Medical Physics and Informatics Original Research

Medical Physics and Informatics Original Research Christner et al. Estimating Effective Dose for CT Medical Physics and Informatics Original Research FOCUS ON: Jodie A. Christner 1 James M. Kofler Cynthia H. McCollough Christner JA, Kofler JM, McCollough CH Keywords: CT, DLP, dose length product, dual-energy CT, effective dose, effective dose equivalent, ICRP publication 26, ICRP publication 60, ICRP publication 103, organ dose, tissue-weighting factors DOI:10.2214/AJR.09.3462 Received August 10, 2009; accepted after revision October 16, 2009. 1 All authors: Mayo Clinic Rochester, 200 First St., SW, Rochester, MN 55905. Address correspondence to C. H. McCollough (mccollough.cynthia@mayo.edu). CME This article is available for CME credit. See www.arrs.org for more information. AJR 2010; 194:881 889 0361 803X/10/1944 881 American Roentgen Ray Society Estimating Effective Dose for CT Using Dose Length Product Compared With Using Organ Doses: Consequences of Adopting International Commission on Radiological Protection Publication 103 or Dual-Energy Scanning OBJECTIVE. The objective of our study was to compare dose length product (DLP) based estimates of effective dose with organ dose based calculations using tissue-weighting factors from publication 103 of the International Commission on Radiological Protection (ICRP) or dual-energy CT protocols. MATERIALS AND METHODS. Using scanner- and energy-dependent organ dose coefficients, we calculated effective doses for CT examinations of the head, chest, coronary arteries, liver, and abdomen and pelvis using routine clinical single- or dual-energy protocols and tissue-weighting factors published in 1991 in ICRP publication 60 and in 2007 in ICRP publication 103. Effective doses were also generated from the respective DLPs using published conversion coefficients that depend only on body region. For each examination type, the same volume CT dose index was used for single- and dual-energy scans. RESULTS. Effective doses calculated for CT examinations using organ dose estimates and ICRP 103 tissue-weighting factors differed relative to ICRP 60 values by 39% ( 0.5 msv, head), 14% (1 msv, chest), 36% (4 msv, coronary artery), 4% (0.6 msv, liver), and 7% ( 1 msv, abdomen and pelvis). DLP-based estimates of effective dose, which were derived using ICRP 60 based conversion coefficients, were less than organ dose based estimates for ICRP 60 by 4% (head), 23% (chest), 37% (coronary artery), 12% (liver), and 19% (abdomen and pelvis) and for ICRP 103 by 34% (head), 37% (chest), 74% (coronary artery), 16% (liver), and 12% (abdomen and pelvis). All results were energy independent. CONCLUSION. These differences in estimates of effective dose suggest the need to reassess DLP to E conversion coefficients when adopting ICRP 103, particularly for scans over the breast. For the evaluated scanner, DLP to E conversion coefficients were energy independent, but ICRP 60 based conversion coefficients underestimated effective dose relative to organ dose based calculations. E ffective dose (E) is a single parameter meant to reflect the relative risk from exposure to ionizing radiation. It reflects the risk of detrimental biologic effects from a nonuniform, partial-body exposure in terms of a whole-body exposure [1, 2]. The risk coefficients used in calculating effective dose were derived from a cohort that included both sexes and all ages and depended primarily on the excess risk observed in survivors of the Japanese atomic bombings. The values are a broad estimate of risk for an average (thin by today s standards) adult hermaphrodite phantom, which is a fairly unrealistic description of the human body (see Fig. 1). Hence, effective dose is not applicable to any single individual. Nonetheless, it is useful for comparing and optimizing imaging procedures that use ionizing radiation, particularly when comparing examinations from different techniques, such as radiography, CT, and nuclear medicine [3 5]. Background Methods for Calculating Effective Dose In this study, two common methods used to estimate effective dose for a CT examination were compared: first, the gold standard method based on organ dose estimates [6, 7] that explicitly uses tissue-weighting coefficients as specified by the International Commission on Radiological Protection (ICRP) [2, 8, 9]; and, second, the computationally more simple method based on the dose length product (DLP) and a DLP to E conversion coefficient, referred to as k, that depends on only the anatomic region examined [10 15]. AJR:194, April 2010 881

Christner et al. Calculation of Organ Doses Monte Carlo simulations The most complete computational method for estimating organ and tissue doses is based on Monte Carlo simulations [6, 7]. The simulations account for many scanner and technique specifics, including scanner geometry, bow-tie filtration, beam collimation, tube potential, and current as well as the CT dose index (CTDI) [5, 16 18] and the scan length for a given CT examination. The Monte Carlo based organ dose coefficient data used for this study were published in 1991 by Jones and Shrimpton [6, 19]. (Similar data are available from the Institute for Radiation Protection [7].) Jones and Shrimpton used a simulated hermaphroditic patient (MIRD-5 phantom) [20 22] having mathematically modeled organs and tissues (Fig. 1). A D The mathematic phantom was divided from head to mid thigh into 208 axial slabs of 5 mm thickness. Then, accounting for tube voltage and using CT scanner specific data for geometry and beam shaping, they simulated a CT scan and calculated absorbed doses to all organs of the body for the irradiation of each axial slab. Summing contributions from all slabs exposed during a particular CT examination yielded the total organ doses. These doses were normalized by the CTDI 100 measured in air at the gantry isocenter, CTDI air [3, 5, 12]. The resultant data tables were published in 1991 in paper [6] and in 1993 in electronic [19] formats by the National Radiological Protection Board (NRPB) in the United Kingdom. (Starting in 2005, the NRPB became the Radiation Protection Division of the Health Protection Agency, UK.) B E Fig. 1 Scan ranges for studied examinations as shown on mathematic model used for National Radiological Protection Board Monte Carlo simulations. (Figure of adult hermaphrodite mathematical phantom reproduced with kind permission from Shrimpton PC, Jones DG, Hillier MC, Wall BF, Le Heron JC, Faulkner K; U.K. Health Protection Agency. Report NRPB-R249 (1991): Survey of CT practice in the U.K. Part II. Dosimetric aspects. Chilton, United Kingdom: National Radiological Protection Board) A E, Drawings show scan ranges. Assessment of CT scanners (ImPACT) spreadsheet and scanner match By 2000, many new scanners were in use. The Im- PACT Group (UK National Health Service CT Evaluation Centre, part of the Medical Physics Department at St. George s Hospital, London, UK) developed an Excel (Microsoft) spreadsheet to provide a convenient user interface for determining organ doses using the NRPB Monte Carlo generated data sets. (The spreadsheet is available free of charge at www.impactscan.org [23]) They further developed a method [24] to map results from the original 23 scanner data sets to new CT scanners through the use of so-called ImPACT factors ; these factors are based on tube voltage dependent CTDI measurements using a standard 100-mm pencil dosimeter in air and either a standard C 882 AJR:194, April 2010

Estimating Effective Dose for CT head or standard body CTDI dose phantom [12]. The ImPACT Group s spreadsheet can be used to determine organ doses for a wide range of relevant examination parameters [5, 17, 18]: scanner type, tube voltage, tube current, rotation time, head or body scan, and either scan width and increment for sequential (nonhelical) scans or pitch for helical scans. Calculation of Effective Dose Method 1: using organ dose estimates and ICRP 26, 60, or 103 tissue-weighting factors Tissue-weighting factors are meant to represent the relative radiation sensitivity of each type of body tissue as determined from population averages over age and sex and are derived primarily from the atomic bomb survivors cohort [3 5, 8]. For partial-body irradiation, effective dose is the weighted summation of the absorbed dose to each specified organ and tissue multiplied by the ICRP-defined tissue-weighting factor for that same organ or tissue [8]: E = { w T H T }, Z where T is all ICRP-specified tissues and organs, w T is the ICRP-specified tissue-weighting factor, H T is the dose to a particular organ or tissue, the inside summation is over all T tissues, and the outside summation is over Z all irradiated slabs. Since 1977, three different sets of tissueweighting factors (Table 1 and Fig. 2) have been defined in publications by the ICRP: ICRP 26, published in 1977 [2]; ICRP 60, in 1991 [8]; and ICRP 103, in 2007 [9]. These revisions were intended to reflect advances in knowledge about the radiation sensitivity of various organs and tissues. In ICRP 26, the term effective dose equivalent was used. In ICRP 60, the name of the summed quantity was changed Fig. 2 Bar graph shows tissue-weighting factors specified by International Commission on Radiological Protection (ICRP) publications 26, 60, and 103. T TABLE 1: Tissue-Weighting Factors for International Commission on Radiological Protection (ICRP) Publications 26, 60, and 103 Tissue or Organ to effective dose in addition to changes to the tissue-weighting factors. (Throughout this article, we use the term effective dose, or E, to mean effective dose equivalent when referring to calculations based on ICRP 26 tissueweighting factors.) Although ICRP 103 assigns different tissue-weighting factors for several primary organs, it retains the name, effective dose. In addition, the three ICRP recommendations differ somewhat in calculation methodology. For example, in ICRP 26, organ doses are defined by a single-point dose in the organ of interest, whereas in ICRP 60, the mean organ dose is to be used. With each publication, the trend has been to specify weighting factors Publication ICRP 26 ICRP 60 ICRP 103 Gonads 0.25 0.20 0.08 Red bone marrow 0.12 0.12 0.12 Lung 0.12 0.12 0.12 Colon 0.12 0.12 Stomach 0.12 0.12 Breast 0.15 0.05 0.12 Bladder 0.05 0.04 Liver 0.05 0.04 Esophagus 0.05 0.04 Thyroid 0.03 0.05 0.04 Skin 0.01 0.01 Bone surface 0.03 0.01 0.01 Brain 0.01 Salivary glands 0.01 Remainder 0.30 0.05 0.12 Total 1.00 1.00 1.00 Tissue-Weighting Factors 0.30 0.25 0.20 0.15 0.10 0.05 0.00 ICRP 26 (1977) ICRP 60 (1991) ICRP 103 (2007) for an increasing number of organs and tissues, which decreased the weighting of remainder tissues. An example is the brain, which was treated as one of the remainder organs in ICRP 26 and ICRP 60. The brain was first listed as a primary organ in ICRP 103. Over time, weighting of specific tissues has also changed. For example, the weighting of the gonads has decreased in each subsequent publication. However, for the breast, weighting was decreased in ICRP 60 but then increased in ICRP 103. As a result of the changes, the estimates of effective dose for the exact same CT examination can differ substantially depending on which ICRP report was used. Gonads Red bone marrow Lung Colon Stomach Breast Bladder Liver Esophagus Thyroid Skin Bone surface Brain Salivary glands Remainder Specific Organs and Tissues AJR:194, April 2010 883

Christner et al. TABLE 2: Published DLP to E k Conversion Coefficients a Method 2: Using DLP and k Coefficients From the European Guidelines DLP is defined as the product of the volume CTDI and the irradicted scan length. DLP = CTDI vol irradiated length, where CTDI vol is the volume CTDI [11, 12]. Comparing effective dose values estimated from DLP for a wide range of scanner models with effective dose values derived from NRPB organ dose calculations and ICRP 60 tissue-weighting coefficients, a linear relationship was found [11] when data sets were restricted to the same anatomic region (e.g., head, neck, chest, and abdomen and pelvis). This led the European Commission to present in 2000 [12] a generic method to quickly estimate effective dose from CT examinations, with updates published in 2004 and 2005 [10, 13, 14]. By this widely used shortcut method, effective dose is calculated as follows: E = k DLP, DLP to E k Conversion Coefficients [msv / (mgy cm)] Jessen et al., [11] (1999) EC [12] (2000) EC Appendix B [10] (2004) where the k coefficient (Table 2) is specific only to the anatomic region scanned. Deviations in estimates of effective dose of ± 15% have been reported using this method relative to the gold standard organ dose based technique for CT scans obtained at 120 kv [25]. In helical CT, this calculation method is apt to underestimate E when DLP is calculated with only the CTDI vol and the prescribed scan range because the irradiated length typically exceeds the prescribed scan length [26, 27]. Because of the widespread use of this method, most manufacturers of CT scanners now compute and display DLP, taking into account the entire irradiated length rather than the lesser prescribed scan length. In spite of these sources of variation in the calculation of effective dose, effective dose is widely used by the academic, clinical, and manufacturing communities. Therefore, the purpose of this investigation was to determine how well estimates of E calculated using DLP agree with calculations based on organ dose estimates after adopting the revised tissueweighting factors of ICRP 103 or when using tube potential values other than 120 kv, such as for dual-energy CT protocols. Materials and Methods Evaluated CT Examinations Clinical protocols (Table 3) were investigated for head, chest, coronary artery, liver, and abdomen and pelvis CT examinations using typical scan ranges scaled to the NRPB mathematic patient model (Fig. 1). All single-energy (120-kV) protocols had a counterpart in which two tube energies were used, neither being 120 kv. Organ doses and DLP were calculated using our clinical technique parameters, where the CTDI vol of the single-energy examination matched that of the corresponding dual-energy examination. For helical coronary artery examinations, ECG gating is required [28, 29]. Because data are typically not reconstructed at all parts of the cardiac cycle, patient dose can be reduced by decreasing the x-ray tube current during unused parts of the cycle. For this study, we assumed a heart rate of 70 beats per minute and an image reconstruction window that resulted in a 30% reduction in irradiation. Evaluated Scanner The CT protocols investigated were in clinical use on our dual-source CT scanners (SOMATOM TABLE 3: Clinical Scan Parameters for the CT Examinations Studied EC Appendix C [13] (2004) and NRPB-W67 [14] (2005) Scan Type Tube Potential (kv) Tube Current Effective mas a mas / rotation Collimation (mm) Pitch Head SE 120 400 2 32 0.6 0.7 1 Rotation Time (s) DE 140 150 2 32 0.6 0.5 0.5 80 500 2 32 0.6 0.5 0.5 Chest SE 120 250 24 1.2 0.5 0.5 DE 140 80 14 1.2 0.7 0.5 80 340 14 1.2 0.7 0.5 Coronary arteries SE 120 240 2 32 0.6 0.22 0.33 DE 140 90 2 32 0.6 0.22 0.33 100 180 2 32 0.6 0.22 0.33 Liver SE 120 450 2 32 0.6 0.7 0.5 DE 140 190 14 1.2 0.5 0.5 80 556 14 1.2 0.5 0.5 Abdomen and pelvis SE 120 240 2 32 0.6 1.2 0.5 Note SE = single energy, DE = dual energy. a Effective mas = mas / pitch. Phantom (cm) Head 0.0021 0.0023 0.0023 0.0021 16 Head and neck 0.0031 16 Neck 0.0048 0.0054 0.0059 32 Chest 0.014 0.017 0.018 0.014 32 Abdomen 0.012 0.015 0.017 0.015 32 Pelvis 0.019 0.019 0.017 0.015 32 Chest, abdomen, and pelvis 0.015 32 Note EC = European Commission, NRPB = National Radiological Protection Board. a E = k DLP, where DLP = dose length product. The phantom size is specified for the volume CT dose index measurements on which DLP is based. DE 140 90 14 1.2 0.6 0.5 80 382 14 1.2 0.6 0.5 884 AJR:194, April 2010

Estimating Effective Dose for CT Definition DS, Siemens Healthcare) at the time of the study. The scanner s so-called z-flying focal spot technique [30] was used for collimations described with the prefix 2 ; for example, 2 32 0.6 mm describes a scanner mode that uses the z-flying focal spot technique to collect two interleaved data sets for a detector having a physical size of 32 rows, each 0.6 mm in length. Measured CTDI values used for scanner matching, through the use of ImPACT factors, were an average over all available quality control data for three identical scanners at our institution. Data were collected over more than 2 years and included, on average, 14 measurements for each set of scan parameters. Specifically, quality control data for CTDI air, CTDI center, and CTDI edge were used for 2 32 0.6 mm collimation at all energies studied (80, 100, 120, and 140 kv) and for collimations of 14 1.2 and 24 1.2 mm at 120 kv. The remaining requisite dose data for collimations of 14 1.2 and 24 1.2 mm at 80, 100, and 140 kv were measured using our standard technique [17, 31] on one scanner. Effective Dose 4.0 3.0 2.0 1.0 0.0 E 26 Fig. 3 Comparison of effective dose (E) estimates by anatomic region according to recommendations of International Commission on Radiological Protection (ICRP) publications 26, 60, or 103 or calculated using dose length product (DLP) and k coefficients. SE = single energy, DE = dual energy. A E, Bar graphs show data for head (A), chest (B), coronary CT angiogram (C), liver (D), and abdomen and pelvis (E) examinations. A Effective Dose Calculation Based on Organ Doses The ImPACT Group s Excel spreadsheet (version 0.99x [23]) was used to calculate organ doses based on the NRPB Monte Carlo data sets [19]. Because data for the studied scanner were not reported by the ImPACT Group, we performed the needed scanner matching according to their method [24] using measurements from our scanners. Data entered into the dose calculator spreadsheet for each examination (Table 4) included clinical CT scan parameters and CTDI air. Additionally, the weighted CTDI 100 defined as n CTDI w = 1/3 CT- DI center + 2/3 CTDI edge [12, 32] and normalized to 100 mas and tube current time product, which is the tube current (in milliamperes) the gantry rotation time (in seconds), were entered to allow the spreadsheet to calculate the CTDI vol for the described examination. Comparison of the spreadsheet-calculated CTDI vol with the scanner-reported CTDI vol served as a verification of correct data entry. After the organ dose calculations, a modified version of the ImPACT Group s spreadsheet was used, along with the tissue-weighting coefficients and calculation methodology specified in ICRP publications 26, 60, or 103 to yield an estimate of effective dose (or effective dose equivalent): E 26, or, respectively. For a dual-energy examination, effective dose was the sum of E values for the lowand high-kv scans. Because use of the dose calculator method results in identical organ doses for any given protocol (e.g., kv, scan region, mas), any differences in the calculated values of effective dose for E 26, and resulted from the use of the different sets of tissue-weighting factors and calculation rules from the three ICRP publications. TABLE 4: Input Parameters for the ImPACT Dose Calculator [23] Spreadsheet Tube Potential (kv) Monte Carlo Set No. a Scan Start b (cm) Scan End b (cm) Scan Length b (cm) CTDI air (mgy) n CTDI w c (mgy) Head 120 20 80.5 94.0 13.5 19.1 12.5 140 20 80.5 94.0 13.5 27.6 18.5 80 9 80.5 94.0 13.5 6.4 3.8 Chest 120 15 42.0 69.5 27.5 19.0 6.2 140 23 42.0 69.5 27.5 27.0 9.6 80 5 42.0 69.5 27.5 7.0 2.0 Coronary arteries 120 20 43.0 55.5 12.5 19.1 5.4 140 22 43.0 55.5 12.5 27.0 9.6 100 5 43.0 55.5 12.5 7.0 2.0 Liver 120 19 19.5 46.0 26.5 18.6 6.4 140 23 19.5 46.0 26.5 27.0 9.6 80 5 19.5 46.0 26.5 7.0 2.0 Abdomen and pelvis 120 19 0.5 46.0 45.5 18.6 6.4 140 23 0.5 46.0 45.5 27.0 9.6 80 5 0.5 46.0 45.5 7.0 2.0 a Resulting from scanner matching, as described in the text. b Scan start, scan end, and scan length refer to the mathematic phantom used for National Radiological Protection Board Monte Carlo calculations as shown in Figure 1. c n CTDI w is the weighted CTDI 100, normalized to 100 mas, as defined in the text. Effective Dose Effective Dose 10 8 6 4 2 0 20 15 10 5 0 E 26 E 26 B D Effective Dose Effective Dose 20 15 10 5 0 15 10 5 0.0 E 26 E 26 C E AJR:194, April 2010 885

TABLE 5: Organ Dose Based Effective Dose (E) Estimates Christner and et Comparisons al. for International Commission on Radiological Protection (ICRP) Publications 26, 60, and 103 Scan Type Effective Dose Calculated From DLP DLP for each CT examination was also calculated using the ImPACT spreadsheet. From this result, we determined an estimate for as the product of DLP and the body region appropriate DLP to E conversion coefficient, k (Table 2). The values used for k were the most recently reported values, published in 2004 [13]. They were based on ICRP 60 and are 0.0021 msv/mgy cm (head), 0.014 msv/ mgy cm (chest), and 0.015 msv/mgy cm (abdomen and pelvis). The estimate of effective dose for a dual-energy examination was the sum of E values for the two energies. Energy Dependence of Effective Dose To assess the influence of CT tube potential (80, 100, 120, or 140 kv) on estimations of effective dose, the values for, and at each tube potential were normalized to CTDI vol to obtain, and / CT- DI vol, respectively. For each examination type, we computed the coefficient of variation as a function of energy for the normalized effective dose values ( or ), where [coefficient of variation = SD / mean]. These coefficients of variation were used to quantitate any energy dependence of the organ dose calculations. Tube Potential (kv) E 26 Range of E a Range of E a / Comparative Assessments Changes from E 26 to were normalized to E 26 and changes from to were normalized to to show the step-wise changes in risk estimates due to changes in the definition of effective dose. Having no knowledge of the true value of E, because it is a nonphysical mathematic construct that cannot be measured, all other normalizations were performed relative to, because ICRP 60 s definitions were used to determine the current k coefficients. Results Comparison of Effective Dose Based on Organ Doses Between ICRP 26, 60, or 103 The relative values of effective dose calculated using ICRP 26, 60, or 103 tissue-weighting factors and organ dose estimates based on Monte Carlo simulations depended on the body region examined (Table 5 and Fig. 3). The range that is, the absolute value (maximum minimum) of the three E values (E 26, and ) was ~ 2 msv (head), 2 msv (chest), 6 7 msv (coronary arteries), 3 msv (liver), and 1 msv (abdomen and pelvis) corresponding, respectively, to 155% (head), 29% (chest), 53% (coronary arteries), 25% (liver), and 7% (abdomen and pelvis) of the values. E 26 ( E 26 ) / E 26 ( ) / Head SE 120 3.0 1.4 0.9 2.2 155 1.6 54 0.5 39 DE 140 1.7 0.8 0.5 1.2 155 0.9 54 0.3 39 80 1.3 0.6 0.4 0.9 153 0.7 53 0.2 39 DE total 3.0 1.4 0.9 2.1 154 1.6 54 0.5 39 Chest SE 120 9.3 7.2 8.2 2.1 29 2.1 23 1.0 14 DE 140 4.4 3.5 3.9 1.0 28 1.0 22 0.5 14 80 4.3 3.2 3.8 1.1 36 1.1 26 0.6 18 DE total 8.8 6.6 7.7 2.1 32 2.1 24 1.0 16 Coronary arteries SE 120 18.0 12.0 16.0 6.2 53 6.2 35 4.2 36 DE 140 10.0 6.5 8.9 3.6 56 3.6 36 2.5 38 100 8.3 5.1 7.3 3.3 64 3.3 39 2.2 43 DE total 18.0 12.0 16.0 6.9 59 6.9 37 4.6 40 Liver SE 120 16.6 13.3 13.8 3.3 25 3.3 20 0.6 4 DE 140 10.7 8.6 8.9 2.2 25 2.2 20 0.4 4 80 5.6 4.5 4.6 1.1 25 1.1 20 0.2 4 DE total 16.3 13.0 13.6 3.3 25 3.3 20 0.6 4 Abdomen and pelvis SE 120 13.3 13.7 12.7 1.0 7 0.4 3 1.0 7 DE 140 7.2 7.5 6.9 0.5 7 0.2 3 0.5 7 Note SE = single energy, DE = dual energy. a Range of E = maximum E minus minimum E for the set E 26,. 80 5.3 5.5 5.1 0.4 7 0.2 3 0.4 7 DE total 12.5 12.9 12.0 0.9 7 0.4 3 0.9 7 For four of the five investigated examinations, E 26 was the largest. The exception was the abdomen and pelvis examination, where there was a relatively small (7%) difference between effective dose estimates. was the smallest of the three estimates of effective dose in three of the five investigated examinations. In changing from ICRP 60 to ICRP 103 tissue-weighting factors, effective dose estimates were least affected for liver (0.6 msv, 4% increase) and abdomen and pelvis ( 1 msv, 7% decrease) examinations. For CT of the head, effective dose decreased by 0.5 msv, or 39%, because the weighting of the brain was reduced in ICRP 103 to 0.01, compared with that for ICRP 60, where half of the remainder weighting factor (0.025) was applied. The largest changes were an increase of 14% for chest (1-mSv increase) and 36% for coronary artery (4-mSv increase) examinations due to the increase in the breast tissue-weighting factors from 0.05 to 0.12. Comparison of Effective Dose Based on Organ Doses or DLP Results were compared for effective dose calculated from organ dose estimates and tissueweighting factors versus DLP and k coefficients 886 AJR:194, April 2010

Estimating Effective Dose for CT (Table 6). underestimated E for all investigated examinations relative to the organ-based calculation of. The percentage differences [100% ( ) / ] for the 120-kV single-energy examination were 4% (head), 23% (chest), 37% (coronary arteries), 12% (liver), and 19% (abdomen and pelvis). Likewise, underestimated E compared with the organ-based calculations of except for the head examination, where it was nearly equivalent (absolute increase of 0.5 msv, relative increase of 34%). The largest differences ( ) were found for the chest ( 2.6 msv, 37%) and coronary artery ( 8.6 msv, 74%) examinations. The per DLP ratio that is, k values for ICRP 103 were computed for each examination (Table 6, see Discussion). Comparison of Effective Dose for Single- or Dual-Energy Protocols Organ dose based estimates of effective dose for single-energy and dual-energy examinations were virtually the same with an observed difference of no more than 0.8 msv, or 8% (Table 5), thus confirming that the experimental design goal was achieved. For dual-energy examinations, effective dose of the high-energy and low-energy components of the examination were split almost equally for head and chest regions, differed by about 20% for coronary artery and abdomen and pelvis regions, and differed nearly 50% for the liver examination. Energy Dependance of Effective Dose The k values for ICRP 103 (Table 6, see Discussion) computed for each examination were energy independent. Normalized values for per CTDI vol (Table 7) were 0.028 msv/mgy (head), 0.39 msv/mgy (chest), 0.18 msv/mgy (coronary arteries), 0.40 msv/mgy (liver), and 0.68 msv/mgy (abdomen and pelvis). These values must be energy independent because each of the input parameters used to compute (CTDI vol, DLP, and the DLP to E (k) conversion coefficients) were fixed, independent of energy (tube potential). For each examination type, the coefficients of variation for and values, which were based on energy-dependent organ-dose calculations, were within 1% of each other and were approximately 3 4% (head), 5 6% (chest), 1 3% (coronary arteries), 7 8% (liver), and 10% (abdomen and pelvis). Thus, for the same total CTDI vol, TABLE 6: Dose Length Product (DLP) Based Effective Dose (E) Estimates and Comparisons for International Commission on Radiological Protection (ICRP) Publications 26, 60, and 103 Tube Potential (kv) CDTI vol (mgy) DLP (mgy cm) k value a / (mgy cm) = k DLP ( ) / ( ) / ( ) / = / DLP / (mgy cm) Head 120 46.9 633 0.0021 1.3 0.01 4 0.5 34 56 0.0013 140 27.8 375 0.0021 0.8 0.00 2 0.3 37 61 0.0013 80 19.1 258 0.0021 0.5 0.01 8 0.2 31 51 0.0014 DE total 46.9 633 0.0021 1.3 0.01 4 0.5 34 56 0.0013 DE SE 0.0 0 Chest 120 14.5 399 0.014 5.6 1.6 23 2.6 37 32 0.021 140 7.7 211 0.014 3.0 0.5 14 1.0 28 25 0.019 80 6.8 187 0.014 2.6 0.6 18 1.1 36 30 0.020 DE total 14.5 398 0.014 5.6 1.1 16 2.1 32 27 0.019 DE SE 0.0 1 Coronary arteries 120 41.5 519 0.014 7.3 4.3 37 8.6 74 54 0.031 140 23.3 291 0.014 4.1 2.4 37 4.8 75 54 0.031 100 18.2 227 0.014 3.2 1.9 38 4.1 81 56 0.032 DE total 41.5 518 0.014 7.3 4.3 37 8.9 77 55 0.031 DE SE 0.0 2 Liver 120 29.3 777 0.015 12.0 1.6 12 2.2 16 16 0.018 140 18.3 484 0.015 7.3 1.3 15 1.7 20 19 0.018 80 11.0 295 0.015 4.4 0.0 1 0.2 4 4 0.016 DE total 29.3 779 0.015 12.0 1.3 10 1.9 14 14 0.017 DE SE 0.0 2 Abdomen and pelvis 120 16.2 737 0.015 11.0 2.6 19 1.7 12 13 0.017 140 8.6 394 0.015 5.9 1.6 21 1.0 14 15 0.018 80 7.6 344 0.015 5.2 0.3 5 0.1 1 2 0.015 DE total 16.2 738 0.015 11.0 1.8 14 0.9 7 8 0.016 DE SE 0.0 1 Note SE = single energy, DE = dual energy. a Based on ICRP 60 [8] as reported by Shrimpton [13, 14]. AJR:194, April 2010 887

Christner et al. TABLE 7: Energy Dependence of, and Normalized to CTDI vol Coefficient of Variation b Tube Potential (kv) CDTI vol (mgy) the tube potential has a minimal effect on estimates of effective dose. (msv / mgy) (msv / mgy) Discussion The results of this study reinforce the fact that effective dose is a derived parameter. It is always computed through multiple steps and approximations. Depending on which set of tissue-weighting coefficients are used, E values may vary substantially. Effective dose is meant as an estimate of relative biologic risk [9] and is not a physical parameter that can be measured., although based on ICRP 60, underestimates (based on organ doses) for the studied scanner. This finding is not surprising because k is based on data averaged over many scanner makes and models and is therefore not specific to this scanner. However, generating many specialized k values is not consistent with the definition of effective dose and its intended use. Adopting ICRP 103 but retaining the current k coefficients would further increase the underestimations of for chest, coronary artery, and abdomen examinations compared with E values derived from organ doses. Hence, to use the DLP-based method of estimating E for ICRP 103 weighting factors requires a comprehensive analysis of a broad variety of scanners to estimate the mean values for DLP to E conversion coefficients. To account for the increased sensitivity assigned to breast tissue by ICRP 103 over ICRP 60, it may be useful to introduce distinct coefficients for cardiac examinations. Martin [4] has reported the inherent relative uncertainties in estimating effective dose (using organ doses) to a reference patient to be about ± 40%. He further reminds readers that because E has been defined by the ICRP [9] as a single parameter to reflect overall risk averaged over all ages and both sexes for a reference patient, neither the Monte Carlo based organ dose coefficients nor the DLP-based k values [13] should be used to calculate E estimates for individual patients [33]. In general, the accuracy of E values derived from Monte Carlo calculations cannot be substantially improved by measuring organ doses using an anthropomorphic phantom and radiation measurement devices because experimental uncertainties will exist, including variations in the physical anthropomorphic phantom used. Further, Monte Carlo methods to estimate absorbed dose have been shown to be extremely accurate (e.g., they are used in radiation therapy treatment planning). Martin s [4] analysis of ± 40% uncertainties in E estimates applies in either case, with Monte Carlo methods being more standardized and reproducible. The results (msv / mgy) Head 120 46.9 0.028 0.030 0.018 4 3 140 27.8 0.028 0.029 0.018 80 19.0 0.028 0.031 0.019 Chest 120 14.5 0.39 0.50 0.57 5 6 140 7.7 0.38 0.45 0.51 80 6.8 0.39 0.47 0.55 Coronary arteries 120 41.5 0.18 0.28 0.38 1 3 140 23.3 0.17 0.28 0.38 100 18.2 0.17 0.28 0.30 Liver 120 29.3 0.40 0.45 0.47 7 8 140 18.3 0.40 0.47 0.49 80 11.0 0.40 0.40 0.42 Abdomen and pelvis 120 16.2 0.68 0.85 0.79 10 10 140 8.6 0.69 0.87 0.81 80 7.6 0.68 0.72 0.67 a k based on ICRP 60 [8]. b Coefficient of variation = 100% (SD) / mean. The coefficient of variation for a given examination indicates any energy dependence. of this study show that estimating effective dose from DLP works about equally well for dual-energy CT as it does for single-energy CT. A limitation of this work is that only one scanner was studied. Additionally, a circa-1990 scanner with similar dose characteristics was used as a surrogate for the studied scanner. However, until Monte Carlo organ dose coefficient values for newer scanners are available, scanner matching to an older scanner model is the only available option. Finally, use of the simplified and relatively small anthropomorphic model makes these estimates of effective dose applicable to scans of a small adult. With today s continued increase in the number of overweight and obese patients, the calculated values of E should be used with caution. They apply to the CT examination generally and cannot be used for any one individual, particularly persons who have a marked difference in body habitus compared with the MIRD phantom. In conclusion, the use of ICRP 103 tissueweighting factors in place of ICRP 60 factors decreased organ dose based estimates of effective dose for CT examinations of the abdomen and pelvis by 7% and head by 39%, but increased estimates of effective dose for scans of the liver by 4%, chest by 14%, and coronary arteries by 36%. The absolute value of 888 AJR:194, April 2010

Estimating Effective Dose for CT the largest change was 4 msv for the coronary artery examination. These changes primarily reflect the increased tissue-weighting factor for breast tissue together with decreased factors for the gonads and brain. For the evaluated CT scanner and examinations,, although based on ICRP 60, underestimated E relative to E60 by 4 37% and relative to E103 by up to 74%. These results were essentially independent of tube potential, suggesting that estimates of E based on DLP work equally well for single-energy and dual-energy CT examinations. References 1. Jacobi W. The concept of effective dose: a proposal for the combination of organ doses. Radiat Environ Biophys 1975; 12:101 109 2. International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. Oxford, UK: Pergamon Press, 1977:ICRP publication no. 26 3. McCollough CH, Schueler BA. Calculation of effective dose. Med Phys 2000; 27:828 837 4. 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