This work has revealed that there is considerable variation among modern MDCT scanners in both CTDIvol and organ dose values. Because these variations are similar, CTDIvol can be used as a normalization factor with excellent results. This demonstrates the feasibility of establishing scanner-independent organ dose estimates by using CTDIvol to account for the differences between scanners.

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 ϭ .543). Normalized fetal dose decreased linearly with increasing patient perimeter (R 2 ϭ 0.681, P Ͻ .001), and a two-factor model with patient perimeter and fetal depth demonstrated a strong correlation with fetal dose (R 2 ϭ 0.799, P Ͻ .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. Note: This copy is for your personal, non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, use the Radiology Reprints form at the end of this article. D iagnostic computed tomographic(CT) imaging is sometimes necessary in a pregnant patient. When a radiologist needs to decide if the diagnostic benefits will outweigh the risks of radiation, it is important to have a reasonably accurate estimate of the radiation dose that the conceptus (fetus or embryo) will receive. Furthermore, in cases in which pregnancy is discovered during or after CT examination, the patient and/or physician may request an estimate of the radiation dose received by the conceptus. For the remainder of this article, the term fetus will be used to refer to either an embryo or a fetus and will therefore be used to describe a conceptus at any gestational age.It is not known definitively how much radiation dose a fetus receives during CT examination, because this cannot be measured directly. Some methods to estimate fetal dose exist, but these estimates are limited by their simplifying assumptions. Existing fetal dose estimation m...

The purpose of this study was to present a method for generating x-ray source models for performing Monte Carlo ͑MC͒ radiation dosimetry simulations of multidetector row CT ͑MDCT͒ scanners. These so-called "equivalent" source models consist of an energy spectrum and filtration description that are generated based wholly on the measured values and can be used in place of proprietary manufacturer's data for scanner-specific MDCT MC simulations. Required measurements include the half value layers ͑HVL 1 and HVL 2 ͒ and the bowtie profile ͑exposure values across the fan beam͒ for the MDCT scanner of interest. Using these measured values, a method was described ͑a͒ to numerically construct a spectrum with the calculated HVLs approximately equal to those measured ͑equivalent spectrum͒ and then ͑b͒ to determine a filtration scheme ͑equivalent filter͒ that attenuates the equivalent spectrum in a similar fashion as the actual filtration attenuates the actual x-ray beam, as measured by the bowtie profile measurements. Using this method, two types of equivalent source models were generated: One using a spectrum based on both HVL 1 and HVL 2 measurements and its corresponding filtration scheme and the second consisting of a spectrum based only on the measured HVL 1 and its corresponding filtration scheme. Finally, a third type of source model was built based on the spectrum and filtration data provided by the scanner's manufacturer. MC simulations using each of these three source model types were evaluated by comparing the accuracy of multiple CT dose index ͑CTDI͒ simulations to measured CTDI values for 64-slice scanners from the four major MDCT manufacturers. Comprehensive evaluations were carried out for each scanner using each kVp and bowtie filter combination available. CTDI experiments were performed for both head ͑16 cm in diameter͒ and body ͑32 cm in diameter͒ CTDI phantoms using both central and peripheral measurement positions. Both equivalent source model types result in simulations with an average root mean square ͑RMS͒ error between the measured and simulated values of approximately 5% across all scanner and bowtie filter combinations, all kVps, both phantom sizes, and both measurement positions, while data provided from the manufacturers gave an average RMS error of approximately 12% pooled across all conditions. While there was no statistically significant difference between the two types of equivalent source models, both of these model types were shown to be statistically significantly different from the source model based on manufacturer's data. These results demonstrate that an equivalent source model based only on measured values can be used in place of manufacturer's data for Monte Carlo simulations for MDCT dosimetry.

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