Attenuation-based size metric for estimating organ dose to patients undergoing tube current modulated CT exams

Pathology-based radiation dose in computed tomography: investigation of the effect of lung lesions on water-equivalent diameter, CTDIVol and SSDE in COVID-19 patients

Lung lesions can increase the CT number and affect the water-equivalent diameter (Dw), Dw-based conversion factor (CFw), and Dw-based size-specific dose estimate (SSDEw). We evaluated the effect of COVID-19 lesions and total severity score (TSS) on radiation dose considering the effect of automatic tube current modulation (ATCM) and fixed tube current (FTC). A total of 186 chest CT scans were categorised into five TSS groups, including healthy, minimal, mild, moderate and severe. The effective diameter (Deff), Dw, CFw, Deff-based conversion factor (CFeff), volume computed tomography dose index (CTDIVol), pathological dose impact factor (PDIF) 1 and SSDEw were calculated. TSS was correlated with Dw (r = 0.29, p-value = 0.001), CTDIVol (ATCM) (r = 0.23, p = 0.001) and PDIF (r = - 0.51, p-value = 0.001). $\overline{{\mathrm{SSDE}}_{\mathrm{w}}}$ (FTC) was significantly different among all groups. $\overline{{\mathrm{SSDE}}_{\mathrm{w}}}$ (ATCM) was greater for moderate (13%) and mild (14%) groups. Increasing TSS increase the Dw and causes a decrease in CFw and $\overline{{\mathrm{SSDE}}_{\mathrm{w}}}$ (FTC), and can increase $\overline{{\mathrm{SSDE}}_{\mathrm{w}}}$ (ATCM) in some Dw ranges.

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 (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% forE_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 onE_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 andE_103,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 andE_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.

AUTOMATED ESTIMATION OF PATIENT'S SIZE USING AUTOWED TOOL AND INDOSECT PROGRAM: A DOSIMETRIC STUDY FOR PAEDIATRIC HEAD CT EXAMINATIONS

Size-specific dose estimate (SSDE), which can be calculated by measuring the effective diameter (De) or water equivalent diameter (Dw) of the patient, is one of the recent approaches for verifying the individual doses during computer tomography (CT) examinations. This work aimed to compare the Dw estimated by the AutoWED tool and IndoseCT software and to investigate CT axial (ARH) and paediatric head (PH) protocols used in southern Saudi Arabia to calculate the dose received by paediatric patients using metrics of volume CT dose index (CTDIvol) and SSDE. The distribution between the ARH and PH protocols was 57.8 and 42.2%, respectively. There was no significant difference in Dw values between the AutoWED tool and the IndoseCT program (0.13%). Including CT table or other objects during estimation of Dw can lead to variation up to 11.4%. The impact of selecting IndoseCT options to identify the border of the patient may be part of the explanation for these variations. A strong linear relationship was obtained between De and Dw in paediatric head size (R2 = 0.96). Using IndoseCT, for 0-1.5, 1.5-5 and 5.0-18 age groups (years), the Dw was found to be 13.2, 15.3 and 16.8 cm, respectively. The SSDE for the PH protocol was substantially lower than that of the ARH protocol. As a result, education of the individuals engaging in paediatric CT examinations is necessary for dose optimization.

Simplified approach to estimation of organ absorbed doses for patients undergoing abdomen and pelvis CT examination

The volumetric computed tomography (CT) dose index (CTDI_vol) is the measure of output displayed on CT consoles relating to dose within a standard phantom. This gives a false impression of doses levels within the tissues of smaller patients in Southeast Asia. A size-specific dose estimate (SSDE) can be calculated from the CTDI_volto provide an assessment of doses at specific positions within a scan using size-specific conversion factors. SSDE is derived using the water equivalent diameter (D_w) of the patient, but calculation ofD_wrequires sophisticated computer software. This study aimed to evaluate relationships betweenD_Wand effective diameter (D_Eff), which can be measured more readily, in order to estimate SSDE at various positions within a routine clinical abdomen and pelvis CT examination for Thai patients. An in-house ImageJ algorithm was developed to measureD_w, effective diameter (D_Eff), and SSDE on CT slices located at the heart, liver, kidneys, colon, and bladder, on 181 CT examinations of abdomen and pelvis. Relationships betweenD_EffandD_wwere determined, and values of organ absorbed dose usingD_Effwere estimated. This approach was validated using a second cohort of 54 patients scanned on a different CT scanner. The results revealed that ratios betweenD_EffandD_wat the heart level were 1.11-1.13 and those for the others were about 1.00. Additionally, the SSDE/CTDI_volratio was estimated for each organ in terms of exponential functions using the relationships betweenD_wandD_Efffor individual organs. In summary, this study proposed a simple method for estimation of organ absorbed doses for Southeast Asian patients undergoing abdomen and pelvis CT examinations where sophisticated computer software is not available.

Estimating Specific Patient Organ Dose for Chest CT Examinations with Monte Carlo Method

Purpose: The purpose of this study was to preliminarily estimate patient-specific organ doses in chest CT examinations for Chinese adults, and to investigate the effect of patient size on organ doses. Methods: By considering the body-size and body-build effects on the organ doses and taking the mid-chest water equivalent diameter (WED) as a body-size indicator, the chest scan images of 18 Chinese adults were acquired on a multi-detector CT to generate the regional voxel models. For each patient, the lungs, heart, and breasts (glandular breast tissues for both breasts) were segmented, and other organs were semi-automated segmented based on their HU values. The CT scanner and patient models simulated by MCNPX 2.4.0 software (Los Alamos National LaboratoryLos Alamos, USA) were used to calculate lung, breast, and heart doses. CTDIvol values were used to normalize simulated organ doses, and the exponential estimation model between the normalized organ dose and WED was investigated. Results: Among the 18 patients in this study, the simulated doses of lung, heart, and breast were 18.15 ± 2.69 mGy, 18.68 ± 2.87 mGy, and 16.11 ± 3.08 mGy, respectively. Larger patients received higher organ doses than smaller ones due to the higher tube current used. The ratios of lung, heart, and breast doses to the CTDIvol were 1.48 ± 0.22, 1.54 ± 0.20, and 1.41 ± 0.13, respectively. The normalized organ doses of all the three organs decreased with the increase in WED, and the normalized doses decreased more obviously in the lung and the heart than that in the breasts. Conclusions: The output of CT scanner under ATCM is positively related to the attenuation of patients, larger-size patients receive higher organ doses. The organ dose normalized by CTDIvol was negatively correlated with patient size. The organ doses could be estimated by using the indicated CTDIvol combined with the estimated WED.

COPD Imaging on a 3rd Generation Dual-Source CT: Acquisition of Paired Inspiratory-Expiratory Chest Scans at an Overall Reduced Radiation Risk

As stated by the Fleischner Society, an additional computed tomography (CT) scan in expiration is beneficial in patients with chronic obstructive pulmonary disease (COPD). It was thus the aim of this study to evaluate the radiation risk of a state-of-the-art paired inspiratory-expiratory chest scan compared to inspiration-only examinations. Radiation doses to 28 organs were determined for 824 COPD patients undergoing routine chest examinations at three different CT systems-a conventional multi-slice CT (MSCT), a 2nd generation (2nd-DSCT), and 3rd generation dual-source CT (3rd-DSCT). Patients examined at the 3rd-DSCT received a paired inspiratory-expiratory scan. Organ doses, effective doses, and lifetime attributable cancer risks (LAR) were calculated. All organ and effective doses were significantly lower for the paired inspiratory-expiratory protocol (effective doses: 4.3 ± 1.5 mSv (MSCT), 3.0 ± 1.2 mSv (2nd-DSCT), and 2.0 ± 0.8 mSv (3rd-DSCT)). Accordingly, LAR was lowest for the paired protocol with an estimate of 0.025 % and 0.013% for female and male patients (50 years) respectively. Image quality was not compromised. Paired inspiratory-expiratory scans can be acquired on 3rd-DSCT systems at substantially lower dose and risk levels when compared to inspiration-only scans at conventional CT systems, offering promising prospects for improved COPD diagnosis.

[Calculation of water equivalent diameter based on anteroposterior localizer CT images]

ObjectiveTo explore a method for calculating water equivalent diameter (D_w) based on localizer CT images for calculation of the size specific dose estimates (SSDE).MethodGE Revolution CT and LightSpeed VCT were used to scan CT dose index phantoms 16 cm and 32 cm in diameter at the tube voltages of 80, 100 and 120 kV to obtain the axial image and anteroposterior localizer radiograph. According to the definition of CT Hounsfield unit, the axial images were used to calculate the conversion factors that convert the phantom thickness to water equivalent thickness. The gray value of the localizer radiograph and the water equivalent thickness were calibrated with a linear equation, and the parameters of the calibration were used to calculate the water equivalent thickness. The method was verified using 2 CT dose index phantoms and in 22 patients undergoing chest and abdominal CT examination.ResultComparison of the water equivalent diameter (D_w) based on the localizer radiograph and axial image of the 2 phantoms showed that the percentage difference between D_w from the axial images and from the localizer radiograph was below 3%. The trend of D_w variations with location in the two methods was sonsistent. The difference in D_w in intermediate region of interest between the axial image and the localizer radiograph from the 22 patients was below 6.6%. With the mean D_w in the ROI, the maximum percentage difference was 7.5%.ConclusionCalibration of the gray value of the localizer radiograph and the water equivalent thickness using the axial image and localizer radiograph of CT dose index phantoms allows quick calculation of the SSDE based on the parameters of calibration.

Patient-Specific Organ and Effective Dose Estimates in Adult Oncologic CT

OBJECTIVE. Patient-specific organ and effective dose provides essential information for CT protocol optimization. However, such information is not readily available in the scan records. The purpose of this study was to develop a method to obtain accurate examination- and patient-specific organ and effective dose estimates by use of available scan data and patient body size information for a large cohort of patients. MATERIALS AND METHODS. The data were randomly collected for 1200 patients who underwent CT in a 2-year period. Physical characteristics of the patients and CT technique were processed as inputs for the dose estimator. Organ and effective doses were estimated by use of the inputs and computational human phantoms matched to patients on the basis of sex and effective diameter. Size-based ratios were applied to correct for patient-phantom body size differences. RESULTS. Patients received a mean of 59.9 mGy to the lens of the eye per brain scan, 10.1 mGy to the thyroid per chest scan, 17.5 mGy to the liver per abdomen and pelvis scan, and 19.0 mGy to the liver per body scan. A factor of 2 difference in dose estimates was observed between patients of various habitus. CONCLUSION. Examination- and patient-specific organ and effective doses were estimated for 1200 adult oncology patients undergoing CT. The dose conversion factors calculated facilitate rapid organ and effective dose estimation in clinics. Compared with nonspecific dose estimation methods, patient dose estimations with data specific to the patient and examination can differ by a factor of 2.

Estimating patient water equivalent diameter from CT localizer images - A longitudinal and multi-institutional study of the stability of calibration parameters

PURPOSE

Water equivalent diameter (WED) is a robust patient-size descriptor. Localizer-based WED estimation is less sensitive to truncation errors resulting from limited field of view, and produces WED estimates at different locations within one localizer radiograph, prior to the initiation of axial scans. This method is considered difficult to implement by the clinical community due to the necessary calibration between localizer pixel values (LPV) and attenuation, and the unknown stability of calibration results across scanners and over time. We investigated the stability of calibration results across 25 computed tomography (CT) scanners from three medical centers, and their stability over 3 ∼ 29 months for 14 of those scanners.

METHODS

Localizer and axial images of ACR and body computed tomography dose index phantoms were acquired, using routine clinical techniques (120 kV and lateral localizers) on each of the 25 CT scanners: 8 GE scanners (CT750HD, VCT, and Revolution), 8 Siemens scanners (Definition AS, Force, Flash, and Edge), 5 Canon scanners (Aquilion-One, Aquilion-Prime80, and Aquilion-64), and 4 Philips scanners (iCT 256, iQon, and Ingenuity). By associating axial images with the corresponding localizer lines, the relationship between the scaled water equivalent area (WEA) and averaged LPV were established through regression analysis.

RESULTS

Linear relationships between the scaled WEA and the averaged LPV were observed in all 25 CT scanners ( R 2 > 0.999 ). Calibration parameters were similar for CT scanners from the same vendor: the coefficients of variation (COV) were ≤ 1% in all four vendor groups for the calibration slope, and < 7% for the intercept. By analyzing the deviation of WED resulted from errors in the calibration slope or intercept alone, we derived the tolerance ranges for the slope or intercept for a given WED error level. The variation of slope and intercept from different CT scanners of the same vendor introduced <±2.5% error in the estimated WED for subjects of 20 and 30-cm WED. The calibration parameters remained stable over time, with the maximum deviations all within the boundary values that introduce ±2.5% error in the estimated WED for subjects of 20 and 30-cm WED.

CONCLUSIONS

The stability in calibration results among CT scanners of the same vendor and over time demonstrated the feasibility of implementing WED estimation for routine clinical use.

Evaluation of CTPA interpreted as limited in pregnant patients suspected for pulmonary embolism

PURPOSE

The purpose of this study is to determine the rates of CT pulmonary angiography (CTPA) interpreted as limited and severely limited in pregnant patients suspected for pulmonary embolism (PE), and to evaluate factors that influence these rates.

METHODS

This is a retrospective study with CTPA for evaluation of PE in pregnancy across a large health system from 2006 to 2017. CTPA was classified as limited from the radiology report with a subset of those studies classified as severely limited. Bivariate and multivariate analysis was performed for limited and severely limited rates with maternal age and patient size as a continuous variable and race, trimester, patient location study priority status, and result of chest radiograph before CTPA as categorical variables.

RESULTS

874 patients with 33% of studies limited and 4% of studies severely limited. Multivariate logistic regression of CTPA studies revealed decreasing patient age (OR 0.967, p = 0.0129) and increasing patient size (OR 1.013, p < 0.0001). Studies performed in the second trimester (OR 1.869, p = 0.0242) and third trimester (OR 2.314, p = 0.0021) were more likely to be reported as limited (each p < 0.05). Increasing patient size (OR 1.017, p = 0.0046) was the only significant predictor of severely limited versus non-severely limited studies.

CONCLUSION

CTPA interpreted as limited in pregnancy are common and may be associated with younger age, larger patient size, and second and third trimesters. However, severely limited interpretations are much less common, with patient size the only significant predictor.

Size-Specific Dose Estimates Based on Water-Equivalent Diameter and Effective Diameter in Computed Tomography Coronary Angiography

BACKGROUND To determine the difference in size-specific dose estimates (SSDEs), separately based on effective diameter (deff) and water equivalent diameter (dw) of the central slice of the scan range in computed tomography coronary angiography (CTCA). MATERIAL AND METHODS There were 134 patients who underwent CTCA examination, were electronically retrieved. SSDEs (SSDEdeff and SSDEdw) were calculated using 2 approaches: deff and dw. The median SSDEs and mean absolute relative difference of SSDEs were calculated. Linear regression model was used to assess the absolute relative difference of SSDEs based on the ratio of deff to dw. RESULTS The median values of SSDEdeff and SSDEdw were 18.26 mGy and 20.56 mGy, respectively (P<0.01). The former was about 10.08% smaller than the latter. The mean absolute relative difference of SSDEs was 10.48%, ranging from 0.33% to 24.16%. A considerably positive correlation was found between the absolute relative difference of SSDEs and the ratio of deff to dw (R²=0.9561, r=0.979, P<0.01). CONCLUSIONS The value of SSDEdeff was smaller by an average of about 10.08% than SSDEdw in CTCA, and the absolute relative difference increased linearly with the ratio of effective diameter to water equivalent diameter.

Predictors of radiation dose for CT pulmonary angiography in pregnancy across a multihospital integrated healthcare network

PURPOSE

There is a large range of published effective radiation dose for CTPA during pregnancy. The purpose of our study is to determine the mean effective radiation dose and predictors of mean effective radiation dose for CTPA in pregnant patients across a multihospital integrated healthcare network.

METHODS

This retrospective study evaluates pregnant women who had a CTPA as the first primary advanced imaging test for evaluation of PE in a multihospital integrated healthcare network from January 2012-April 2017. Patient and CT-related data were obtained from the electronic health record and Radimetrics server (Radimetrics Inc, Bayer). DLP was recorded and effective radiation dose in mSv was determined using a conversation factor of 0.014 mSv·mGy^-¹·cm^-¹. Patient size was determined by water equivalent diameter. Bivariate and multivariate analysis were performed for effective radiation dose based on patient and CT factors.

RESULTS

In the 534 CTPA exams, the mean effective radiation dose was 3.96 mSv. Bivariate analysis showed significant differences in radiation dose by trimester, p = 0.042: first trimester 4.52 mSv, second trimester 3.73 mSv, and third trimester 3.95 mSv. Multivariable analysis demonstrated CTPA during first trimester, increasing mAs, kVp, scan length, patient size, and use of mAs modulation, as well as decreasing pitch, to be predictive of higher effective radiation dose.

CONCLUSION

Mean effective radiation dose was on the lower end of published studies. Trimester was a statistically significant predictor of effective radiation dose when accounting for known predictors of radiation dose.

Comparative Study of Volume Computed Tomography Dose Index and Size-Specific Dose Estimate Head in Computed Tomography Examination for Adult Patients Based on the Mode of Automatic Tube Current Modulation

BACKGROUND The aim of this study was to compare the metrics of volume computed tomography index (CTDIvol) and size-specific dose estimate (SSDE), and quantity the differences in head CT examinations of adult patients. MATERIAL AND METHODS A total of 157 patients underwent head CT examination were enrolled in this retrospective study. Pearson correlation analysis and linear regression correlation analysis were performed to observe the correlation between the dose metrics of CTDIvol and SSDEaver versus tube current product (mAs) and water equivalent diameter (WED). Correlated factors of CTDIvol and SSDEaver were analyzed by multivariate linear stepwise regression analysis. RESULTS A sum of 4239 data settings were measured: slices with WED >16 cm was 71.05%, and the slices with f <1 was 72.64%. The average value of the absolute difference between WED and the diameter of AAPM head phantom was 2.24±1.42 cm. Statistically significant difference was found between the values of CTDIvol and SSDEaver (P=0.000). The dispersion degree of the CTDIvol values was greater than that of SSDEaver. Strong positive correlation was shown between CTDIvol and mAs (P=0.000), as well as CTDIvol and WED (P=0.000). Strong positive correlation was shown between SSDEaver and mAs (P=0.000), and moderate correlation for SSDEaver and WED (P=0.000). Both the metrics of mAs and WED were included in the multivariate linear stepwise regression equation to observe the effect of related factors on the value of SSDEaver. CONCLUSIONS SSDEaver with better representative can reproduce the radiation dosage of the specific adult patients in head CT examination.

Organ doses evaluation for chest computed tomography procedures with TL dosimeters: Comparison with Monte Carlo simulations

Purpose

To evaluate organ doses in routine and low‐dose chest computed tomography (CT) protocols using an experimental methodology. To compare experimental results with results obtained by the National Cancer Institute dosimetry system for CT (NCICT) organ dose calculator. To address the differences on organ dose measurements using tube current modulation (TCM) and fixed tube current protocols.

Methods

An experimental approach to evaluate organ doses in pediatric and adult anthropomorphic phantoms using thermoluminescent dosimeters (TLDs) was employed in this study. Several analyses were performed in order to establish the best way to achieve the main results in this investigation. The protocols used in this study were selected after an analysis of patient data collected from the Institute of Radiology of the School of Medicine of the University of São Paulo (InRad). The image quality was evaluated by a radiologist from this institution. Six chest adult protocols and four chest pediatric protocols were evaluated. Lung doses were evaluated for the adult phantom and lung and thyroid doses were evaluated for the pediatric phantom. The irradiations were performed using both a GE and a Philips CT scanner. Finally, organ doses measured with dosimeters were compared with Monte Carlo simulations performed with NCICT.

Results

After analyzing the data collected from all CT examinations performed during a period of 3 yr, the authors identified that adult and pediatric chest CT are among the most applied protocol in patients in that clinical institution, demonstrating the relevance on evaluating organ doses due to these examinations. With regards to the scan parameters adopted, the authors identified that using 80 kV instead of 120 kV for a pediatric chest routine CT, with TCM in both situations, can lead up to a 28.7% decrease on the absorbed dose. Moreover, in comparison to the standard adult protocol, which is performed with fixed mAs, TCM, and ultra low‐dose protocols resulted in dose reductions of up to 35.0% and 90.0%, respectively. Finally, the percent differences found between experimental and Monte Carlo simulated organ doses were within a 20% interval.

Conclusions

The results obtained in this study measured the impact on the absorbed dose in routine chest CT by changing several scan parameters while the image quality could be potentially preserved.

DEVELOPMENT OF RADIATION DOSE CALCULATION SOFTWARE USING THE SIZE-SPECIFIC DOSE ESTIMATE

We aimed to develop a software for facilitating absorbed dose per pixel (pixel dose) calculation using a size-specific dose estimate (SSDE). We calculated the pixel dose at nine equal points inserted into the radiophotoluminescence glass dosemeter (RPLD) and compared the pixel dose with the measured doses using RPLD. With this method, the relative errors of average pixel dose was -0.1% for adults and 2.86, 3.36 and 1.17% for those aged 10, 5 and 1 years without tube current modulation, respectively. In contrast, the relative error of SSDE was 17.37% for adults and 20.38, 20.73 and 19.20% for those aged 10, 5 and 1 years, respectively. In other words, the pixel dose was almost equal to the measured doses. Therefore, our software can be useful for determining arbitrary point.

Comparison of methods to estimate water-equivalent diameter for calculation of patient dose

Modern CT systems seek to evaluate patient-specific dose by converting the CT dose index generated during a procedure to a size-specific dose estimate using conversion factors that are related to patient attenuation properties. The most accurate way to measure patient attenuation is to evaluate a full-field-of-view reconstruction of the whole scan length and calculating the true water-equivalent diameter (Dw ) using CT numbers; however, due to time constraints, less accurate methods to estimate Dw using patient geometry measurements are used more widely. In this study we compared the accuracy of Dw values calculated from three different methods across 35 sample scans and compared them to the true Dw . These three estimation methods were: measurement of patient lateral dimension from a pre-scan localizer radiograph; measurement of the sum of anteroposterior and lateral dimensions from a reconstructed central slice; and using CT numbers from a central slice only. Using the localizer geometry method, 22 out of 35 (62%) samples estimated Dw within 20% of the true value. The middle slice attenuation and geometry methods gave estimations within the 20% margin for all 35 samples.

Radiation dose dependence on subject size in abdominal computed tomography: Water phantom and patient model comparison

PURPOSE

Development of patient organ dose evaluation method in computed tomography (CT) needs to model the correlation between organ dose and patient size, under various conditions of scan length, tube current lineshape, and organ location. To facilitate this task, this work was to perform a comprehensive study of the relationship between the dose to water phantom and its diameter under various settings of phantom axis, scan length, and the location across or beyond the scanned range.

METHODS

A dose calculation algorithm and the published data by Li et al. [Med. Phys. 39, 5347-5352 (2012); 40, 031903 (2013); 43, 5878-5888 (2016)] were used to calculate longitudinal dose distribution D_L (z) in 10- to 50-cm diameter water phantoms undergoing constant tube current scans. The relationship between dose and phantom diameter was examined on three phantom axes (center, cross-sectional average, periphery), at seven scan lengths from 15 to 70 cm, and at eight longitudinal locations within or beyond each scan range. The water phantom results were compared to those of patient models of eight previous studies.

RESULTS

For the water phantoms matching the abdominal perimeters (36.3-124.5 cm) of the GSF family of voxelized phantoms, the median and range of D_L (z)(water) across scan range were consistent with those of the organ doses from the GSF phantom abdominal scans of a previous study. In 41 water phantoms (diameters 10-50 cm), D_L (z)(water) at locations inside scan range decreased with increasing phantom diameters. Exponential regression analysis of the above trend yielded regression parameters approximately consistent with those of phantom or patient models of eight previous studies. However, the usual exponential function might not be optimal for modeling the dose dependence on subject size. Inside scan range, the log(dose) vs diameter curve was non-linear on a semilogarithmic graph. Outside of scan range, dose might increase with larger subject sizes, contradicting to the exponential attenuation law. In the CT examinations of a patient population, direct modeling of organ dose dependence on patient size would be more challenging due to varying scan lengths and changing organ distances to the scan range centers.

CONCLUSION

An efficient approach to take into account the abdominal organ dose dependences on other factors is to calculate D_L (z)(water) with the water equivalent diameter, scan length, and tube current lineshape from the patient examinations, and to evaluate the organ dose to D_L (z)(water) ratio, where z is at the organ's longitudinal location. The ratio may be used for abdominal organ dose evaluation in the patient examinations. How to make use of D_L (z)(water) for organ dose evaluation in other body regions may be explored in the future.

Automatic calculation of patient size metrics in computed tomography: What level of computational accuracy do we need?

Objectives

To compare the effectiveness of two different patient size metrics based on water equivalent diameter (D_w), the mid‐scan water equivalent diameter D_{w_c}, and the mean (average) water equivalent diameter in the imaged region, D_{w_ave}, for automatic detection of accidental changes in computed tomography (CT) acquisition protocols.

Methods

Patient biometric data (height and weight) were available from a previous survey for 80 adult chest examinations, and 119 adult single‐acquisition chest–abdomen–pelvis (CAP) examinations for two 16 slice scanners (GE LightSpeed and Toshiba Aquilion RXL) equipped with automatic tube current modulation (ATCM). D_{w_c} and D_{w_ave} were calculated from the archived CT images. Size‐specific dose estimates (SSDE) were obtained from volume CT dose index (CTDI_vol), using the conversion factors for a patient diameter of D_{w_c}.

Results

CTDI_vol and SSDE correlate better with D_{w_ave} than with D_{w_c}. R‐squared values of linear fits to CTDI_vol of CAP examinations were 0.81–0.89 for D_{w_c} and 0.93–0.94 for D_{w_ave} (SSDE: 0.69–080 for D_{w_c}, 0.87–0.92 for D_{w_ave}). Percentage differences between D_{w_c} and D_{w_ave} were −4 ± 4% for chest and +5 ± 4% for CAP examinations (in % of D_{w_ave}). However, small D_w variations translated as larger variations in CTDI_vol for these ATCM systems (e.g., a 24% increase in D_w doubled CTDI_vol). The dependence of CTDI_vol on D_{w_ave} was similar for chest and CAP examinations performed with similar ATCM parameters, while use of D_{w_c} resulted in a clear separation of the same data according to examination type. Maximum D_w variation in the imaged region was 5.6 ± 1.6 cm for chest and 6.5 ± 1.4 cm for CAP examinations.

Conclusions

D_{w_ave} is a better metric than D_{w_c} for binning similar‐sized patients in dose comparison studies, despite the additional computational effort required for its calculation Therefore, when implementing automatic determination of D_w for SSDE calculations, automatic calculation of D_{w_ave} should be considered.

Physical validation of a Monte Carlo-based, phantom-derived approach to computed tomography organ dosimetry under tube current modulation

PURPOSE

To physically validate the accuracy of a Monte Carlo-based, phantom-derived methodology for computed tomography (CT) dosimetry that utilizes organ doses from precomputed axial scans and that accounts for tube current modulation (TCM).

METHODS

The output of a Toshiba Aquilion ONE CT scanner was modeled, based on physical measurement, in the Monte Carlo radiation transport code MCNPX (v2.70). CT examinations were taken of two anthropomorphic phantoms representing pediatric and adult patients (15-yr-old female and adult male) at various energies, in which physical organ dose measurements were made using optically stimulated luminescence dosimeters (OSLDs). These exams (chest-abdomen-pelvis) were modeled using organ dose data obtained from the computationally equivalent phantom of each anthropomorphic phantom. TCM was accounted for by weighting all organ dose contributions by both the relative attenuation of the phantom and the image-derived mA value (from the DICOM header) at the same z-extent (cranial-caudal direction) of the axial dose data.

RESULTS

The root mean squares of percent difference in organ dose when comparing the physical organ dose measurements to the computational estimates were 21.2, 12.1, and 15.1% for the uniform (no attenuation weighting), weighted (computationally derived), and image-based methodologies, respectively.

CONCLUSIONS

Overall, these data suggest that the Monte Carlo-based dosimetry presented in this work is viable for CT dosimetry. Additionally, for CT exams with TCM, local attenuation weighting of organ dose contributions from precomputed axial dosimetry libraries increases organ dose accuracy.

THE SIZE-SPECIFIC DOSE ESTIMATE (SSDE) FOR TRUNCATED COMPUTED TOMOGRAPHY IMAGES

The purpose of this study is to investigate truncated axial computed tomography (CT) images in the clinical environment and to produce correction factors for abdomen, thoracic and head regions based on clinical data, in order to accurately predict the water-equivalent diameter (DW) and size-specific dose estimate (SSDE). We investigated axial images of 75 patients who underwent CT examinations. Truncated axial images were characterized by the truncation percentage (TP). Correction factors were calculated by using the value of DW for a certain TP (truncated image) divided by the value of DW for TP = 0% (the non-truncated image). Most of the thorax images acquired for this study were truncated images (86.2%), in the abdomen region about half of the images were truncated (48.1%), and in the head region only a small portion were truncated (9.1%). In the thorax region the value of TP for the truncated images varied up to 50%, in the abdomen region it varied up to 35%, and in the head region it was smaller than 10%. We have shown how to accurately estimate DW and SSDE by applying a correction factor to the truncated images. The correction factors increase exponentially with increasing TP. The corrected DW and SSDE for the truncated images were significant in the thoracic region, but were not significant in the abdomen and head regions.

Estimating patient dose from CT exams that use automatic exposure control: Development and validation of methods to accurately estimate tube current values

PURPOSE

The vast majority of body CT exams are performed with automatic exposure control (AEC), which adapts the mean tube current to the patient size and modulates the tube current either angularly, longitudinally or both. However, most radiation dose estimation tools are based on fixed tube current scans. Accurate estimates of patient dose from AEC scans require knowledge of the tube current values, which is usually unavailable. The purpose of this work was to develop and validate methods to accurately estimate the tube current values prescribed by one manufacturer's AEC system to enable accurate estimates of patient dose.

METHODS

Methods were developed that took into account available patient attenuation information, user selected image quality reference parameters and x-ray system limits to estimate tube current values for patient scans. Methods consistent with AAPM Report 220 were developed that used patient attenuation data that were: (a) supplied by the manufacturer in the CT localizer radiograph and (b) based on a simulated CT localizer radiograph derived from image data. For comparison, actual tube current values were extracted from the projection data of each patient. Validation of each approach was based on data collected from 40 pediatric and adult patients who received clinically indicated chest (n = 20) and abdomen/pelvis (n = 20) scans on a 64 slice multidetector row CT (Sensation 64, Siemens Healthcare, Forchheim, Germany). For each patient dataset, the following were collected with Institutional Review Board (IRB) approval: (a) projection data containing actual tube current values at each projection view, (b) CT localizer radiograph (topogram) and (c) reconstructed image data. Tube current values were estimated based on the actual topogram (actual-topo) as well as the simulated topogram based on image data (sim-topo). Each of these was compared to the actual tube current values from the patient scan. In addition, to assess the accuracy of each method in estimating patient organ doses, Monte Carlo simulations were performed by creating voxelized models of each patient, identifying key organs and incorporating tube current values into the simulations to estimate dose to the lungs and breasts (females only) for chest scans and the liver, kidney, and spleen for abdomen/pelvis scans. Organ doses from simulations using the actual tube current values were compared to those using each of the estimated tube current values (actual-topo and sim-topo).

RESULTS

When compared to the actual tube current values, the average error for tube current values estimated from the actual topogram (actual-topo) and simulated topogram (sim-topo) was 3.9% and 5.8% respectively. For Monte Carlo simulations of chest CT exams using the actual tube current values and estimated tube current values (based on the actual-topo and sim-topo methods), the average differences for lung and breast doses ranged from 3.4% to 6.6%. For abdomen/pelvis exams, the average differences for liver, kidney, and spleen doses ranged from 4.2% to 5.3%.

CONCLUSIONS

Strong agreement between organ doses estimated using actual and estimated tube current values provides validation of both methods for estimating tube current values based on data provided in the topogram or simulated from image data.

Estimating organ doses from tube current modulated CT examinations using a generalized linear model

PURPOSE

Currently, available Computed Tomography dose metrics are mostly based on fixed tube current Monte Carlo (MC) simulations and/or physical measurements such as the size specific dose estimate (SSDE). In addition to not being able to account for Tube Current Modulation (TCM), these dose metrics do not represent actual patient dose. The purpose of this study was to generate and evaluate a dose estimation model based on the Generalized Linear Model (GLM), which extends the ability to estimate organ dose from tube current modulated examinations by incorporating regional descriptors of patient size, scanner output, and other scan-specific variables as needed.

METHODS

The collection of a total of 332 patient CT scans at four different institutions was approved by each institution's IRB and used to generate and test organ dose estimation models. The patient population consisted of pediatric and adult patients and included thoracic and abdomen/pelvis scans. The scans were performed on three different CT scanner systems. Manual segmentation of organs, depending on the examined anatomy, was performed on each patient's image series. In addition to the collected images, detailed TCM data were collected for all patients scanned on Siemens CT scanners, while for all GE and Toshiba patients, data representing z-axis-only TCM, extracted from the DICOM header of the images, were used for TCM simulations. A validated MC dosimetry package was used to perform detailed simulation of CT examinations on all 332 patient models to estimate dose to each segmented organ (lungs, breasts, liver, spleen, and kidneys), denoted as reference organ dose values. Approximately 60% of the data were used to train a dose estimation model, while the remaining 40% was used to evaluate performance. Two different methodologies were explored using GLM to generate a dose estimation model: (a) using the conventional exponential relationship between normalized organ dose and size with regional water equivalent diameter (WED) and regional CTDIvol as variables and (b) using the same exponential relationship with the addition of categorical variables such as scanner model and organ to provide a more complete estimate of factors that may affect organ dose. Finally, estimates from generated models were compared to those obtained from SSDE and ImPACT.

RESULTS

The Generalized Linear Model yielded organ dose estimates that were significantly closer to the MC reference organ dose values than were organ doses estimated via SSDE or ImPACT. Moreover, the GLM estimates were better than those of SSDE or ImPACT irrespective of whether or not categorical variables were used in the model. While the improvement associated with a categorical variable was substantial in estimating breast dose, the improvement was minor for other organs.

CONCLUSIONS

The GLM approach extends the current CT dose estimation methods by allowing the use of additional variables to more accurately estimate organ dose from TCM scans. Thus, this approach may be able to overcome the limitations of current CT dose metrics to provide more accurate estimates of patient dose, in particular, dose to organs with considerable variability across the population.

Automated Calculation of Water-equivalent Diameter (DW) Based on AAPM Task Group 220

The purpose of this study is to accurately and effectively automate the calculation of the water‐equivalent diameter (DW) from 3D CT images for estimating the size‐specific dose. DW is the metric that characterizes the patient size and attenuation. In this study, DW was calculated for standard CTDI phantoms and patient images. Two types of phantom were used, one representing the head with a diameter of 16 cm and the other representing the body with a diameter of 32 cm. Images of 63 patients were also taken, 32 who had undergone a CT head examination and 31 who had undergone a CT thorax examination. There are three main parts to our algorithm for automated DW calculation. The first part is to read 3D images and convert the CT data into Hounsfield units (HU). The second part is to find the contour of the phantoms or patients automatically. And the third part is to automate the calculation of DW based on the automated contouring for every slice (DW,all). The results of this study show that the automated calculation of DW and the manual calculation are in good agreement for phantoms and patients. The differences between the automated calculation of DW and the manual calculation are less than 0.5%. The results of this study also show that the estimating of DW,all using DW,n=1 (central slice along longitudinal axis) produces percentage differences of −0.92%±3.37% and 6.75%±1.92%, and estimating DW,all using DW,n=9 produces percentage differences of 0.23%±0.16% and 0.87%±0.36%, for thorax and head examinations, respectively. From this study, the percentage differences between normalized size‐specific dose estimate for every slice (nSSDEall) and nSSDEn=1 are 0.74%±2.82% and −4.35%±1.18% for thorax and head examinations, respectively; between nSSDEall and nSSDEn=9 are 0.00%±0.46% and −0.60%±0.24% for thorax and head examinations, respectively.

PACS number(s): 87.57.Q‐, 87.57.uq‐

Comparison of organ-specific-radiation dose levels between 70 kVp perfusion CT and standard tri-phasic liver CT in patients with hepatocellular carcinoma using a Monte-Carlo-Simulation-based analysis platform

Purpose

The aim of this study was to systematically compare organ-specific-radiation dose levels between a radiation dose optimized perfusion CT (dVPCT) protocol of the liver and a tri-phasic standard CT protocol of the liver using a Monte-Carlo-Simulation-based analysis platform.

Methods and materials

The complete CT data of 52 patients (41 males; mean age 65 ± 12) with suspected HCC that underwent dVPCT examinations on a 3rd generation dual-source CT (Somatom Force, Siemens) with a dose optimized tube voltage of 70 kVp or 80 kVp were exported to an analysis platform (Radimetrics, Bayer). The dVPCT studies were matched with a reference group of 50 patients (35 males; mean age 65 ± 14) that underwent standard tri-phasic CT (sCT) examinations of the liver with 130 kVp using the calculated water-equivalent-diameter of the patients. The analysis platform was used for the calculation of the organ-specific effective dose (ED) as well as global radiation-dose parameters (ICRP103).

Results

The organ-specific ED of the dVPCT protocol was statistically significantly lower when compared to the sCT in 14 of 21, and noninferior in a total of 18 of 21 examined items (all p < 0.05). The EDs of the dVPCT examinations were especially in the dose sensitive organs such as the red marrow (17.3 mSv vs 24.6 mSv, p = < 0.0001) and the liver (33.3 mSv vs 46.9 mSv, p = 0.0003) lower when compared to the sCT.

Conclusion

Our results suggest that dVPCT performed at 70 or 80 kVp compares favorably to sCT performed with 130 kVp with regard to effective organ dose levels, especially in dose sensitive organs, while providing additional functional information which is of paramount importance in patients undergoing novel targeted therapies.