Development of a generalized method to allow the estimation of doses to the ICRP reference adults from CT, on the basis of normalized organ and CTDI dose data determined by Monte Carlo calculation for a range of contemporary scanners.

Objective. Development of a method to provide organ and effective dose coefficients to reference adults for any CT scanner based on values ofCTDImeasured 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 toCTDIfree-in-air), for each slab of 3 reference phantoms (ICRP Male/Female, and AH hermaphrodite), to similarly normalized values ofCTDIin standard CT dosimetry phantoms. Three methods have been investigated to apply the models to values ofCTDIfor 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 (nE103,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 tonE103,phanis overall the approach preferred in practice, yielding an uncertainty of around 6% in estimated values ofnE103,phan. The present methodology also provides superior performance when compared against some other common normalization factors forE103,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.

CT scanner-specific organ dose coefficients generated by Monte Carlo calculation for the ICRP adult male and female reference computational phantoms.

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 (E103,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% forE103,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 onE103,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 andE103,phanby 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 andE103,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.

Selection of bone dosimetry models for application in Monte Carlo simulations to provide CT scanner-specific organ dose coefficients.

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.

Development of Monte Carlo simulations to provide scanner-specific organ dose coefficients for contemporary CT.

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.

Updated estimates of typical effective doses for common CT examinations in the UK following the 2011 national review.

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 effect of angular and longitudinal tube current modulations on the estimation of organ and effective doses in x-ray computed tomography.

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.

Validation of a Monte Carlo tool for patient-specific dose simulations in multi-slice computed tomography.

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.