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Estimating the dose delivered to the patient in X-ray computed tomography (CT) examinations is not a trivial task. Monte Carlo (MC) methods appear to be the method of choice to assess the 3D dose distribution. The purpose of this work was to extend an existing MC-based tool to account for arbitrary scanners and scan protocols such as multi-slice CT (MSCT) scanners and to validate the tool in homogeneous and heterogeneous phantoms. The tool was validated by measurements on MSCT scanners for different scan protocols under known conditions. Quantitative CT Dose Index (CTDI) measurements were performed in cylindrical CTDI phantoms and in anthropomorphic thorax phantoms of various sizes; dose profiles were measured with thermoluminescent dosimeters (TLD) in the CTDI phantoms and compared with the computed dose profiles. The in-plane dose distributions were simulated and compared with TLD measurements in an Alderson-Rando phantom. The calculated dose values were generally within 10% of measurements for all phantoms and all investigated conditions. Three-dimensional dose distributions can be accurately calculated with the MC tool for arbitrary scanners and protocols including tube current modulation schemes. The use of the tool has meanwhile also been extended to further scanners and to flat-detector CT. [ABSTRACT FROM AUTHOR]

Objective . Development of a method to provide organ and effective dose coefficients to reference adults for any CT scanner based on values of CTDI measured both in air and in standard CT dosimetry phantoms. Approach . Results from previous Monte Carlo simulations for a range of contemporary CT scanners have been analyzed to provide linear models relating values of organ dose (normalized to CTDI free-in-air ), for each slab of 3 reference phantoms (ICRP Male/Female, and AH hermaphrodite), to similarly normalized values of CTDI in standard CT dosimetry phantoms. Three methods have been investigated to apply the models to values of CTDI for a 'new' scanner not previously simulated: a Generic approach using averaged normalized organ dose profiles for whole body exposure of the phantoms; and two processes for matching the scanner, on the basis of normalized organ doses or effective dose ( n E 103, phan ), to one of the 102 sets of dose coefficients previously calculated for 12 contemporary CT scanner models, from 4 manufacturers, operating under a range of conditions. Main results . The merit of each method has been quantitatively assessed when applied to both the present contemporary scanners with each test data set being excluded in turn during the matching process, and also to 3 previously-simulated older scanners. Whereas all three methods appear viable, with all doses being within 1% and 10% for the contemporary and old scanners respectively, matching to n E 103, phan is overall the approach preferred in practice, yielding an uncertainty of around 6% in estimated values of n E 103, phan . The present methodology also provides superior performance when compared against some other common normalization factors for E 103, phan . Significance . The CT dose model and the data sets will be incorporated into a new CT dosimetry tool that will be made available from UKHSA in support of facilitating improvements in patient protection., (© 2023 Crown Copyright, UK Health Security Agency.)

Objective. Provide analyses of new organ dose coefficients (hereafter also referred to as normalized doses) for CT that have been developed to update the widely-utilized collection of data published 30 years ago in NRPB-SR250. Approach. In order to reflect changes in technology, and also ICRP recommendations concerning use of the computational phantoms adult male (AM) and adult female (AF), 102 series of new Monte Carlo simulations have been performed covering the range of operating conditions for 12 contemporary models of CT scanner from 4 manufacturers. Normalized doses (relative to free air on axis) have been determined for 39 organs, and for every 8 mm or 4.84 mm slab of AM and AF, respectively. Main results. Analyses of results confirm the significant influence (by up to a few tens of percent), on values of normalized organ (or contributions to effective dose ( E 103,phan )), for whole body exposure arising from selection of tube voltage and beam shaping filter. Use of partial (when available) rather than a Full fan beam reduced both organ and effective dose by up to 7%. Normalized doses to AF were larger than corresponding figures for AM by up to 30% for organs and by 10% for E 103,phan . Additional simulations for whole body exposure have also demonstrated that: practical simplifications in the main modelling (point source, single slice thickness, neglect of patient couch and immobility of phantom arms) have sufficiently small (<5%) effect on E 103,phan ; mis-centring of the phantom away from the axis of rotation by 5 mm (in any direction) leads to changes in normalized organ dose and E 103,phan by up to 20% and 6%, respectively; and angular tube current modulation can result in reductions by up to 35% and <15% in normalized organ dose and E 103,phan , respectively, for 100% cosine variation. Significance. These analyses help advance understanding of the influence of operational scanner settings on organ dose coefficients for contemporary CT, in support of improved patient protection. The results will allow the future development of a new dose estimation tool., (© 2022 Crown Copyright, UK Health Security Agency.)

This is the second paper arising from a project concerning the application of Monte Carlo simulations to provide scanner-specific organ dose coefficients for modern CT scanners. The present focus is centred on the bone dosimetry models that have been developed. Simulations have been performed in photon only transport mode, with the assumption of electron equilibrium. This approximation breaks down for doses to active marrow and endosteum since the target cells are localised within tens of micrometre from bone tissue and dose enhancement functions are necessary to correct for the additional dose from photoelectric electrons created in adjacent material. The dose enhancement models used previously in publications NRPB-SR250 (Jones and Shrimpton 1993 Software Report NRPB-SR250, National Radiological Protection Board, Chilton, UK) and ORNL-TM8381 (Cristy and Eckerman 1987 Technical Report Oak Ridge National Laboratory, Oak Ridge, TN) have been implemented and compared with the contemporary approaches of Johnson et al (2011 Phys. Med. Biol. 56 2347-65) and ICRP Publication 116 (ICRP 2010 Ann. ICRP 40 1-257) that are being adopted in the present project. In addition, the calculation of dose to endosteum in the medullary cavity is reviewed and updated using electron mode simulations. For the purposes of quality assurance and comparison, the various dose enhancement functions have been applied in relation to the NRPB18+DJ and HPA18+  stylised hermaphrodite phantoms and also the adult male and female voxel phantoms recommended in ICRP Publication 110 (ICRP 2009 Ann. ICRP 39 1-165), for exposure from three CT scanners modelled previously. Contemporary results for standard examinations on the head and trunk calculated for these latter phantoms demonstrate moderate increases (modal value  +18%) in active marrow dose coefficients relative to values derived from data published in NRPB-SR250. A similar analysis in relation to endosteum dose coefficients shows larger reductions (modal value  -46%), owing at least in part to changes in assumed location of the target cells. Even larger changes are apparent for both of these dose coefficients in relation to examination of the upper legs (-39% and  -94%, respectively). However, resultant changes in any values of effective dose will be less owing to the low weighting factors applied for these tissues.

The ImPACT (imaging performance assessment of CT scanners) CT patient dosimetry calculator is still used world-wide to estimate organ and effective doses (E) for computed tomography (CT) examinations, although the tool is based on Monte Carlo calculations reflecting practice in the early 1990's. Subsequent developments in CT scanners, definitions of E, anthropomorphic phantoms, computers and radiation transport codes, have all fuelled an urgent need for updated organ dose conversion factors for contemporary CT. A new system for such simulations has been developed and satisfactorily tested. Benchmark comparisons of normalised organ doses presently derived for three old scanners (General Electric 9800, Philips Tomoscan LX and Siemens Somatom DRH) are within 5% of published values. Moreover, calculated normalised values of CT Dose Index for these scanners are in reasonable agreement (within measurement and computational uncertainties of  ±6% and  ±1%, respectively) with reported standard measurements. Organ dose coefficients calculated for a contemporary CT scanner (Siemens Somatom Sensation 16) demonstrate potential deviations by up to around 30% from the surrogate values presently assumed (through a scanner matching process) when using the ImPACT CT Dosimetry tool for newer scanners. Also, illustrative estimates of E for some typical examinations and a range of anthropomorphic phantoms demonstrate the significant differences (by some 10's of percent) that can arise when changing from the previously adopted stylised mathematical phantom to the voxel phantoms presently recommended by the International Commission on Radiological Protection (ICRP), and when following the 2007 ICRP recommendations (updated from 1990) concerning tissue weighting factors. Further simulations with the validated dosimetry system will provide updated series of dose coefficients for a wide range of contemporary scanners.

Objective: To investigate the impact of evolving International Commission on Radiological Protection (ICRP) recommendations concerning calculation of effective dose (E) and compare updated typical UK values for common CT examinations with previous data., Methods: Monte Carlo simulations have provided normalized organ doses relating to 15 CT scanner models and 5 virtual reference adults. Series of representative E/dose-length product (DLP) coefficients were derived for common examinations on the separate bases of not only older stylized mathematical phantoms and voxel phantoms presently recommended by ICRP, but also the 1977, 1990 and 2007 formulations for E. Updated E/DLP coefficients were applied to typical values of DLP from the 2011 UK survey., Results: Changes in ICRP recommendations that have arisen from improving evidence on stochastic risk, influence values of E by up to a factor two for CT examinations of the head and neck, although differences for the trunk typically amount to ±10%. Adoption of the voxel rather than the mathematical phantoms used previously can lead to further changes in E by a few tens of percent. Updated typical values of E for UK CT examinations range from 2 to 20 mSv. Increases by 20-400% since 2003 arise not only from increases by 30-160% in typical values of DLP, but also increases by 30-90% in relation to E/DLP coefficients for examinations of the trunk., Conclusion: Values of E, including updated typical data for UK CT, should be compared with caution in relation to their purpose and underlying factors concerning their calculation., Advances in Knowledge: Updated E/DLP coefficients and typical values of E for UK CT, and an appreciation of factors influencing these data.

The radiation risks from a range of medical x-ray examinations (radiography, fluoroscopy, and computed tomography) were assessed as a function of the age and sex of the patient using risk models described in Publication 103 (ICRP, 2007) and UNSCEAR (2006, Annex A). Such estimates of risk based on typical organ doses were compared with those derived from effective doses using the International Commission on Radiological Protection's nominal risk coefficients. Methodologically similar but not identical dose and risk calculations were performed independently at the Institute of Radiation Hygiene (Russia) and the Health Protection Agency (UK), and led to similar conclusions. The radiogenic risk of stochastic health effects following various x-ray procedures varied significantly with the patient's age and sex, but to differing degrees depending on which body organs were irradiated. In general, the risks of radiation-induced stochastic health effects in children are estimated to be higher (by a factor of ≤ 4) than in adults, and risks in senior patients are lower by a factor of ≥ 10 relative to younger people. If risks are assessed on the basis of effective dose, they are underestimated for children of both sexes by a factor of ≤ 4. This approach overestimates risks by a factor of ≤ 3 for adults and about an order of magnitude for senior patients. The significant sex and age dependence of radiogenic risk for different cancer types is an important consideration for radiologists when planning x-ray examinations. Whereas effective dose was not intended to provide a measure of risk associated with such examinations, it may be sufficient to make simple adjustments to the nominal risk per unit effective dose to account for age and sex differences., (Copyright © 2012. Published by Elsevier Ltd.)

Purpose: Tube current modulation (TCM) is one of the recent developments in multislice CT that has proven to reduce the patient radiation dose without affecting the image quality. Presently established methods and published coefficients for estimating organ doses from the dose measured free in air on the axis of rotation or in the CT dose index (CTDI) dosimetry phantoms do not take into account this relatively new development in CT scanner design and technology. Based on these organ dose coefficients effective dose estimates can be made. The estimates are not strictly valid for CT scanning protocols utilizing TCM. In this study, the authors investigated the need to take TCM into account when estimating organ and effective dose values., Methods: A whole-body adult anthropomorphic phantom (Alderson Rando) was scanned with a multislice CT scanner (Somatom Definition, Siemens, Forchheim, Germany) utilizing TCM (CareDose4D). Tube voltage was 120 kV, beam collimation 19.2 mm, and pitch 1. A voxelized patient model was used to define the tissues and organs in the phantom. Tube current values as a function of tube angle were obtained from the raw data for each individual tube rotation of the scan. These values were used together with the Monte Carlo dosimetry tool IMPACTMC (VAMP GmbH, Erlangen, Germany) to calculate organ dose values both with and without account of TCM. Angular and longitudinal modulations were investigated separately. Finally, corresponding effective dose conversion coefficients were determined for both cases according to the updated 2007 recommendations of the ICRP., Results: TCM amplitude was greatest in the shoulder and pelvic regions. Consequently, dose distributions and organ dose values for particular cross sections changed considerably when taking angular modulation into account. The effective dose conversion coefficients were up to 11% lower for a single rotation in the shoulder region and 17% lower in the pelvis when taking angular TCM into account. In the head, neck, thorax, and upper abdominal regions, conversion coefficients changed similarly by only 5% or less. Conversion coefficients for estimating effective doses for scans of complete regions, e.g., chest or abdomen, were approximately 8% lower when taking angular and longitudinal TCMs into account., Conclusions: The authors conclude that for accurate organ and effective dose estimates in individual cross sections in the shoulder or pelvic regions, the angular tube current modulation should be taken into account. In general, using the average of the modulated tube current causes an overestimation of the effective dose.

A review of patient doses from CT examinations in the UK for 2003 has been conducted on the basis of data received from over a quarter of all UK scanners, of which 37% had multislice capability. Questionnaires were employed to collect scan details both for the standard protocols established at each scanner for 12 common types of CT examination on adults and children, and for samples of individual patients. This information was combined with published scanner-specific CT dose index (CTDI) coefficients to estimate values of the standard dose indices CTDI(w) and CTDI(vol) for each scan sequence. Knowledge of each scan length allowed assessment of the dose-length product (DLP) for each examination, from which effective doses were then estimated. When compared with a previous UK survey for 1991, wide variations were still apparent between CT centres in the doses for standard protocols. The mean UK doses for adult patients were in general lower by up to 50% than those for 1991, although doses were slightly higher for multislice (4+) (MSCT) relative to single slice (SSCT) scanners. Values of CTDI(vol) for MSCT were broadly similar to European survey data for 2001. The third quartile values of these dose distributions have been used to derive UK national reference doses for examinations on adults (separately for SSCT and MSCT) and children as initial tools for promoting patient protection. The survey has established the PREDICT (Patient Radiation Exposure and Dose in CT) database as a sustainable national resource for monitoring dose trends in CT through the ongoing collation of further survey data.

Monte Carlo simulations of CT examinations have been performed to estimate effective doses, normalized to axial air kerma, for six mathematical phantoms representing ages from newborn to adult, and for three CT scanner models covering a range of designs. Organ doses were calculated for CT exposures of contiguous, 1 cm wide, transverse slices in each phantom and summed to give normalized effective doses for scans of four regions of the trunk and head. In all cases an inverse trend is observed between normalized effective dose and phantom age, with the dose to the newborn from head and neck scans being 2.2-2.5 times higher than that to the adult, depending on scanner model. Corresponding increases for scans of the trunk region are more variable between scanners and range from a factor of 1.3 to 2.4. If typical clinical exposure conditions for adults are also utilized for children, then, for example, the effective dose to the newborn from a chest scan could be above 15 mSv. It is concluded that CT has the potential to deliver significantly greater radiation doses to children than to adults and in view of their greater susceptibility to radiation effects, special efforts should be made in clinical practice to reduce doses to children by the use of size-specific scan protocols.

The complex conditions of irradiation in computed tomography (CT), involving highly-collimated X-ray beams, necessitate the use of specially-defined dose descriptors such as the computed tomography dose index (CTDI). When used in a weighted form (CTDIW), this concept can describe the absorbed dose from a single slice in standard head and body phantoms. The model can easily be extended to characterise patient exposure for a complete examination by means of the reference dose quantity dose-length product (DLP). Effective dose can also be estimated from DLP, when required.

Surveys in the U.K. have identified the patterns of patient exposure from diagnostic medical radiological procedures and have led to the development of structured advice to promote optimisation of patient protection, including diagnostic reference levels for some common conventional X-ray examinations. Trends for reductions in individual patient and collective doses from these particular procedures have been offset by increasing application of computed tomography. Practice in diagnostic nuclear medicine is conducted by authorized physicians on the basis of recommended maximum usual activities of radiopharmaceutical for specific procedures.

The mean energy imparted to the patient has been proposed as a practical quantity which may be useful for assessing somatic risk in X-ray diagnostic radiology. A technique is described for estimating energy imparted during most common X-ray examinations of the trunk or complete head from a measurement of exposure-area product conveniently provided by the Diamentor transmission ionisation chamber. The necessary energy imparted calibration for the Diamentor has been deduced from consideration of incident spectral energy fluence and Monte Carlo calculations of the fraction of total beam energy imparted to mathematical phantoms representing the patient. The relationship between energy imparted and exposure-area product depends primarily on the applied potential and total filtration of the X-ray beam, and to a lesser extent on voltage waveform and X-ray target angle. Direct measurements of energy imparted to an Alderson Rando phantom for a range of irradiation conditions provided an excellent verification of the technique developed. Using the energy imparted per exposure-area product factors presented, overall uncertainties of less than +/- 15% and +/- 20% should be possible for measurements of energy imparted during examinations of the trunk and head respectively using the Diamentor.

Surveys have been conducted in Britain, France and Italy, using essentially the same techniques, to establish the level of provision of diagnostic radiology services, the frequency of X-ray examinations and examples of the radiation doses delivered to patients in each country. Different national strategies for conducting some types of X-ray examination and marked differences in the general availability of this aspect of health care indicate that the justification and optimization of medical exposures is not interpreted in the same way in these countries.

A collaborative survey between the National Radiological Protection Board and the Hospital Physicists' Association has been conducted to ascertain current levels of exposure for patients undergoing 10 routine types of X-ray examination in England. The main part of this study consisted of measurements on nearly 3200 patients attending 20 randomly selected English hospitals. The energy imparted to each patient was determined from a measurement of the total exposure-area product for the examination. In addition, thermoluminescent dosemeters were attached to the patient's skin to enable the derivation of doses to the major radiosensitive organs, either directly or using appropriate conversion factors calculated for a mathematical phantom by a Monte Carlo technique. Histograms are presented showing the wide distributions often observed in the doses for each type of examination. Mean values of exposure-area product, energy imparted to the patient, entrance skin dose per film and organ dose are reported, together with coefficients of variation. Comparison of the results with those from similar surveys in the UK and abroad is complicated by inconsistencies in the reporting of such data, but substantial differences are sometimes apparent, particularly for the estimates of organ doses. The present measurements will provide a useful baseline for future measurements and will be used to evaluate the collective dose to the population from medical exposures and the radiation risks from the various radiological procedures.