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...

Tube current modulation was designed to reduce radiation dose in CT imaging while maintaining overall image quality. This study aims to develop a method for evaluating the effects of tube current modulation (TCM) on organ dose in CT exams of actual patient anatomy. This method was validated by simulating a TCM and a fixed tube current chest CT exam on 30 voxelized patient models and estimating the radiation dose to each patient’s glandular breast tissue. This new method for estimating organ dose was compared with other conventional estimates of dose reduction. Thirty detailed voxelized models of patient anatomy were created based on image data from female patients who had previously undergone clinically indicated CT scans including the chest area. As an indicator of patient size, the perimeter of the patient was measured on the image containing at least one nipple using a semi-automated technique. The breasts were contoured on each image set by a radiologist and glandular tissue was semi-automatically segmented from this region. Previously validated Monte Carlo models of two multidetector CT scanners were used, taking into account details about the source spectra, filtration, collimation and geometry of the scanner. TCM data were obtained from each patient’s clinical scan and factored into the model to simulate the effects of TCM. For each patient model, two exams were simulated: a fixed tube current chest CT and a tube current modulated chest CT. X-ray photons were transported through the anatomy of the voxelized patient models, and radiation dose was tallied in the glandular breast tissue. The resulting doses from the tube current modulated simulations were compared to the results obtained from simulations performed using a fixed mA value. The average radiation dose to the glandular breast tissue from a fixed tube current scan across all patient models was 19 mGy. The average reduction in breast dose using the tube current modulated scan was 17%. Results were size dependent with smaller patients getting better dose reduction (up to 64% reduction) and larger patients getting a smaller reduction, and in some cases the dose actually increased when using tube current modulation (up to 41% increase). The results indicate that radiation dose to glandular breast tissue generally decreases with the use of tube current modulated CT acquisition, but that patient size (and in some cases patient positioning) may affect dose reduction.

Objective-The aims of this study were to estimate the dose to radiosensitive organs (glandular breast and lung) in patients of various sizes undergoing routine chest CT examinations with and without tube current modulation; to quantify the effect of tube current modulation on organ dose; and to investigate the relation between patient size and organ dose to breast and lung resulting from chest CT examinations.Materials and Methods-Thirty voxelized models generated from images of patients were extended to include lung contours and were used to represent a cohort of women of various sizes. Monte Carlo simulation-based virtual MDCT scanners had been used in a previous study to estimate breast dose from simulations of a fixed-tube-current and a tube current-modulated chest CT examinations of each patient model. In this study, lung doses were estimated for each simulated examination, and the percentage organ dose reduction attributed to tube current modulation was correlated with patient size for both glandular breast and lung tissues.Results-The average radiation dose to lung tissue from a chest CT scan obtained with fixed tube current was 23 mGy. The use of tube current modulation reduced the lung dose an average of 16%. Reductions in organ dose (up to 56% for lung) due to tube current modulation were more substantial among smaller patients than larger. For some larger patients, use of tube current modulation for chest CT resulted in an increase in organ dose to the lung as high as 33%. For chest CT, lung dose and breast dose estimates had similar correlations with patient size. On average the two organs receive approximately the same dose effects from tube current modulation. Conclusion-The dose to radiosensitive organs during fixed-tube-current and tube currentmodulated chest CT can be estimated on the basis of patient size. Organ dose generally decreases with the use of tube current-modulated acquisition, but patient size can directly affect the dose reduction achieved. In discussion of individual risk during CT, organ dose is a better measure for estimating patient risk than is effective dose because effective dose is intended for estimating radiation exposure of entire populations, not for individual dose estimates [6,7]. Most existing methods for estimating radiation dose to the radiosensitive organs during chest CT are based on measurements or simulations of a single patient model or phantom with unrealistic anatomic features (e.g., breasts modeled as a homogeneous material located directly anterior to the thoracic region) [2,[8][9][10][11]. It is not known how well these methods serve for estimation of organ doses in an actual patient population, which includes natural variations in patient size and breast composition. KeywordsThe primary limitation of most previous tools used for estimation of organ dose during chest CT may be that those methods do not account for the effects of tube current modulation. Tube current modulation is an acquisition technique often used in CT scanners to decrease the ov...

No abstract

No abstract