Department of Radiology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, CA, 90024, USA.
Physics and Biology in Medicine Graduate Program, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, CA, 90024, USA.
Med Phys. 2019 Jun;46(6):2729-2743. doi: 10.1002/mp.13499. Epub 2019 Apr 24.
The purpose of this work was to estimate scanner-independent CTDI -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.
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 ) 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 , was obtained by dividing D from the MC simulations by CTDI . Patient size was described using water equivalent diameter (D ) measured at the three-dimensional geometric centroid of the fetus. Fetal depth (DE ) was measured from the anterior skin surface to the anterior part of the fetus. nD and D 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 with D and fetal depth (DE ). For one scanner model, nD 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 from FTC simulations was averaged across all scanners for each patient . was then compared with SSDE f-factors and correlated with D using an exponential model and with D and DE using a bivariate linear model.
For TCM, the coefficient of determination (R ) of nD and D was observed to be 0.73 using an exponential model. Using the bivariate linear model with D and DE , an R of 0.78 was observed. For the TCM technology modeled, TCM yielded nD values that were on average 6% and 17% higher relative to FTC and SSDE f-factors, respectively. For FTC, the R of with respect to D was observed to be 0.64 using an exponential model. Using the bivariate linear model, an R of 0.75 was observed for with respect to D and DE . A mean difference of 0.4% was observed between and SSDE f-factors.
Good correlations were observed for nD from TCM and FTC scans using either an exponential model with D or a bivariate linear model with both D and DE . 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 and (b) D as a patient size metric, in addition to (c) DE if available. These results also suggest that SSDE f-factors may provide a reasonable (within ±25%) estimate of nD for TCM and FTC abdomen/pelvis CT exams.
本研究旨在估算不同孕周孕妇行腹部/盆腔 CT 检查时，使用管电流调制（TCM）和固定管电流（FTC）技术的 CT 剂量指数（CTDI）与胎儿剂量之间的转换系数。
对 24 例妊娠 <5 周至 36 周行腹部/盆腔 CT 检查的孕妇，采用蒙特卡罗（MC）方法对腹部/盆腔螺旋扫描进行模拟，对母体和胎儿（或胚胎）的解剖结构进行体素化建模。利用 MC 模拟对 TCM 和 FTC 扫描的绝对胎儿剂量（D）进行估算。为每个患者模型生成估计的 TCM 方案，使用经过验证的方法，该方法考虑了一个扫描仪模型的患者衰减和扫描仪输出限制，并将其纳入 MC 模拟中。对于每个患者模型，使用来自四个制造商的多排探测器 CT 扫描仪模拟 FTC 扫描。通过将 MC 模拟中的 D 除以 CTDI 得到归一化胎儿剂量估计值 nD。使用在三维几何中心测量的胎儿水当量直径（D）描述患者的大小。从胎儿的前皮肤表面到前部分测量胎儿深度（DE）。使用指数模型对 nD 和 D 进行相关性分析，为 TCM 和 FTC 腹部/盆腔 CT 检查开发胎儿剂量转换系数方程。此外，还进行了二元线性回归分析，以分析 nD 与 D 和胎儿深度（DE）的相关性。对于一个扫描仪模型，将 TCM 的 nD 与 FTC 和美国医学物理学家协会（AAPM）报告 204 中的剂量面积乘积（SSDE）转换系数（f 因子）进行比较。对所有扫描仪的每个患者进行 FTC 模拟的 nD 平均值。然后使用指数模型将 与 SSDE f 因子进行比较，并使用指数模型与 D 和 DE 进行相关性分析。
对于 TCM，nD 与 D 的相关性采用指数模型，决定系数（R）为 0.73。使用具有 D 和 DE 的二元线性模型，观察到 R 为 0.78。对于所建模的 TCM 技术，与 FTC 和 SSDE f 因子相比，TCM 分别平均产生 6%和 17%更高的 nD 值。对于 FTC，采用指数模型，与 D 的 R 为 0.64。使用二元线性模型，与 D 和 DE 的 R 为 0.75。观察到与 SSDE f 因子的 之间的平均差异为 0.4%。
使用 D 作为患者大小指标的指数模型或同时使用 D 和 DE 的二元线性模型，均可以很好地对 TCM 和 FTC 扫描的 nD 进行相关性分析。这些结果表明，对于不同孕周孕妇的腹部/盆腔 CT 检查，可使用包括（a）扫描仪报告的 CTDI 和（b）D 作为患者大小指标，以及（c）如果可用的话，还可使用 DE，对胎儿剂量进行合理估计。这些结果还表明，SSDE f 因子可能可以合理地（在±25%范围内）估计 TCM 和 FTC 腹部/盆腔 CT 检查的 nD。
