Diagnostic reference levels as a quality assurance tool

Australian Institute of Radiography The Radiographer 2009; 56 (3): 32 37 Literature review Diagnostic reference levels as a quality assurance tool KD Edmonds Medical Physics Section, Medical Radiation Branch, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Victoria 3085, Australia. Correspondence keith.edmonds@arpansa.gov.au Abstract The objective of diagnostic reference levels (DRL) in radiology is to assist in the optimisation of radiation dose to patients, while maintaining diagnostic image quality, and to detect unusually high doses that do not contribute significantly to the clinical outcome of a medical imaging examination. DRL have been in existence overseas for more than a decade and its influence has contributed to a steady decline in dose for general radiography and fluoroscopic procedures. High dose modalities such as CT and interventional procedures are increasing dramatically both locally and internationally resulting in the unwanted outcome of a significant increase in population cumulative effective dose. This calls for urgent dose reduction and dose constraint measures. Utilising DRL is one method of optimising patient dose. Some local and international DRL dose levels for some common radiographic, interventional and CT examinations are presented as a suggestion for the application of this methodology in Australian radiology practice. Keywords: diagnostic reference levels, guidance levels, radiation dose, reference values, quality assurance. Introduction The objective of establishing diagnostic reference levels (DRL) in diagnostic imaging (also previously known as Guidance Levels or Reference Values 1,2 ) is to provide radiology and other departments that use x-ray imaging with a convenient DRL dose comparison to ensure that radiation doses to patients are kept within reasonable limits. The main task of radiation protection is not only to minimise the stochastic risks but also to avoid deterministic injuries. Stochastic refers to effects whose probability increases with increasing dose and for which there is no threshold dose. Any dose, no matter how small, has the potential to cause harm and this becomes apparent years after the exposure. Examples are leukaemia and hereditary effects. Deterministic effects are those in which the severity of the effect, rather than the probability, increase with increasing dose and for which there is a threshold dose. Examples are epilation, erythema and hematologic damage and are known as early effects. 3 A DRL, as defined by the International Commission on Radiological Protection (ICRP), is a form of investigation level, applied to an easily measured quantity, usually the absorbed dose in air, or tissue-equivalent material at the surface of a simple phantom or a representative patient. 4 The ICRP recommends the establishment of reference levels as a method of optimising the radiation exposure to patients. This is accomplished by comparison between the numerical value of the diagnostic reference level (derived from relevant national, regional or local data) and the mean or other appropriate value observed in practice for a suitable reference group of patients or a suitable reference phantom. Another definition by the Council of the European Union in its Council Directive 97/43 defines DRL as dose levels in medical radiodiagnostic practices or, in the case of radio-pharmaceuticals, levels of activity, for typical examinations for groups of standardsized patients or standard phantoms for broadly defined types of equipment. These levels are expected not to be exceeded for standard procedures when good and normal practice regarding diagnostic and technical performance is applied. 36 It reinforces the concept of references doses applying only to standard or representative patients. DRL therefore are not dose limits but a guide of good practice. It is not a dose constraint and the DRL values are not used for regulatory or commercial purposes. DRL act as an investigation trigger if the numerical values are consistently exceeded. Background The need for DRL Patient exposures in diagnostic radiology are increasing at a disquieting rate for certain radiographic, fluoroscopic and CT examinations. Regulla and Elder 5 pointed out that data obtained from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNCEAR) show that there are significant differences in national radiation exposures and a very uneven distribution of patient doses among world population for the same or similar procedures. Mean annual x-ray effective dose of the population can vary by up to a factor of 60. In the United States, studies such as the Nationwide Exposure X-ray Trends (NEXT) surveys also showed that patient doses in radiology vary considerably from one facility to the next. 6 Gray, et al. 1 posed the question, why one radiology facility should use an exposure that is 10, 20 or 126 times greater than another facility to produce a radiographic image? Johnston and Brennan 7 and Carroll and Brennan 8 also reported wide variations in patient doses for the same radiographic examinations among hospitals in the UK and Europe. These patient doses are attributed to a wide range of factors such as type of image receptor, exposure factors, fluoroscopic times, number of images, type of anti-scatter grid and level of quality control as reviewed by Seeram, 9 Bushong 10 and Parry, et al. 11 In Australia, the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) Code of Practice, Section 3.1.8

Diagnostic reference levels as a quality assurance tool The Radiographer 33 (Radiation Protection Series No.14) states that the Responsible Person must establish a program to ensure that radiation doses administered to a patient for diagnostic purposes are: 1 Periodically compared with DRL for diagnostic procedures for which DRL have been established in Australia 2 If DRL are consistently exceeded, reviewed to determine whether radiation has been optimised. 12 In addition, the ARPANSA Safety Guide, Section 7.8 (Radiation Protection Series No.14.1), suggests that as part of the QA program, patient dose surveys are undertaken periodically to establish that the doses are acceptable when compared with published DRL. It also recommends that accrediting bodies such as RANZCR and the Australian Council on Healthcare Standards consider including compliance with DRL for a core set of examinations. If the radiology department observes dose values consistently exceeding the DRL, then this warrants further investigation however some flexibility should be allowed if higher doses are indicated by sound clinical judgement. 13,14 However, at this point in time, there are no DRL published in the Code of Practice or the Safety Guides. It would seem logical therefore to use published values from the literature from extensive surveys in countries with similar healthcare settings e.g. similar levels of education and training for imaging technologists, radiologists and similar provision of imaging equipment. 15 History of DRL National surveys of patient doses from x-ray examinations in Europe and the USA since the 1950s have demonstrated wide variations in doses between radiology departments and illustrated the need for quantitative guidance on patient exposure. It was only at this stage that dose measurements to patients began in earnest. National surveys in the USA and UK concentrated on measuring entrance surface doses with or without backscatter for common radiographic projections. The Nationwide Evaluation of X-ray Trends in the USA in the 1970s measured entrance skin exposure free-in-air for average exposure technique factors or used a standard phantom. The NRPB national patient dose survey in the UK in the 1980s measured entrance surface dose directly on the surface of the patient (including backscatter) using thermoluminescence dosemeters. A European trial supporting the Quality Criteria for Diagnostic Radiographic Images in 1991 used the same technique. 37 Dose guidelines began to appear in late 1980s. First was the USA, promoted by the Centre for Devices and Radiological Health (CDRH) in conjunction with the Conference of Radiation Control Program Directors Inc. Then in the UK it was conducted by the National Radiation Protection Board in collaboration with relevant professional bodies. Europe then followed with reference doses incorporated into Working Documents by EC Study Groups. 38 International recommendations then appeared on how to measure and set reference dose levels based on the initiatives led by the USA and the UK. The ICRP Publication 60 first made mention of the concept of investigation levels in 1990 followed by the current definition of DRL in ICRP Publication 73 in 1996 4 and the EC Medical Exposure Directive in 1997. 36 The United Kingdom introduced DRL in 1990 for common diagnostic examinations based on a national patient dose survey in the mid-1980s conducted by the NRPB, now known as the Health Protection Agency (HPA). They are now based on the five-yearly reviews of the National Patient Dose Database and are currently in their third review. 16 The International Atomic Energy Agency (IAEA) in 1996 and 2002 also issued advice on the use of DRL (or guidance levels) in their safety standard series and included guidance levels for typical adult patient doses for general radiography, CT, fluoroscopy and mammography. 2,14, Many countries worldwide have now incorporated the European Community Directive in national legislative documents. 1 Several organisations currently providing guideline documents for establishing DRL include the ICRP, 4,20 the Health Protection Agency in the UK, 16,21 the Commission of European Communities 22 and the American College of Radiology. 23 Aim The overall aim of DRL are to better manage patient dose in diagnostic radiology using the principle of optimisation which is defined as exposure to radiation from justified activities should be kept as low as reasonably achievable, social and economic factors being taken into account. The European Commission and the ICRP provide a range of tools to achieve this. 15,24 Data obtained from patient dose surveys show that typical patient doses for the same type of x-ray examinations can vary considerably from one radiology practice to another. The establishment of DRL therefore is to give an indication of unusually high values. The DRL are usually set at the third quartile value of the distribution of typical doses derived from dose surveys both nationally and internationally. Using the third quartile or 75th percentile is a compromise between being overly stringent and overly complacent. 3 Essentially, if mean doses exceed a reference level dose an investigation should take place to establish the cause and take corrective action, unless the dose was clinically justified. Reference doses were also used to provide a trigger for practices in need of investigation and hopefully lead to dose optimisation. ICRP 73 4 recommended that DRL values be selected by professional bodies, be reviewed at regular intervals and be specific to a country or region. Wide variations in patient doses are to be expected and it is only sensible to compare mean or median values, which is less influenced by extreme outliers, on representative groups of patients to monitor trends with time, equipment or technique. Method From a practical perspective, the DRL should be expressed as a readily measurable patient-related quantity for the specified procedure. For example, 1 General radiographic examinations either entrance skin dose (ESD) or the dose area product (DAP) 2 Fluoroscopic examinations dose area product (DAP) 3 CT examinations computed tomography dose index ( CTDI w or CTDI vol ) and the dose length product (DLP). New CT scanners in accordance with Australian Standards, AS/NZS 32002.4, 25 should display the volume CTDI vol and/or the DLP on the operator s console after the selection of technique factors and prior to the initiation of x-rays. 13 DRL used for film-screen technology should not necessarily be used for new digital radiography without prior adjustment. 26 Dosimetry methods International guidance on patient dosimetry techniques for x-rays used in medical imaging is published by the International Commission on Radiation Units and Measurements in ICRU Report 74. 27 This report contains advice on the relevant dosimetric quantities and how to measure or calculate them in a clinical setting which is directly applicable to the patient dose surveys needed to estimate population exposure.

34 The Radiographer KD Edmonds There are various methods of recording dose levels. Thermoluminescent dosimeters (TLD) are often used for plain film examinations and include dose contribution from backscatter if placed on the patient or phantom surface. The small TLD sachets are usually placed in the centre of the irradiated field on the entrance surface of the patient or phantom. The TLD can be stuck directly and unobtrusively to the patient s skin with very little interference in patient mobility or comfort. They do not interfere with the examination or obscure important diagnostic information on the radiographic image. They need to be calibrated with respect to radiation qualities used in diagnostic radiology. TLD are also prone to some inaccuracy due to signal fade, nonlinear response and dependency on beam energy. 3 Ionisation chambers are bulky and more difficult to attach to patients. The parallel plate ionisation chambers measure back scatter but the lead backed solid state detectors do not. They are not recommended for direct measurement of entrance surface dose on the skin of the patient. They can, however, be used to make measurements of the absorbed dose to air, in free air, without a patient or phantom present. The measurements can then be corrected using appropriate backscatter factors and the inverse square law to estimate the entrance surface dose. Newer technology such as optically stimulated luminescence dosimeters (OSLs) and radiochromic film may replace TLD. Radiochromic film is currently being evaluated by ARPANSA. Alternatively, the entrance skin dose may also be calculated from x-ray tube output measurements (mgy/mas) and the exposure parameters, kvp, filtration and mas. The incident air kerma is calculated from the tube output using the inverse square law and then multiplied by the backscatter factor to obtain the entrance skin dose. A dose area product (DAP) meter consists of an ionisation meter that is usually attached to the x-ray tube collimator and measures the dose in Gy. square centimetre (Gy cm 2 ) which is proportional to the beam area and incident air kerma. The unit unfortunately does not measure backscatter which is important in higher dose examinations such as cardiac and vascular interventional procedures. However DAP meters can be used for radiographic and fluoroscopic procedures such as barium meals, angiography and on mobile image intensifiers. 2,3,28 For CT machines, the CTDI w and /or CTDI vol (mgy) and the DLP values(mgy cm) are conveniently provided at the operator console before or after the examination. The CTDI w is the weighted sum of the CT dose (or air kerma ) index measured in the centre and periphery (1 cm under the surface) of a 16 cm diameter (head) or a 32 cm diameter (trunk) standard polymethylmethacrylate (PMMA) CT dosimetry phantom. The CT dose index is measured with a 100mm long pencil ionization chamber inside a standard PMMA CT dosimetry phantom. CTDI w = CTDI c + CTDI p where c is the centre position 3 3 and p is the peripheral position of the phantom. Units: mgy CTDI w corrected for pitch is the CTDI vol. CTDI vol = 1 CTDI w pitch 2 Units: mgy The DLP is the product of CTDI vol and the scan length of the examination. Thus DLP = CTDI vol x length irradiated. Units: mgy cm 15 As a starting point it is suggested that the UK 2000 survey review of DRL for general radiography for adults (Table 1) and fluoroscopy for adults (Table 2), Paediatric procedures (Table 3), the UK 2003 CT survey (Table 4), and mammography (Table 5), be adopted and /or adapted in the Australian context as there are currently no established national DRL. 29 Some state regulators though have provided local DRL guidance on radiography, fluoroscopy and CT. 34 Dose values should be reviewed as computed/digital radiography becomes more widespread in order to minimise the detrimental influence of exposure creep. 30 This phenomenon occurs after the change over from film-screen radiography to digital radiography where exposure factors may actually increase in order to reduce image noise. Uncoupling of display from acquisition in digital radiography introduces the potential for systematic overexposure without necessarily compromising image quality. 31 The wide exposure latitude and linear response to x-ray energy provides an image appearance that remains consistent throughout the exposure range and this in turn provides little feedback to the technologist. Underexposed images typically have a grainy, mottled appearance that causes radiologists to reject images. Over-exposed images, on the other hand, have a crisp, sharp appearance. In order to prevent repeating the image, the technologist may increase exposure factors especially for manual and mobile radiography. Exposure indices or exposure indicators provided by the various CR/DR manufacturers also have a wide range of acceptable values and are currently not standardised throughout the industry. In the case of CT examinations, care should be taken when following overseas DRL because of the wide variety of CT scanners and local examination protocols employed. In addition, the rapid advances in CT technology have also resulted in constantly changing scanning protocols. Nevertheless, the DRL provided in Tables 1 5, serve as a rough guide until new DRL emerge from current surveys in Australia. In future, a web based interactive dose survey software program will be provided by ARPANSA where each radiology department can access it to calculate their dose levels and compare them with DRL. Discussion The development of DRL practice in diagnostic radiology within Australia is still at an early stage as no national surveys have been carried out for any radiological examinations for the express purpose of establishing national DRL. At a local level, various organisations, regulatory authorities and individual practices have carried out limited general radiography, fluoroscopy and CT surveys. 34 There is a clear need to manage (optimise) the radiation doses from diagnostic radiology in order to minimise the risks from radiation induced cancers. The establishment and use of DRL is recommended by international radiation protection organisations as an important component of the management of these doses and many countries have incorporated them into their radiation protection regulations 12,36 Data from European countries shows a wide variation in common DRL which may be due to differences in socio-economic conditions, regulatory regime, activeness of professional bodies and health care implementation (private/public mix etc). 7,32 International radiation protection bodies such as the IAEA and ICRP therefore recommend that each country carry out its own national wide scale DRL survey. It is for this reason that Australia must develop its own set of common national DRL. The introduction of computed and digital radiography in recent years has had a significant impact on the potential for higher dose

Diagnostic reference levels as a quality assurance tool The Radiographer 35 Table 1: Recommended diagnostic reference doses for individual radiographs on adult patients. Radiograph ESD per radiograph (mgy) DAP per radiograph (Gy cm 2 ) Skull AP/PA 3 - Skull LAT 1.5 - Chest PA 0.2 0.12 Chest LAT 1 - Thoracic spine AP 3.5 - Thoracic spine LAT 10 - Lumbar spine AP 6 1.6 Lumbar spine LAT 14 3 Lumbar spine LSJ 26 3 Abdomen AP 6 3 Pelvis AP 4 3 Adopted from the UK 2000 DRL survey review. 29 Note: Adult is defined as a person of average size (70 80 kg) ESD = Entrance Skin Dose, DAP = Dose Area Product. Table 2: Recommended diagnostic reference doses for fluoroscopic/interventional examinations on adult patients. Examination DAP per exam (Gy cm 2 ) Fluoroscopy time per exam (mins) Barium (or water soluble) swallow 11 2.3 Barium meal 13 2.3 Barium follow through 14 2.2 Barium (or water soluble) enema 31 2.7 Small bowel enema 50 10.7 Biliary drainage/intervention 54 17 Femoral angiogram 33 5 Hickman line 4 2.2 Hysterosalpingogram 4 1 IVU 16 - MCU 17 2.7 Nephrostogram 13 4.6 Nephrostomy 19 8.8 Retrograde pyelogram 13 3 Sialogram 1.6 1.6 T-tube cholangiogram 10 2 Venogram (leg) 5 2.3 Coronary angiogram 36 5.6 Oesophageal dilation 16 5.5 Pacemaker implant 27 10.7 Adopted from the UK 2000 survey review. 29 DAP = Dose Area Product. delivery. 30,31 In addition, the exponential increase in CT examinations has lead to the unwanted outcome of a significant increase in population cumulative effective dose. 35 Other causal agents that are linked to high doses include type of image receptor, exposure factors, fluoroscopic time, number of images, type of antiscatter grid and level of quality control. 9,10,11 DRL depend significantly on local practice and equipment. They may also change with time as optimisation strategies become successful. The UK experience over the past 20 years has shown that the implementation of DRL together with a dose optimisation program has resulted in a gradual reduction of doses. 29,32 For DRL to succeed, acceptance and application of the concept is required across as many radiology departments as possible. Complex calculations will only discourage participation. Furthermore, setting DRL is a resource intensive activity and requires a national response. Priority should be given to procedures with greatest dose implications, i.e. CT and Interventional procedures. DRL should be owned by the professions such as the Australian Institute of Radiography and the Royal Australian New Zealand College of Radiology. ARPANSA will assist in facilitating their development. Conclusion Overseas experience has shown that the use of DRL have proven be a useful quality assurance tool in optimising patient dose in

36 The Radiographer KD Edmonds Table 3: Recommended diagnostic reference doses for complete examinations on paediatric patients. Examination Standard age (y) DAP per exam (Gy cm 2 ) MCU 0 0.4 1 1.0 5 1.0 10 2.1 15 4.7 Barium meal 0 0.7 1 2.0 5 2.0 10 4.5 15 7.2 Barium swallow 0 0.8 1 1.5 5 1.5 10 2.7 15 4.6 Adopted from the UK 2000 survey review. 29 Table 4: Recommended diagnostic reference levels for CT examinations (CTDI vol and DLP). Patient group Scan region CTDI vol (mgy) single slice/multi slice DLP (mgy cm) Single slice/multi slice Adults Post fossa Cerebrum 65/100 55/65 760/930 Abdomen (liver metastases ) 13/14 460/470 Abdomen and pelvis (abscess) 13/14 510/560 Chest, abdomen and pelvis (lymphoma staging or follow up). 22/26 760/940 Chest (lung cancer) 10/13 430/580 Chest Hi-res 3/7 80/170 Children 0 1 year-old Head (post fossa) Head (cerebrum) Thorax 35 30 12 270 (whole exam) 200 5-year-old Head (post fossa) Head (cerebrum) Thorax 50 45 13 470 (whole exam) 230 10-year-old Head (post fossa) Head (cerebrum) Thorax 65 50 20 620 (whole exam) 370 Adopted from UK 2003 CT dose survey. 33 Dose values for adults relate to the 16 cm diameter CT dosimetry phantom for examinations of the head and the 32 cm diameter CT dosimetry phantom for examinations of the trunk.all dose values for children relate to the 16 cm diameter CT dosimetry phantom. CTDI vol = Computed Tomography Dose Index Volume, DLP = Dose Length Product.

Diagnostic reference levels as a quality assurance tool The Radiographer 37 Table 5: Recommended diagnostic reference level for mammography for a typical adult patient. For film screen examinations using a grid, the mean glandular dose (MGD) is 2 mgy based on the 4.2 cm acrylic American College of Radiologists phantom. 17,18,19 For a 50% adipose, 50% glandular 5 cm thick phantom the MGD is 3 mgy 17 Note: For digital mammography, the values quoted above represent an upper limit diagnostic radiology. It is recommended that local dose surveys be performed annually while national surveys every five years. 32,33 The imaging technologist is the main person who decides on the exposure factors and the visual image quality for the radiologist to make a diagnosis. The technologist should therefore be aware of the exposure options that minimise radiation doses while still maintaining good image quality and monitoring dose levels. DRL would therefore serve as an important means of minimising radiation doses as well as dose variations at minimal cost to radiology departments. They also increase staff awareness and imaging technologists will be better equipped to deal with patient enquiries. 3 ARPANSA is responsible for carrying out national DRL surveys in consultation with relevant stakeholders such as the Royal Australian & New Zealand College of Radiology, The Australian Institute of Radiography, Australasian College of Physical Scientists & Engineers in Medicine, Australian & New Zealand Society of Nuclear Medicine, Department of Health and Aging and the various State/Territory Regulators. Acknowledgements The author thanks Anthony Wallace for his advice in preparing this article. The author KD Edmonds DCR BHA Grad Dip Pub Health References 1 Gray JE, Archer BR, Butler PF, Hobbs BB, Mettler FA, Pizzutiello RJ, et al. Reference values for diagnostic radiology: application and impact. Radiology 2005; 235: 354 8. 2 IAEA 2002.Radiological Protection for Medical Exposure to Ionizing Radiation. IAEA Safety Standards Series. Safety Guide No RS-G-1.5. Vienna, Austria: International Atomic Energy Agency. 3 Seeram,E, Brennan P C, Diagnostic reference levels in radiology. Radiologic Technology 2006: 77: 373 84. 4 ICRP. Radiological Protection and Safety in Medicine. ICRP Publication 73, Annals of the ICRP 1996; 26 (2). 5 Regulla DF, Eder H. Patient exposures in medical x-ray imaging in Europe. Radiat Prot Dosimet 2005; 14: 11 25. 6 Conference of Radiation Control Program Directors. Nationwide Evaluation of X-ray Trends (NEXT): Tabulation and Graphical Summary of 1995 Abdomen and Lumbosacral Spine Survey 2000. CRCPD Publication 00-2. 7 Johnston DA, Brennan PC. Reference dose levels for patients undergoing common diagnostic x-ray examinations in Irish hospitals. Br J Radiol 2000: 73: 396 402. 8 Carroll EM, Brennan PC. Radiation doses for barium enema and barium meal examinations in Ireland: potential diagnostic reference levels. Br J Radiol 2003; 76: 393 7. 9 Seeram E. Rad Tech s Guide to Radiographic Protection. Boston: Blackwell Science; 2001. 10 Bushong S. Radiologic Science for Technologists 8th ed. St Louis: Mosby; 2004. 11 Parry RA, Glaze SA, Archer BJ. Typical patient radiation doses in diagnostic radiology. Radiographics 1999; 19: 1289 1302. 12 ARPANSA. Code of Practice for Radiation Protection in the Medical Applications of Ionizing Radiation, Radiation Protection Series 2008 RPS 14. 13 ARPANSA. Safety Guide for Radiation Protection in Diagnostic and Interventional Radiology Radiation Protection Series 2008 RPS 14.1. 14 IAEA 1996. International Atomic Energy Agency, International Basic safety standards for protection against ionizing radiation and for the safety of radiation sources. Safety Series No. 115, IAEA, Vienna. 15 European Commission. European guidance on estimating population doses from medical x-ray procedures.2008. Radiation Protection Publication No 154. TREN/04/NUCL/S07.39241. 16 National Radiological Protection Board. Guidelines on patient dose to promote the optimisation for diagnostic medical exposures. Documents of the NRPB 1999; 10 (1). 17 Craig AR, Heggie JCP, McLean ID, Coakley KS, Nicoll JJ. Recommendations for a mammography quality assurance program ACPSEM Position paper. Australas Phys Eng Sci Med 2001; 24 (107): 131. 18 Mammographic Quality Control Manual. Royal Australian and New Zealand College of Radiologists 2002. 19 National Accreditation Standards. Breastscreen Australia 2002; pp 157. 20 International Commission on Radiological Protection. 1990 recommendations of the International Commission on Radiological Protection. ICRP Publication No. 60. Oxford: Pergamon; 1991. 21 HPA 2008. National protocol for patient dose measurements in diagnostic radiology. Dosimetry Working Party of the Institute of Physical Sciences in Medicine. Health Protection Agency.Chilton.Didcot. 22 Commission of European Communities. European Guidelines on Quality Criteria For Diagnostic Radiographic Images. The European Commission 1996. 23 ACR Technical Standard for Diagnostic Medical Physics Performance Monitoring of Radiographic and Fluoroscopic Equipment. Revised. Reston: American College of Radiology; 2001: 729 32. 24 International Commission on Radiological Protection. Diagnostic reference levels in medical imaging: review and additional advice. Available online at: www.icrp.org/docs/drl_for_web.pdt [verified 21st March]. 25 AS/NZS 2005. Standards Australia and Standards New Zealand. Medical electrical equipment Particular requirements for safety-x-ray equipment for computed tomography, AS/NZS 3200.2.44 Ed 2.1 26 ICRP. Managing patient dose in digital radiography. ICRP Publication 93 Annals of the ICRP 2004; 34 (1). 27 ICRU. Patient Dosimetry for x-rays used in medical imaging. ICRU Report 74. Journal of the ICRU 2005; 5 (2). 28 IPSM (1992). Dosimetry Working Party of the Institute of Physical Sciences in Medicine. National protocol for patient dose measurements in diagnostic radiology. Chilton, UK: NRPB. 29 Hart D, Hillier MC, Wall BF. Doses to patients from medical x-ray examinations in the UK-2000 review. Chilton: NRPB 2002; NRPB-W14. 30 Warren-Forward H, Arthur L, Hobson L, et al. An assessment of exposure indices in computed radiography for the posterior-anterior chest and the lateral lumbar spine. Br J Radiol 2007; 80: 26 31. 31 Willis C. Strategies for dose reduction in ordinary radiographic examinations using CR and DR. Pediatr Radiol 2004; 34 (Suppl 3): S196 S200. 32 Hart D, Hillier MC,Wall BF. National reference doses for common radiographic, fluoroscopic and dental x-ray examinations in the UK. Br J Radiol 2009: 82: 1 12. 33 Shrimpton PC,Hillier MC,Lewis MA,Dunn M. National survey of doses from CT in the UK: 2003. Br J Radiol 2006: 79: 968 80. 34 Boal T, Hedt J, Einsiedel P. A survey of patient dose and image quality factors for CT scanners in Victoria. Aust Phys Engineer Sci Med 1999; 22 (3): 103 12. 35 Brenner DJ, Hall EJ. Computed tomography an increasing source of radiation exposure. New Engl J Med 2007; 357 (22): 2277 84. 36 Office for Official Publications of European Communities, Luxemburg, 1997. European Commission, Council Directive 97/43/Euroatom of June 1997 on the health protection of individuals against the dangers of ionizing radiation in relation to medical exposure. Official Journal of the European Communities L180/22 1997. 37 Wall BF, Shrimpton PC. The Historical development of reference doses in diagnostic radiology. Radiat Prot Dosimet 1998; 80 (1 3): 15 9. 38 EC Study Group. Quality Criteria for Diagnostic Radiographic Images. Working Document, 2nd ed, CEC XII/173/90 (Brussels: EC) 1990.