Estimating fetal dose from tube current‐modulated (TCM) and fixed tube current (FTC) abdominal/pelvis CT examinations

Abstract

Purpose The purpose of this work was to estimate scanner‐independent CTDIvol‐to‐fetal‐dose coefficients for tube current‐modulated (TCM) and fixed tube current (FTC) computed tomography (CT) examinations of pregnant patients of various gestational ages undergoing abdominal/pelvic CT examinations. Methods For 24 pregnant patients of gestational age from <5 to 36 weeks who underwent clinically indicated CT examinations, voxelized models of maternal and fetal (or embryo) anatomy were created from abdominal/pelvic image data. Absolute fetal dose (Dfetus) was estimated using Monte Carlo (MC) simulations of helical scans covering the abdomen and pelvis for TCM and FTC scans. Estimated TCM schemes were generated for each patient model using a validated method that accounts for patient attenuation and scanner output limits for one scanner model and were incorporated into MC simulations. FTC scans were also simulated for each patient model with multidetector row CT scanners from four manufacturers. Normalized fetal dose estimates, nDfetus, was obtained by dividing Dfetus from the MC simulations by CTDIvol. Patient size was described using water equivalent diameter (Dw) measured at the three‐dimensional geometric centroid of the fetus. Fetal depth (DEf) was measured from the anterior skin surface to the anterior part of the fetus. nDfetus and Dw were correlated using an exponential model to develop equations for fetal dose conversion coefficients for TCM and FTC abdominal/pelvic CT examinations. Additionally, bivariate linear regression was performed to analyze the correlation of nDfetus with Dw and fetal depth (DEf). For one scanner model, nDfetus from TCM was compared to FTC and the size‐specific dose estimate (SSDE) conversion coefficients (f‐factors) from American Association of Physicists in Medicine (AAPM) Report 204. nDfetus from FTC simulations was averaged across all scanners for each patient (nDfetus¯). nDitalicfetus¯ was then compared with SSDE f‐factors and correlated with Dw using an exponential model and with Dw and DEf using a bivariate linear model. Results For TCM, the coefficient of determination (R2) of nDfetus and Dw was observed to be 0.73 using an exponential model. Using the bivariate linear model with Dw and DEf, an R2 of 0.78 was observed. For the TCM technology modeled, TCM yielded nDfetus values that were on average 6% and 17% higher relative to FTC and SSDE f‐factors, respectively. For FTC, the R2 of nDitalicfetus¯ with respect to Dw was observed to be 0.64 using an exponential model. Using the bivariate linear model, an R2 of 0.75 was observed for nDitalicfetus¯ with respect to Dw and DEf. A mean difference of 0.4% was observed between nDitalicfetus¯ and SSDE f‐factors. Conclusion Good correlations were observed for nDfetus from TCM and FTC scans using either an exponential model with Dw or a bivariate linear model with both Dw and DEf. These results indicate that fetal dose from abdomen/pelvis CT examinations of pregnant patients of various gestational ages may be reasonably estimated with mod...

Organ doses from CT localizer radiographs: Development, validation, and application of a Monte Carlo estimation technique

“…The localizer organ doses for all XCAT organs were studied as a function of patient chest diameter by fitting an exponential relationship to characterize the dependence of organ doses on patient characteristics. An exponential relationship was chosen to match the relationship with exponential nature of x‐ray attenuation in matter and is consistent with other works that use exponential fits to describe dosimetry variation with patient thickness . The exponential function used in the present study was of the formwhere a and b are fitting parameters and d is the mean patient chest diameter in the region of the scan which was calculated directly from the XCAT phantoms.…”

Reference dataset for benchmarking fetal doses derived from Monte Carlo simulations of CT exams

“…The scanning parameters for both scenarios are given in Table III. 17 For TCM simulations, the tube current information provided in the dataset is based on the methodology developed by McMillan et al 22 This method approximates the TCM algorithm of one manufacturer (CAREDose4D 23 , Siemens Healthineers, Forchheim Germany), and provides the tube current information in a format similar to the TCM data as extracted from raw projections. Specifically, for each patient, the tube current information [given in mA, as illustrated in Fig.…”

“…The dose values included in the dataset are from published results of TCM and FTC fetal doses from Hardy et al (Scenarios A and B). 17 In addition, the dose values for the supplementary Scenarios C and D are also given. The fetal dose results for both TCM and FTC simulations presented in this dataset are from the *F4 energy fluence tally of MCNPX, which records energy fluence on a per voxel basis (MeV/ cm 2 /photon).…”

“…By default, all voxelized models are centered about the source-to-isocenter distance (SID) in the x-y plane and in the z direction. D w,topo (z) is presented in a comma-delimited text file in which each column contains the table position (z, in mm, from the simulated topogram performed at 1 mm increments 17,22 ), AP(z), LAT(z), and D w (z) (in cm), respectively, from the simulated topogram. D w,image (z) is given as a single-column text file with D w estimates from each image in the series wherein each entry in the file is the D w estimate for each image slice.…”

“…[Color figure can be viewed at wile yonlinelibrary.com] TABLE III. Scanning parameters used for Scenarios A and B, that is, the TCM and FTC, respectively, scans on the Siemens Definition AS64 used in the Hardy et al study 17. The TCM protocol is based on CAREDose4D.…”