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 CTDI_vol -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 (D_fetus ) 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, nD_fetus , was obtained by dividing D_fetus from the MC simulations by CTDI_vol . Patient size was described using water equivalent diameter (D_w ) measured at the three-dimensional geometric centroid of the fetus. Fetal depth (DE_f ) was measured from the anterior skin surface to the anterior part of the fetus. nD_fetus and D_w 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 nD_fetus with D_w and fetal depth (DE_f ). For one scanner model, nD_fetus 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. nD_fetus from FTC simulations was averaged across all scanners for each patient $\left(\overline{n{D}_{\mathrm{fetus}}}\right)$ . $\overline{n{D}_{\mathrm{fetus}}}$ was then compared with SSDE f-factors and correlated with D_w using an exponential model and with D_w and DE_f using a bivariate linear model.

Results: For TCM, the coefficient of determination (R² ) of nD_fetus and D_w was observed to be 0.73 using an exponential model. Using the bivariate linear model with D_w and DE_f , an R² of 0.78 was observed. For the TCM technology modeled, TCM yielded nD_fetus values that were on average 6% and 17% higher relative to FTC and SSDE f-factors, respectively. For FTC, the R² of $\overline{n{D}_{\mathrm{fetus}}}$ with respect to D_w was observed to be 0.64 using an exponential model. Using the bivariate linear model, an R² of 0.75 was observed for $\overline{n{D}_{\mathrm{fetus}}}$ with respect to D_w and DE_f . A mean difference of 0.4% was observed between $\overline{n{D}_{\mathrm{fetus}}}$ and SSDE f-factors.

Conclusion: Good correlations were observed for nD_fetus from TCM and FTC scans using either an exponential model with D_w or a bivariate linear model with both D_w and DE_f . These results indicate that fetal dose from abdomen/pelvis CT examinations of pregnant patients of various gestational ages may be reasonably estimated with models that include (a) scanner-reported CTDI_vol and (b) D_w as a patient size metric, in addition to (c) DE_f if available. These results also suggest that SSDE f-factors may provide a reasonable (within ±25%) estimate of nD_fetus for TCM and FTC abdomen/pelvis CT exams.

Keywords: Monte Carlo simulations; computed tomography; conceptus dose; embryo dose; fetal dose; radiation dose; tube current modulation.

Figure 1

Images in the first row, from left to right, represent early‐term, mid‐term, and late‐term pregnant patients. Images in the second row show the uterus (yellow), gestational sac (green), and fetus (red) segmented from the images of these pregnant patients. Adapted with permission from Fig. 3 of Angel et al. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

(a) Voxelized representation of patient model (sagittal view) and (b) simulated computed tomography (CT) radiograph anterior–posterior (AP view). The simulated CT radiograph was generated by simulating projections at 1‐mm increments along the length of the voxelized patient model and dividing the resulting projections by a reference air scan. The legend below (a) is color coded for the material designations for each voxel. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Estimated TCM scheme for a pregnant patient who received a clinically indicated computed tomography (CT) examination. The TCM scheme is overlaid on an image of the simulated CT localizer radiograph anterior–posterior (AP orientation) of the pregnant patient. The portion of the scan range in which the fetus is located is indicated with yellow dashed lines. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

nD _fetus,1 (D _w ) for the tube current‐modulated ( TCM) and fixed tube current (FTC) scans from the AS64. nD _fetus,1 (D _w ) represents the exponential model using nD _fetus and D _w for TCM and FTC scans. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

The results here are only for fixed tube current (FTC) scans. nD _fetus,1 (D _w ) for the (a) LightSpeed VCT, (b) Brilliance 64, (c) Aquilion 64, and (d) AS64. The CTDI _vol values for the four scanners were 17.7 mGy for the LightSpeed VCT, 12.5 mGy for the Brilliance, 24.6 mGy for the Aquilion, and 15.6 mGy for the AS64. nD _fetus,1 (D _w ) represents the exponential model using nD _fetus and D_w for each of the four scanners. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

Regression analysis for $\overline{n{D}_{fetus,1}}(D_w)$ using the exponential model. $\overline{n{D}_{fetus,1}}(D_w)$ represents the exponential model using $\overline{n{D}_{\mathit{fetus}}}$ and D _w .

Figure 7

The same regression analyses shown in Fig. 4 accompanied by the size‐specific dose estimate (SSDE) f‐factors from American Association of Physicists in Medicine (AAPM) Report 204 as a point of reference and shaded areas corresponding to ±20% and ±25% of the SSDE f‐factors. A summary of the doses that fall within ±20% and ±25% of the SSDE f‐factors are tabulated in Table 10. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8

nD _fetus,1 (D_w) for the (a) LightSpeed VCT, (b) Brilliance 64, (c) Aquilion 64, and (d) Definition AS64 shown in Fig. 5 with the size‐specific dose estimate (SSDE) f‐factors from American Association of Physicists in Medicine (AAPM) Report 204 included as a point of reference. In addition, shaded areas corresponding to ±20% and ±25% of the SSDE f‐factors are also shown. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 9

$\overline{n{D}_{fetus,1}}(D_w)$ accompanied with the size‐specific dose estimate (SSDE) f‐factors and the shaded regions corresponding to ±20% and ±25% of the SSDE f‐factors for comparison. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 10

Axial and sagittal images showing the variability in early‐term maternal anatomy. For ID1 in (a) and ID4 in (b), the greater DE _f means that the uterus (yellow) and gestational sac (green), respectively, are situated deeper within the pelvis and hence provided the fetus more shielding. For ID5 in (c), the uterus and gestation sac extend anteriorly and for ID3 in (d), a distended bladder (outlined in cyan) pushes uterus more anteriorly. [Color figure can be viewed at wileyonlinelibrary.com]