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Radiation Dose to the Fetus for Pregnant Patients Undergoing Multidetector CT Imaging: Monte Carlo Simulations Estimating Fetal Dose for a Range of Gestational Age and Patient Size¹

¹ From the Departments of Radiology and Radiation Oncology, David A. Geffen School of Medicine, University of California at Los Angeles, 924 Westwood Blvd, Suite 650, Los Angeles, CA 90024 (E.A., N.Y., J.J.D., C.H.C., J.W.S., M.F.M.); Department of Diagnostic Radiology, Mayo Clinic, Scottsdale, Ariz (C.V.W.); Department of Radiology, University of Michigan Hospitals, Ann Arbor, Mich (M.M.G.); Division of Diagnostic Imaging, University of Texas M. D. Anderson Cancer Center, Houston, Tex (D.D.C., D.M.S.); and Department of Radiology, Mayo Clinic College of Medicine, Rochester, Minn (A.N.P., C.H.M.). From the 2006 RSNA Annual Meeting. Received September 24, 2007; revision requested December 10; revision received January 28, 2008; accepted April 7; final version accepted April 21. Supported by grants R01EB004898 and T32EB002101 from the National Institute of Biomedical Imaging and Bioengineering. Address correspondence to E.A. (e-mail: EAngel{at}mednet.ucla.edu).

Purpose: To use Monte Carlo simulations of a current-technology multidetector computed tomographic (CT) scanner to investigate fetal radiation dose resulting from an abdominal and pelvic examination for a range of actual patient anatomies that include variation in gestational age and maternal size.

Materials and Methods: Institutional review board approval was obtained for this HIPAA-compliant retrospective study. Twenty-four models of maternal and fetal anatomy were created from image data from pregnant patients who had previously undergone clinically indicated CT examination. Gestational age ranged from less than 5 weeks to 36 weeks. Simulated helical scans of the abdominal and pelvic region were performed, and a normalized dose (in milligrays per 100 mAs) was calculated for each fetus. Stepwise multiple linear regression was performed to analyze the correlation of dose with gestational age and anatomic measurements of maternal size and fetal location. Results were compared with several existing fetal dose estimation methods.

Results: Normalized fetal dose estimates from the Monte Carlo simulations ranged from 7.3 to 14.3 mGy/100 mAs, with an average of 10.8 mGy/100 mAs. Previous methods yielded values of 10–14 mGy/100 mAs. The correlation between gestational age and fetal dose was not significant (P = .543). Normalized fetal dose decreased linearly with increasing patient perimeter (R² = 0.681, P < .001), and a two-factor model with patient perimeter and fetal depth demonstrated a strong correlation with fetal dose (R² = 0.799, P < .002).

Conclusion: A method for the estimation of fetal dose from models of actual patient anatomy that represented a range of gestational age and patient size was developed. Fetal dose correlated with maternal perimeter and varied more than previously recognized. This correlation improves when maternal size and fetal depth are combined.

© RSNA, 2008