat., 1978, Bd 190, S. 560–565.

Received August 27, 1982.