DÄ internationalArchive3/2008Radiation Protection in Diagnostic Radiology

Review article

Radiation Protection in Diagnostic Radiology

Dtsch Arztebl Int 2008; 105(3): 41-6. DOI: 10.3238/arztebl.2008.0041

Shannoun, F; Blettner, M; Schmidberger, H; Zeeb, H

Introduction: The use of ionizing radiation is regulated by legislative bodies to limit both individuals' and the population's exposure to radiation. Germany has implemented the European 97/43/EURATOM directive in national law by updating the existing radiation protection regulations. The German Commission on Radiological Protection regularly publishes statements and recommendations on radiation protection in medicine and diagnostic radiology, such as the introduction of diagnostic reference levels and referral guidelines for radiological and nuclear medicine imaging.
Methods: Review of selected literature, national and international recommendations and legal texts.
Results: From a radiological protection perspective, clear justification for radiological examinations and techniques aimed at minimizing radiation dose while providing the required diagnostic information, are essential.
Discussion: Referring doctors should be sure to use existing guidelines for medical imaging, and liaise with radiologists and nuclear medicine specialists, with whom they share the responsibility for choosing appropriate imaging modalities.
Dtsch Arztebl Int 2008; 105(3): 41–6
DOI: 10.3238/arztebl.2008.0041

Key words: radiation protection in medicine, radiation dosimetry, radiation risk, German Commission on Radiological Protection, Directive 97/43/EURATOM
LNSLNS Radiation protection is almost as old as the invisible rays discovered by Wilhelm Röntgen on 8 November 1895. The damaging effects of X-rays were discovered shortly afterwards. Physicians and patients who had been exposed to radiation for a protracted period often developed erythema. Although it follows that ionizing radiation can have adverse effects on health, radiological investigational procedures are now an accepted part of clinical practice, as the advantages for the patient far outweigh the risks of radiation exposure.

To restrict the radiation exposure of individual persons and of the overall population, work and contact with ionizing radiation is regulated by recommendations, directives, ordinances, and laws. As a contract state of the European Atomic Community EURATOM, in the 1957 Rome agreements, Germany undertook to convert the EURATOM directives into national law. The first of these directives dealt with protection of employees, not of patients (1). The 1984 directive 84/466/EURATOM (2) specified basic measures for radiation protection in medical investigations and treatments and was the first directive to deal with radiation protection of patients at the European level. This directive laid down for the first time that a justification must be given for each medical use of radiation. This was incorporated in 1987 in the X-ray Ordinance (Röntgenverordnung, RöV) and in 1987 in the Radiation Protection Ordinance (Strahlenschutzverordnung, StrlSchV).

Directive 97/43/EURATOM (5) – also known as the Patient Protection Directive (Patientenschutzrichtlinie, PatSRL) – was converted into national law in Germany through amendments in these two ordinances (3, 4) in 2001 and 2002. The Council of the European Community had already issued the PatSRL in 1997, with the aims of creating harmonized legislation in Europe, as this could stimulate and enhance the protection of patients from ionizing radiation throughout Europe. Recommendations of the International Commission on Radiological Protection (ICRP) were then adopted which greatly tightened the requirements for justification, optimization, training, as well as for equipment and quality control of X-ray systems (6).

The Radiological Protection Committee (Strahlenschutzkommission, SSK) was founded in 1974, with the aims of supporting and advising the federal ministries with responsibility for protection from ionizing and non-ionizing radiation. It currently consists of seven subcommittees, one of which deals with radiation protection in medicine. The SSK regularly publishes statements and recommendations on various themes related to radiation protection, most of which are freely accessible on the internet (www.SSK.de [in German, with a list of international links]).

The recommendations of the SSK and new laws related to diagnostic radiology will now be discussed, accompanied by a refresher on the basic knowledge required for physicians dealing with ionizing radiation, as revealed by analysis of selected literature.

Radiation dosimetry
Radiation dose parameters were defined with the aim of permitting quantification of the dose of ionizing radiation and its effects on tissue. The dose parameters summarized here are based on the definitions published by the ICRP and the International Commission on Radiation Units and Measurements (ICRU) and which have been adopted internationally in the radiation protection legislation in most countries. The dose units are SI units.

Energy dose
The energy dose is the basic physical parameter in radiation dosimetry. This describes energy transfer from ionizing radiation to materials other than air. The unit is the gray (Gy), which corresponds to 1 joule/kg. The energy dose can hardly be determined on a routine basis and must usually be calculated from the ion dose with an ionization chamber. Table 1 (gif ppt)summarizes the formulas and units for the various dose parameters.

Equivalent dose
The equivalent dose is the most important dose parameter for evaluating the effects of radiation and to assess radiation risk, as it incorporates the dimensionless radiation weighting factor wR to allow for the different biological activity of various types of radiation. Multiplication of the applied energy dose in an organ or tissue by the corresponding weighting factor gives the equivalent dose. The weighting factor is equal to unity (wR = 1) for the types of radiation used in radiological diagnosis and nuclear medicine (gamma rays and X-rays), so that the energy dose and equivalent dose are numerically the same. The weighting factor wR for neutron, proton, and alpha radiation is 5- to 20-fold greater than for photon radiation (gamma or X-radiation) or for electron radiation (beta radiation). To avoid confusion with the energy dose, the unit for the equivalent dose is the sievert (Sv).

Effective dose
The effective dose is used to quantify radiation exposure in individuals. Exposure of the individual organs and tissues in the body triggers radiation effects with different probabilities, depending on the organ. The combined damage in all organs and tissues in the body is estimated by multiplying the equivalent dose in each organ and tissue with a tissue weighting factor wT and then summing the results over the whole body to give the effective dose. The unit for effective dose is again the sievert (Sv).

The weighting factors wT published by the ICRP (6) are average values for the overall population, for both genders and with the age distribution from 0 to 75 years. The effective dose was introduced by the ICRP to allow estimation of the nominal stochastic risk after radiation exposure. This is largely based on findings on the victims of the atomic bomb attacks of Hiroshima and Nagasaki, regularly updated with the newest findings of the Radiation Effects Research Foundation (RERF).

Use of the effective dose to quantify doses and risks in medicine is intended to improve the comparability of different procedures for radiological investigation (7).

Diagnostic reference levels
The ALARA principle (see box gif ppt) demands that, during work with ionizing materials, every reasonable effort should be made to minimize exposure of man, animals, and material – even below the dose limits. In spite of the obligation to comply with the ALARA principle and the precept of optimizing patient protection during medical exposure, it has been found that the radiation exposure of patients during comparable investigations can vary by several orders of magnitude (8, 9). For this reason, "dose limits" or "reference values" have been specified for the most frequent procedures in medical diagnosis. These "reference dose values" should be easy to determine. They are intended to be practical aids to help in the easy recognition of situations in which the administered activity or radiation is on average unusually high for the patients.

The dose parameters discussed above are intended for the determination of patient exposure, but can only be calculated with complex measurement procedures and conversion factors. They are therefore rather unsuited as "reference values" in diagnostic radiology. For routine work, dose parameters have become established which are physically easier to determine, such as the surface dose or dose-area product (DAP) for projection radiography and the CTDIw value and the dose-length product (DLP) for computed tomography (CT). These dose parameters can be directly measured or read off (10).

The diagnostic reference values are standard values which must be complied with in nuclear medicine. They may be exceeded in X-ray diagnosis in individual cases, if this is justified. If they are permanently exceeded, measures must be taken to reduce the dose, in accordance with the requirement of optimization in radiation protection. These reference values are specified and published by the Federal Office for Radiation Protection (Bundesamt für Strahlenschutz, BfS) (3); routine checks of compliance are a medical responsibility. The SSK has recently published a guideline for investigations in radiology and nuclear medicine. This gives typical effective doses from medical radiation exposure (review in table 2 gif ppt) (11). This leads to the conclusion that a chest CT leads to 400-fold greater radiation exposure for the patient than conventional chest projection radiography and 16-fold greater radiation exposure than a normal two-plane bilateral mammography.

Effect of ionizing radiation and radiation risk
Radiation protection is necessary because ionizing radiation has biological effects that affect the organism.

A distinction is made between non-stochastic (deterministic) effects and stochastic (random) effects. In addition, somatic effects (such as radiation sickness or cancer) are differentiated from genetic effects in the offspring.

Stochastic and deterministic damage
There is a "deterministic" and non-stochastic effect if the extent of damage depends on the applied dose and its spatial and temporal distribution. In this case, there is a threshold value. Once this is exceeded, the effects of radiation are observed. Deterministic effects include cell, tissue, and organ damage in radiation erythema, skin necroses, and acute radiation sickness.

If the probability that an effect occurs depends on the dose, but the severity of the effect does not, the effect is said to be "stochastic." The probability of the effect is very low at low doses. It is now assumed that there is no threshold under which damage is absolutely excluded. Cancers and genetic damage are, for example, consequences of stochastic effects. Direct dose-dependent radiation damage is extremely rare in X-ray diagnosis. At most, stochastic effects are observed (12).

Radiation risk
The radiation risk is the quantification of the potential damage which can arise from ionizing radiation. The emphasis here is mainly on malignant disease; there are limited data on cardiovascular effects. Risk assessment is mainly based on epidemiological studies. Reliable risk values can mostly only be determined for the intermediate and high dose ranges. For the low dose range – under 100 mSv – the risk values are extrapolated. This is carried out using different dose-response relationships.

The calculations are still largely based on data from atomic bomb survivors from Hiroshima and Nagasaki. On the basis of the observed linearity in the higher dose range, it has been assumed that the dose-action relationship is also linear in the low dose range. The data for leukemia are better described with a linear-quadratic function. Using empirical evidence and theoretical principles on the effects of lower doses, risk coefficients for the low dose range have been corrected with a dose reduction factor – a controversial procedure. The risk coefficients calculated from the Japanese data were adopted by the ICRP in 1990 (6).

For the risk assessment for the population (adults and children), the ICRP proposes the following so-called lifetime risk coefficients for cancer mortality: 5% per Sv for low doses and 10% per Sv for high doses. A risk coefficient of 10% per Sv means that a radiation exposure of 10 mSv for 10 000 persons leads to 10 additional deaths from cancer or leukemia. (Without the effects of radiation, about 2500 of 10 000 people die of cancer). The leukemia risk after radiation of the red bone marrow is 0.5% per Sv, corresponding to one tenth of the total cancer risk in the lower dose range. It becomes clear that the risk coefficients are very different in the different organs. The genetic radiation risk – the probability that future generations will suffer from severe genetic damage – has been given as 1% per Sv. Thus the genetic radiation risk is five times less than the risk of fatal cancer. New considerations will probably lead to even lower figures in the future.

Radiation doses to healthy normal tissue from radiation scatter during radiotherapy are markedly greater than the normal dose values in X-ray diagnosis. For this reason, these values cannot be extrapolated to assess the risks, e.g., of a second malignancy or of genetic damage after radiotherapy (13).

When evaluating the risk of medical radiation exposure, the age at time of exposure and the patient's life expectancy in comparison to the general population must be considered. The risk of mortality from ionizing greatly decreases with increasing age. This is linked to the fact that most tumors have a long latency period, so that they cannot be identified within the exposed person's life-time (12).

Radiation exposure in radiology
In industrial countries, most radiation exposure linked to civilization is from medical diagnosis (14). For this reason, the European PatSRL, the StrlSchV and the RöV demand that the medical radiation exposure of the general population and relevant reference groups should be regularly determined (3, 4, 5).

According to the BfS, about 148 million X-ray investigations were performed in 2001 – one third being in dentistry. This corresponds to 1.9 investigations per inhabitant (14). Although there was essentially no change in the number of X-ray investigations per 1000 inhabitants between 1996 and 2001, the mean effective dose per inhabitant rose from 1.6 mSv to 1.8 mSv within this period. The reason for this is evidently the increasing use of CT and interventional radiography. Although CTs make up only about 7% of all X-ray investigations in Germany, they make a disproportionate contribution – 47.2% – to patients' radiation exposure (figure gif ppt). This trend towards an increasing contribution of CT to medical radiation exposure is also observed in other countries (table 3 gif ppt). For these reasons, CT should be an important and worthwhile aspect of radiological protection.

In 1999, a review on CT exposure in practice was performed throughout Germany (19). This review, as well as other publications (20, 21) makes it clear that the patients' radiological exposure during CT is highly dependent on the technical parameters set. Potential dose reductions of 50% were identified in half the participants in the survey (19). The introduction and maintenance of dose reference values for CT investigations thus seems to be both sensible and necessary.

It has been estimated that medical diagnosis in Germany leads to an additional "attributable" cancer mortality of 1.5 to 2% (22, 23). The risk is similar in other countries with comparably high patient exposure, such as Luxembourg and Belgium, but markedly lower in England, the Netherlands, and Switzerland (0.6%, 0.7%, and 1%, respectively). Nevertheless, Berrington de Gonzalez and Darby, the authors of the English study, do not exclude the possibility that the risk from ionizing radiation in diagnostic radiology is being overestimated. In any case, it is quite clear that there are major differences in the risks in the 15 countries compared (23).

Résumé
It is essential for radiological protection that ionizing radiation is only used when there is a clear justification. The qualification in radiological protection is acquired by suitable training in the relevant area and appropriate practical experience, together with theoretical knowledge. This must be updated at least every five years by successful participation in a course which is recognized by the responsible authority – usually the local Medical Association. Only a qualified physician can justify the medical use of ionizing radiation and establish that the expected benefit of the investigation outweighs the radiation risk (3, 4).

As is evident in the SSK recommendations and in the BfS reports, rules for the referral for radiological investigation can influence the type and scope of the procedures used (24). Exposure may be reduced by optimizing the procedure and by avoiding unnecessary imaging, particularly repeats, or inexpertly performed tests. Particularly in exclusion diagnoses by imaging procedures, ultrasound and magnetic resonance tomography should be the procedures of first choice, in so far as this is possible, as these supply information for further diagnostic and therapeutic measures without using ionizing radiation (25).

Communication between the referring physicians and specialists in radiology and nuclear medicine is of essential importance in justifying the use of ionizing radiation and in identifying the optimal procedure. In 2001, the SSK recommended the preparation of "Guidelines for Referral for Imaging Techniques" (24). This was published in summer 2006 under the title "Orientierungshilfe für radiologische und nuklearmedizinische Untersuchungen“ (Guideline for Investigations in Radiology and Nuclear Medicine) (11). This guideline covers pregnancy and protection of the fetus together with the necessary optimization of radiation exposure in investigations of children and adolescents. It particularly emphasizes the selection of suitable imaging procedures to avoid unnecessary patient exposure.

Technical radiological protection in isolation therefore is not the most effective method to reduce radiation exposure in medicine. Physicians must ensure that unnecessary investigations are not performed. The referring physician bears special responsibility and should use guidelines for diagnosis to a greater extent. In difficult cases, they should also consult with their colleagues in radiology and nuclear medicine in the selection of the most suitable procedure. If cooperation between these two groups of physicians in the selection of imaging procedures for common diseases could be enhanced, this would be a major contribution towards radiological protection.

Conflict of interest statement
The authors declare that no conflict of interest exists according to the guidelines of the International Committee of Medical Journal Editors.

Manuscript received on 28 February 2007, revised version accepted on 24 August 2007.

Translated from the original German by Rodney A. Yeates, M.A., Ph.D..


Corresponding author
Prof. Dr. rer. nat. Maria Blettner
Institut für Medizinische Biometrie, Epidemiologie und Informatik
Johannes Gutenberg-Universität
55101 Mainz, Germany
1.
Richtlinie zur Festlegung der Grundnormen für den Gesundheitsschutz der Bevölkerung und der Arbeitskräfte gegen die Gefahren ionisierender Strahlungen; Amtsblatt der Europäischen Gemeinschaften. Nr. 11 vom 20/02/1959; 221–39.
2.
Rat der Europäischen Gemeinschaften: Richtlinie 84/466/ EURATOM zur Festlegung der grundlegenden Maßnahmen für den Strahlenschutz bei ärztlichen Untersuchungen und Behandlungen; Amtsblatt der Europäischen Gemeinschaften. Nr. L 265 vom 03/09/1984; 1–3.
3.
Bundesgesetzblatt vom 8. Januar 1987: Verordnung über den Schutz vor Schäden durch Röntgenstrahlen (Röntgenverordnung) in der Fassung vom 30/04/2003.
4.
Bundesgesetzblatt vom vom 30. Juni 1989: Verordnung über den Schutz vor Schäden durch ionisierende Strahlen (Strahlenschutzverordnung), in der Fassung vom 18/06/2002.
5.
Rat der Europäischen Gemeinschaften: Richtlinie 97/43/EURATOM des Rates vom 30/06/1997 über den Gesundheitsschutz von Personen gegen die Gefahren ionisierender Strahlung bei medizinischer Exposition und zur Aufhebung der Richtlinie 84/466/Euratom; Amtsblatt der Europäischen Gemeinschaften. Nr. L 180 vom 09/07/1997; 22–7.
6.
International Commission on Radiological Protection: Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Oxford: Pergamon Press 1991.
7.
Strahlenschutzkommission: Anwendung der effektiven Dosis bei medizinischen Untersuchungen; Im Jahresbericht 1997 der Strahlenschutzkommission, Heft 12. Stuttgart: Gustav Fischer Verlag 1998.
8.
Bernhardt JH, Veit R, Bauer B: Erhebungen zur effektiven Dosis und zur Kollektivdosis bei der Röntgendiagnostik in den alten Bundesländern; Veröffentlichungen der Strahlenschutzkommission, Band 30. Stuttgart: Gustav Fischer Verlag 1995.
9.
Vaño E, Valesco A, Moran P, Gonzalez L, Pedrosa CSA: Evalution of diagnostic radiology in a big hospital during a 5 year period, and the derived collective dose. Br J Radiol 1993; 66: 892–98. MEDLINE
10.
Veit R, Bauer B, Bernhardt HJ: Proposed procedure for the establishment of diagnostic reference levels in Germany. Radiat Prot Dosimetry 1998; 80: 117–20.
11.
Strahlenschutzkommission: Orientierungshilfe für radiologische und nuklearmedizinische Untersuchungen. Heft 30. Berlin: H. Hoffmann 2006.
12.
Jung H: Die Risiken der Röntgendiagnostik. Röntgenstrahlen 1991; 66: 46–53.
13.
Müller WU: Genetische Effekte und Kanzerogenese. In: Bamberg M, Molls M, Sack H (Hrsg.): Radioonkologie Grundlagen. München: Zuckerschwerdt Verlag 2003; 266–74.
14.
Brix G, Nekolla E, Griebel J: Strahlenexposition von Patienten durch diagnostische und interventionelle Röntgenanwendungen – Fakten, Bewertung und Trends. Radiologe 2005; 45: 340–9. MEDLINE
15.
Linton OW, Mettler FA: National conference on dose reduction in CT, with an emphasis on pediatric patients. Am J Roentgenol 2003; 181: 321–9. MEDLINE
16.
Nishizawa K, Matsumoto M, Iwai K, Maruyama T: Nishizawa survey of CT practice in Japan and collective effective dose estimation. Nippon Acta Radiol 2004; 64: 151–8. MEDLINE
17.
Shannoun F, Zeeb H, Back C, Blettner M: Medical exposure of the population from diagnostic use of ionizing radiation in Luxembourg between 1994 and 2002. Health Phys 2006; 91: 154–62. MEDLINE
18.
Vanmarcke H, Paridaens J, Eggermont G, Mol H, Brouwers J: Ioniserende straling, In: Milieu – en natuurrapport Vlaanderen, Vlaamse Milieu-maatschappij. Garant 2002: MIRAT Report 2001.
19.
Galanski M, Nagel HD, Stamm G: CT-Expositionspraxis in der Bundesrepublik Deutschland. Fortschr Röntgenstr 2001; 173: 61–6. MEDLINE
20.
European Commission: European guidelines on quality criteria for computed tomography. Luxembourg: Office for Official Publication of the European Communities 2000.
21.
Koller F, Roth J: Die Bestimmung der effektiven Dosen bei CT-Untersuchungen und deren Beeinflussung durch Einstellparameter. Fortschr Röntgenstr 2007; 179: 38–45. MEDLINE
22.
Kaul A, Bauer B, Bernhardt J, Nosske D, Veit R: Effective doses to members of the public from the diagnostic application of ionizing radiation in Germany. Eur Radiol 1997; 7: 1127–32. MEDLINE
23.
Berrington de Gonzalez A, Darby S: Risk of cancer from diagnostic X-rays: estimates for the UK and 14 other countries. Lancet 2004; 363: 345–51. MEDLINE
24.
Strahlenschutzkommission: Notwendigkeit der Erstellung von Überweisungskriterien für die Durchführung bildgebender Verfahren. Heft 30. München: Urban & Fischer 2002.
25.
Busch HP: Qualitäts- und Dosismanagement in der Digitalen Projektionsradiographie. Fortschr Röntgenstr 2003; 175: 32–7. MEDLINE
Institut für Medizinische Physik und Strahlenschutz (IMPS), Fachhochschule Friedberg-Gießen: Dr. Shannoun
Institut für Medizinische Biometrie, Epidemiologie und Informatik (IMBEI),
Johannes Gutenberg-Universität, Mainz: Prof. Dr. rer. nat. Blettner, Prof. Dr. med. Zeeb
Klinik und Poliklinik für Radioonkologie sowie Strahlentherapie, Johannes
Gutenberg-Universität Mainz: Prof. Dr. med. Schmidberger
1. Richtlinie zur Festlegung der Grundnormen für den Gesundheitsschutz der Bevölkerung und der Arbeitskräfte gegen die Gefahren ionisierender Strahlungen; Amtsblatt der Europäischen Gemeinschaften. Nr. 11 vom 20/02/1959; 221–39.
2. Rat der Europäischen Gemeinschaften: Richtlinie 84/466/ EURATOM zur Festlegung der grundlegenden Maßnahmen für den Strahlenschutz bei ärztlichen Untersuchungen und Behandlungen; Amtsblatt der Europäischen Gemeinschaften. Nr. L 265 vom 03/09/1984; 1–3.
3. Bundesgesetzblatt vom 8. Januar 1987: Verordnung über den Schutz vor Schäden durch Röntgenstrahlen (Röntgenverordnung) in der Fassung vom 30/04/2003.
4. Bundesgesetzblatt vom vom 30. Juni 1989: Verordnung über den Schutz vor Schäden durch ionisierende Strahlen (Strahlenschutzverordnung), in der Fassung vom 18/06/2002.
5. Rat der Europäischen Gemeinschaften: Richtlinie 97/43/EURATOM des Rates vom 30/06/1997 über den Gesundheitsschutz von Personen gegen die Gefahren ionisierender Strahlung bei medizinischer Exposition und zur Aufhebung der Richtlinie 84/466/Euratom; Amtsblatt der Europäischen Gemeinschaften. Nr. L 180 vom 09/07/1997; 22–7.
6. International Commission on Radiological Protection: Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Oxford: Pergamon Press 1991.
7. Strahlenschutzkommission: Anwendung der effektiven Dosis bei medizinischen Untersuchungen; Im Jahresbericht 1997 der Strahlenschutzkommission, Heft 12. Stuttgart: Gustav Fischer Verlag 1998.
8. Bernhardt JH, Veit R, Bauer B: Erhebungen zur effektiven Dosis und zur Kollektivdosis bei der Röntgendiagnostik in den alten Bundesländern; Veröffentlichungen der Strahlenschutzkommission, Band 30. Stuttgart: Gustav Fischer Verlag 1995.
9. Vaño E, Valesco A, Moran P, Gonzalez L, Pedrosa CSA: Evalution of diagnostic radiology in a big hospital during a 5 year period, and the derived collective dose. Br J Radiol 1993; 66: 892–98. MEDLINE
10. Veit R, Bauer B, Bernhardt HJ: Proposed procedure for the establishment of diagnostic reference levels in Germany. Radiat Prot Dosimetry 1998; 80: 117–20.
11. Strahlenschutzkommission: Orientierungshilfe für radiologische und nuklearmedizinische Untersuchungen. Heft 30. Berlin: H. Hoffmann 2006.
12. Jung H: Die Risiken der Röntgendiagnostik. Röntgenstrahlen 1991; 66: 46–53.
13. Müller WU: Genetische Effekte und Kanzerogenese. In: Bamberg M, Molls M, Sack H (Hrsg.): Radioonkologie Grundlagen. München: Zuckerschwerdt Verlag 2003; 266–74.
14. Brix G, Nekolla E, Griebel J: Strahlenexposition von Patienten durch diagnostische und interventionelle Röntgenanwendungen – Fakten, Bewertung und Trends. Radiologe 2005; 45: 340–9. MEDLINE
15. Linton OW, Mettler FA: National conference on dose reduction in CT, with an emphasis on pediatric patients. Am J Roentgenol 2003; 181: 321–9. MEDLINE
16. Nishizawa K, Matsumoto M, Iwai K, Maruyama T: Nishizawa survey of CT practice in Japan and collective effective dose estimation. Nippon Acta Radiol 2004; 64: 151–8. MEDLINE
17. Shannoun F, Zeeb H, Back C, Blettner M: Medical exposure of the population from diagnostic use of ionizing radiation in Luxembourg between 1994 and 2002. Health Phys 2006; 91: 154–62. MEDLINE
18. Vanmarcke H, Paridaens J, Eggermont G, Mol H, Brouwers J: Ioniserende straling, In: Milieu – en natuurrapport Vlaanderen, Vlaamse Milieu-maatschappij. Garant 2002: MIRAT Report 2001.
19. Galanski M, Nagel HD, Stamm G: CT-Expositionspraxis in der Bundesrepublik Deutschland. Fortschr Röntgenstr 2001; 173: 61–6. MEDLINE
20. European Commission: European guidelines on quality criteria for computed tomography. Luxembourg: Office for Official Publication of the European Communities 2000.
21. Koller F, Roth J: Die Bestimmung der effektiven Dosen bei CT-Untersuchungen und deren Beeinflussung durch Einstellparameter. Fortschr Röntgenstr 2007; 179: 38–45. MEDLINE
22. Kaul A, Bauer B, Bernhardt J, Nosske D, Veit R: Effective doses to members of the public from the diagnostic application of ionizing radiation in Germany. Eur Radiol 1997; 7: 1127–32. MEDLINE
23. Berrington de Gonzalez A, Darby S: Risk of cancer from diagnostic X-rays: estimates for the UK and 14 other countries. Lancet 2004; 363: 345–51. MEDLINE
24. Strahlenschutzkommission: Notwendigkeit der Erstellung von Überweisungskriterien für die Durchführung bildgebender Verfahren. Heft 30. München: Urban & Fischer 2002.
25. Busch HP: Qualitäts- und Dosismanagement in der Digitalen Projektionsradiographie. Fortschr Röntgenstr 2003; 175: 32–7. MEDLINE