Journal of the ICRU Vol 5 No 2 (2005) Report 74 Oxford University Press
doi:10.1093/jicru/ndi022
EXECUTIVE SUMMARY quantity are discussed in Section 4. Quantities are also recommended for establishment and use of diagnostic reference levels (DRLs). It should be noted that the recommendations made here of dosimetric quantities for CT are of a provisional nature. Methods for assessing the patient dose of x rays used in medical imaging are presented in Section 4. Such methods are required for the determination of normalization quantities used in dose-conversion coefficients and of quantities recommended for establishment and use of DRLs. Measurements of these quantities rely mainly on the use of ionization chambers or solid-state dosimeters, including TLDs. For the low- and medium-energy x rays used in medical imaging, the air kerma free-in-air is the desired quality for calibration. Examples are given of x-ray beam qualities recommended for calibration, but it is noted that an international code of practice for patient dosimetry in diagnostic radiology is presently being developed by the International Atomic Energy Agency, including practical details of calibrations. Methods for determining organ and tissue doses are the subject of Section 5. It is concluded that assessment of organ and tissue doses, as well as doses to the most heavily irradiated regions of the body, mainly relies on Monte Carlo calculations. Therefore, specific information is provided on the application of Monte Carlo calculations of radiation transport as employed for patient dosimetry in medical x-ray imaging. This section includes comparisons of dose-conversion coefficients calculated at different institutes as well as comparisons of dose measurements and calculations. It is noted that procedures for medical x-ray imaging vary from country to country. Consequently, dose-conversion coefficients calculated by various authors commonly refer to national or regional imaging procedures. For similar exposure conditions, similar results are obtained from calculations at different institutes and from measurements. When a dose-conversion coefficient is needed for a specific situation, the best approach is to select a value from the available
Ó International Commission on Radiation Units and Measurements 2005
Downloaded from http://jicru.oxfordjournals.org/ at Selcuk University on January 26, 2015
In Section 1 it is emphasized that medical imaging was virtually the first application of x rays. The health risks of irradiation became evident only later. Appropriate quantities to measure the amount of irradiation of an object had to be developed, leading to quantities like exposure, absorbed dose, and kerma. It is furthermore stressed that there is a need for harmonization of quantities and terminology for different applications in medical imaging using x rays. The two purposes of patient dosimetry of x rays used in medical imaging are to set and check standards of good practice and to assess the risks of detriment or harm. Image quality is stressed to be of paramount importance in medical imaging but is the subject of other ICRU Reports. Specifications of x-ray beams used for medical imaging are presented in Section 2. It is recommended to characterize the radiation quality of x-ray beams used for medical imaging by a combination of various parameters, including first and second half-value layer, HVL1 and HVL2, the ratio of HVL1 and HVL2, the tube voltage, and the total filtration. In most cases a combination of three of these parameters will be sufficient for characterization. The radiation intensity is also an important characteristic of an x-ray tube (including filtration). For this purpose the x-ray tube output is defined. Quantities and units for dose measurement and calculation in medical x-ray imaging are dealt with in Section 3. Relevant basic dosimetric qualities are presented in first instance. Several application-specific qualities have been found useful for measurements in medical x-ray imaging, but ambiguity remains in the names of quantities and their use. Therefore, application-specific quantities and new symbols are defined. Concerning risk-related quantities, mean organ and tissue doses are defined as well as absorbed dose to the more heavily irradiated regions of tissues in relation to deterministic effects. Dose-conversion coefficients relate the specified dosimetric quantities to a normalization quality. Both types of dosimetric
PATIENT DOSIMETRY FOR X RAYS USED IN MEDICAL IMAGING
data based on similarities in exposure conditions (projection, view, and radiation quality) and a patient model. Appendix A presents information on backscatter factors for irradiation conditions relevant for dosimetry for medical imaging using x rays. Appendices B, C, and D provide dose-conversion coefficients that
reflect the differences in medical imaging using x rays in the USA, Germany, and the UK, respectively. Appendix E treats dose-conversion coefficients for mammography. Appendix F describes a PCbased Monte Carlo program for calculating patient doses in radiography (excluding mammography) and in fluoroscopy.
Downloaded from http://jicru.oxfordjournals.org/ at Selcuk University on January 26, 2015
8
doi:10.1093/jicru/ndi022
EXECUTIVE SUMMARY quantity are discussed in Section 4. Quantities are also recommended for establishment and use of diagnostic reference levels (DRLs). It should be noted that the recommendations made here of dosimetric quantities for CT are of a provisional nature. Methods for assessing the patient dose of x rays used in medical imaging are presented in Section 4. Such methods are required for the determination of normalization quantities used in dose-conversion coefficients and of quantities recommended for establishment and use of DRLs. Measurements of these quantities rely mainly on the use of ionization chambers or solid-state dosimeters, including TLDs. For the low- and medium-energy x rays used in medical imaging, the air kerma free-in-air is the desired quality for calibration. Examples are given of x-ray beam qualities recommended for calibration, but it is noted that an international code of practice for patient dosimetry in diagnostic radiology is presently being developed by the International Atomic Energy Agency, including practical details of calibrations. Methods for determining organ and tissue doses are the subject of Section 5. It is concluded that assessment of organ and tissue doses, as well as doses to the most heavily irradiated regions of the body, mainly relies on Monte Carlo calculations. Therefore, specific information is provided on the application of Monte Carlo calculations of radiation transport as employed for patient dosimetry in medical x-ray imaging. This section includes comparisons of dose-conversion coefficients calculated at different institutes as well as comparisons of dose measurements and calculations. It is noted that procedures for medical x-ray imaging vary from country to country. Consequently, dose-conversion coefficients calculated by various authors commonly refer to national or regional imaging procedures. For similar exposure conditions, similar results are obtained from calculations at different institutes and from measurements. When a dose-conversion coefficient is needed for a specific situation, the best approach is to select a value from the available
Ó International Commission on Radiation Units and Measurements 2005
Downloaded from http://jicru.oxfordjournals.org/ at Selcuk University on January 26, 2015
In Section 1 it is emphasized that medical imaging was virtually the first application of x rays. The health risks of irradiation became evident only later. Appropriate quantities to measure the amount of irradiation of an object had to be developed, leading to quantities like exposure, absorbed dose, and kerma. It is furthermore stressed that there is a need for harmonization of quantities and terminology for different applications in medical imaging using x rays. The two purposes of patient dosimetry of x rays used in medical imaging are to set and check standards of good practice and to assess the risks of detriment or harm. Image quality is stressed to be of paramount importance in medical imaging but is the subject of other ICRU Reports. Specifications of x-ray beams used for medical imaging are presented in Section 2. It is recommended to characterize the radiation quality of x-ray beams used for medical imaging by a combination of various parameters, including first and second half-value layer, HVL1 and HVL2, the ratio of HVL1 and HVL2, the tube voltage, and the total filtration. In most cases a combination of three of these parameters will be sufficient for characterization. The radiation intensity is also an important characteristic of an x-ray tube (including filtration). For this purpose the x-ray tube output is defined. Quantities and units for dose measurement and calculation in medical x-ray imaging are dealt with in Section 3. Relevant basic dosimetric qualities are presented in first instance. Several application-specific qualities have been found useful for measurements in medical x-ray imaging, but ambiguity remains in the names of quantities and their use. Therefore, application-specific quantities and new symbols are defined. Concerning risk-related quantities, mean organ and tissue doses are defined as well as absorbed dose to the more heavily irradiated regions of tissues in relation to deterministic effects. Dose-conversion coefficients relate the specified dosimetric quantities to a normalization quality. Both types of dosimetric
PATIENT DOSIMETRY FOR X RAYS USED IN MEDICAL IMAGING
data based on similarities in exposure conditions (projection, view, and radiation quality) and a patient model. Appendix A presents information on backscatter factors for irradiation conditions relevant for dosimetry for medical imaging using x rays. Appendices B, C, and D provide dose-conversion coefficients that
reflect the differences in medical imaging using x rays in the USA, Germany, and the UK, respectively. Appendix E treats dose-conversion coefficients for mammography. Appendix F describes a PCbased Monte Carlo program for calculating patient doses in radiography (excluding mammography) and in fluoroscopy.
Downloaded from http://jicru.oxfordjournals.org/ at Selcuk University on January 26, 2015
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