Purpose: The purpose of this work was to estimate scanner independent CTDIvol-to-fetal-dose coefficients for both tube current modulated (TCM) and fixed tube current (FTC) CT examinations of pregnant patients of various gestational ages undergoing abdominal/pelvic CT examinations.
Methods: For 24 pregnant patients of gestational age from less than 5 to 36 weeks who underwent clinically-indicated CT examinations, voxelized models of maternal and fetal anatomy were created from abdominal/pelvic image data. Estimated TCM schemes were generated for each patient model using a validated method that accounts for both patient attenuation and scanner model characteristics. FTC scans were simulated for each patient model with multidetector row CT scanners from four manufacturers. Fetal dose was estimated using Monte Carlo (MC) simulations of helical scans covering the abdomen/pelvis for TCM and FTC scenarios. Normalized fetal dose, nDfetus, was obtained by dividing fetal doses by CTDIvol. Patient size was described using water equivalent diameter (Dw) measured at the geometric centroid of the fetus. Normalized fetal dose 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 normalized fetal dose with Dw and fetal depth (DEf).
Results: For TCM, the RÂ² of nDfetus and Dw was observed to be 0.73 using an exponential model. Using the bivariate linear model with Dw and DEf, an RÂ² of 0.78 was observed. For FTC scans, when nDfetus was averaged across the four scanners, the RÂ² of with respect to Dw was observed to be 0.64 using an exponential model. Using the bivariate linear model, an RÂ² of 0.75 was observed with respect to Dw and DEf.
Conclusion: Fetal dose from abdomen/pelvis CT examinations may be reasonably estimated with models that include CTDIvol, Dw, and DEf.
Funding Support, Disclosures, and Conflict of Interest: 1) Michael McNitt-Gray, PhD - Departmental master research agreement with Siemens Healthineers 2) Erin Angel, PhD - Employee of Canon Medical Systems