Room: Room 202
Purpose: To date, fetal dose estimates have been limited to fixed tube current exams. However, tube current modulation (TCM) is used routinely in clinical practice. The purpose of this study is to examine the effects of TCM on fetal dose for abdomen/pelvis (A/P) CT exams.
Methods: Data from seven publicly available pregnant patient models having gestational ages ranging from less than 5 to 24 weeks were used in this study. Voxelized patient models of maternal and fetal anatomy were used in Monte Carlo (MC) simulations. Image data from these models were used to simulate the CT scan radiograph (i.e. topogram) so that the desired attenuation characteristics could be estimated. From these characteristics, predicted TCM schemes were generated for each patient model using a validated method that accounts for both patient attenuation and scanner machine limitations. Fetal doses were obtained by incorporating each TCM scheme into a MC source model of a 64-slice MDCT scanner. For patients at early gestational ages, the gestational sac was used as a substitute for the fetus. Water equivalent diameter (Dw) at the three-dimensional centroid of the fetus (or gestational sac) was used as the size metric. All fetal doses were normalized by scan-specific 32 cm CTDIvol values (based upon the average tube current across the entire simulated scan) and compared to the water equivalent diameter. In addition, these results were compared with size-specific dose estimates (SSDE) from AAPM Report 204.
Results: Normalized fetal dose values varied from 1.18 to 1.42 in this limited dataset. The differences between normalized fetal dose and SSDE were within Â± 12% across this range of patient sizes.
Conclusion: The effects of TCM on fetal dose estimates were investigated and these preliminary results indicate that SSDE conversion coefficients may provide reasonably accurate estimates of fetal doses in these exams.
Funding Support, Disclosures, and Conflict of Interest: Michael McNitt-Gray, Ph.D Departmental master research agreement, Siemens Healthineers Erin Angel, Ph.D Employee of Canon Medical Systems