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Radiation Dose to the Fetus from Body MDCT During Early Gestation

¹ Department of Radiology, Duke University Medical Center, DUMC 3808, Durham, NC 27710.
² Radiation Safety Division, Duke University Health System, Durham, NC 27710.

Received December 17, 2004; accepted after revision February 7, 2005.

 
Address correspondence to L. M. Hurwitz.

Abstract

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OBJECTIVE. The purpose of our study was to determine radiation dose to the fetus at early gestation when contemporary MDCT scanners are used for common clinical indications.

MATERIALS AND METHODS. Anthropomorphic phantoms were constructed to reflect a pregnant woman. Thermoluminescence dosimeters (TLDs) and metal oxide semiconductor field effect transistor (MOSFET) detectors were placed in appropriate locations to determine real-time radiation exposure to the fetus at 0 and 3 months' gestation. Imaging was performed on a 16-MDCT scanner using current institutional CT protocols: renal stone (140 kVp, 160 mA, rotation time of 0.5 sec, 16 x 0.625 mm), appendix (140 kVp, 340 mA, rotation time of 0.5 sec, 16 x 0.625 mm), and pulmonary embolus (140 kVp, 380 mA, rotation time of 0.8 sec, 16 x 1.25 mm).

RESULTS. The radiation dose to the fetus at 0 and 3 months, respectively, was as follows: renal stone protocol, 0.8–1.2 and 0.4–0.7 cGy; appendix protocol, 1.52–1.68 and 2–4 cGy; and pulmonary embolus protocol, 0.024–0.047 and 0.061–0.066 cGy.

CONCLUSION. Radiation doses to the fetus from institutional MDCT protocols that may be used during pregnancy (for pulmonary embolus, appendicitis, and renal colic) are below the level thought to induce neurologic detriment to the fetus. Imaging the mother for appendicitis theoretically may double the fetal risk for developing a childhood cancer. Radiation doses to the fetus from pulmonary embolus chest CT angiography are of the same magnitude as ventilation–perfusion (V/Q) scanning.

Keywords: CT • fetal imaging • radiation dose • women's imaging

Introduction

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The American College of Radiology recommends that when imaging is required in the evaluation of the pregnant woman, nonionizing techniques such as sonography and MRI be used as the first choice [1, 2]. However, CT is often used for such indications as suspected pulmonary embolus, appendicitis, renal colic, or trauma including pregnancy in some situations [3–6]. The use of CT in this setting likely stems from its familiarity and wide availability and from the fact that sonography is often limited in the pregnant woman. Recent reports regarding physician practice and perception when using CT in the pregnant woman illustrate a varied understanding about the proper use and risks of this imaging technique [7–9]. For example, differences between community-based and university-centered radiologists and clinicians exist when evaluating pulmonary embolus, with some favoring ventilation–perfusion (V/Q) radionuclide scanning and others favoring CT [7, 9]. Confusion also exists regarding the potential teratogenic effects on the fetus resulting from abdominal CT during pregnancy.

Fetal radiation doses from maternal body CT have been measured for conventional axial CT scanners and single-detector helical CT scanners [10–12]. Radiation doses to the fetus from increasingly used 16-MDCT scanners using contemporary clinical CT protocols have not been reported, to our knowledge. Major design differences exist between single-detector CT and MDCT machines, and care must be exercised in developing scanning protocols to avoid additional radiation to the fetus [13]. Radiation doses from MDCT protocols have been reported to be greater than those from single-detector CT [14]. The purpose of this study was to determine fetal radiation doses resulting from 16-MDCT of the chest, abdomen, and pelvis during early pregnancy using direct measurements of radiation-absorbed dose in an anthropomorphic phantom designed to simulate a gravid woman.

Materials and Methods

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Phantom
A commercially available female anthropomorphic phantom (model 702, CIRS) was modified to model the gravid woman. The phantom specifications were height, 160 cm; weight, 55 kg; thoracic dimensions, 20 x 25 cm; and abdominal dimensions, 19 x 33 cm. This phantom is made of human-tissue-equivalent material and has been validated to have the same X-ray interaction as living human beings [15]. We modified the phantom to reflect the possible expected location of the fetus at conception and at 3 months' gestation, because this is the time of organogenesis and neural crest development and therefore the time of greatest radiosensitivity for the developing fetus [16]. Landmarks for uterine location and shape for 0 and 3 months' gestation were determined on the basis of parameters of the MIRD (medical internal radiation dosimetry) mathematic phantom series for the pregnant woman [17].

Absorbed radiation dose to the fetal location was determined using our institutional contemporary clinical body MDCT protocols for pulmonary embolus, appendicitis, and renal stone imaging (Table 1). Anatomic landmarks for our imaging studies are as follows: chest pulmonary embolus CT: above the level of the aortic arch to the diaphragms; appendix CT: above the dome of the liver to the pubic symphysis; and renal stone CT: level of the kidneys to the pubic symphysis. Imaging was performed on a 16-MDCT scanner (LightSpeed, GE Healthcare).

Detector Descriptions and Placement
MOSFET (metal oxide semiconductor field effect transistor) model 1002RD (Thomson-Nielsen) dosimeters were placed at five locations representing various fetal locations in the uterus at 0 and 3 months' gestation (Fig. 1A). MOSFET dosimeters provide a real-time readout capability [18]; however, because of a less sensitive lower limit of detection, they were used only for abdominopelvic protocols, in which a fetus is directly exposed to the primary CT beam. For chest CT protocols, thermoluminescence dosimeters (TLDs), model TLD-100 (Thermo Electron) were used because a fetus is exposed to only scatter radiation outside the primary CT beam, and this small amount of radiation is more accurately detected with TLD methodology. The lower limits of detection for the TLD-100 and MOSFET model 1002RD are 0.20 and 1.50 mGy, respectively [19].

View larger version (72K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —Anthropomorphic phantom of a woman during early pregnancy. Drawing shows detector representing fetal location in anthropomorphic phantom of woman during early pregnancy. Points A and B are maximal most anterior and cranial locations at 3 months and points C, D, and E are fetal location at 0 months. Reprinted with permission from Vladimir Varchena, Senior Engineer, CIRS, Inc.

View larger version (14K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —Anthropomorphic phantom of a woman during early pregnancy. Bar graphs show absorbed fetal radiation dose at 0 and 3 months' gestation from 16-MDCT of chest, pulmonary embolus protocol (B); of abdomen and pelvis, renal calculus protocol (C); and of abdomen and pelvis, appendix protocol (D).

View larger version (15K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C —Anthropomorphic phantom of a woman during early pregnancy. Bar graphs show absorbed fetal radiation dose at 0 and 3 months' gestation from 16-MDCT of chest, pulmonary embolus protocol (B); of abdomen and pelvis, renal calculus protocol (C); and of abdomen and pelvis, appendix protocol (D).

View larger version (13K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D —Anthropomorphic phantom of a woman during early pregnancy. Bar graphs show absorbed fetal radiation dose at 0 and 3 months' gestation from 16-MDCT of chest, pulmonary embolus protocol (B); of abdomen and pelvis, renal calculus protocol (C); and of abdomen and pelvis, appendix protocol (D).

Effective Dose Calculation
Effective doses from these three imaging protocols were computed according to the International Commission on Radiological Protection (ICRP) 60 and ICRP 26 methods [21, 22] and are listed in Table 2. This permits a comparison of imaging protocols we performed with those of other investigations.

Results

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Radiation doses to the fetus from pulmonary embolus, appendix, and renal stone MDCT examinations are shown in Figures 1B, 1C, and 1D. For the chest pulmonary embolism protocol, the fetus was exposed to only scatter radiation. The radiation recorded from this protocol measured less than 700 µGy (240–660 Gy) (Fig. 1B). The radiation dose was greater for both the appendix and renal stone protocols, but the renal stone protocol was lower. Fetal doses at 0 months ranged from 1.52 to 1.68 cGy for the appendix MDCT protocol and from 0.4 to 0.72 cGy for the renal stone MDCT protocol. Fetal doses at 3 months were 1.99–3.22 cGy for the appendix protocol and 0.85–1.17 cGy for the renal stone MDCT protocol (Figs. 1C and 1D). Maternal effective doses were also measured for each protocol (Table 2).

Discussion

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Our study of contemporary 16-MDCT imaging protocols provides a range of fetal radiation doses to the first trimester fetus. The dose to the fetus in the first trimester for the chest pulmonary embolus protocol is low, in agreement with other investigations. The fetal dose is close to that of a V/Q scan at the beginning of the first trimester, and approximately double that of the V/Q scan at the change from the first to the second trimester. The dose to the fetus in this time period for CT protocols that directly irradiate the uterus is higher than those that limit direct irradiation to the thoracic cavity of the mother.

For pulmonary embolus MDCT examinations, there is no direct radiation to the fetus. Only scatter radiation is involved, and the dose to the fetus is relatively small (240–660 µGy). The fact that the fetal dose is small is in agreement with the findings of other investigators; however, these other studies indicated even lower fetal radiation levels from CT of pulmonary embolus, 3.3–20.2 µGy in the first trimester and up to 76.7 µGy in the second trimester [10]. This difference could be due to differences in imaging protocols, because the CT technique consisted of a lower peak kilovoltage of 120, lower tube current of 100 mA, and thicker collimation of 2.5 mm, and were performed on single-detector helical scanners compared with our protocols that were performed using a 16-MDCT scanner. In addition, the scanners were from different manufacturers. Furthermore, differences in how radiation dose was determined—Monte Carlo geometric model design versus direct measurements with an anthropomorphic phantom—between the two studies also account for some change in the radiation dose calculation. Although they were still relatively low, our investigation did find that the radiation doses received by the fetus were greater by a factor of 16 at 0 months (320 vs 20 µGy) and by almost a factor of 10 at 3 months (660 vs 77 µGy) compared with prior reports in the literature.

Our results refute the assertion that a pulmonary embolus MDCT protocol will yield lower radiation doses to the fetus than a V/Q scan [10], which provides a fetal dose of 320–360 µGy. In our institution, fetal radiation exposure at early gestation from a pulmonary embolus MDCT protocol is approximately 320 µGy. As a comparison, our institutional clinical protocol for a V/Q scan during pregnancy uses 2.0 mCi of technetium-99m macroaggregated albumin (MAA) and 10 mCi of xenon-133 gas and results in a fetal absorbed radiation dose of approximately 320–360 µGy. Many institutions reduce the radiation dose from V/Q scans when imaging a pregnant woman by eliminating ventilation imaging, reducing ventilation imaging to a single breath-hold view, or reducing the amount of the perfusion agent [23]. During the third month of gestation, the absorbed radiation dose (660 µGy) for MDCT is approximately twice that resulting from V/Q scanning (320 µGy) [24–26] (Table 3). We realize that differences in clinical MDCT protocols among institutions and among individual scanners may result in a fetal dose that is greater than or equivalent to that of V/Q scanning but is unlikely to be significantly lower, as has been reported with single-detector CT scanners.

For the CT protocols (renal stone and appendicitis assessment) resulting in direct fetal exposure, the doses are obviously higher than for the thoracic imaging protocols. Similar to other institutions, our renal calculus protocol uses a lower tube current setting to reduce radiation exposure because this has been validated for renal calculus imaging [27]. Our institution uses a full abdominal and pelvic protocol for appendix imaging because differentiating appendicitis from other abdominal and pelvic disorders can be difficult during pregnancy. Options for reducing radiation dose for appendix imaging by limiting z-axis coverage have been suggested [28]. To our knowledge, there are no reports of directly measured radiation doses to the fetus based on the abdominal protocols designed for imaging for suspected appendicitis and renal colic.

Knowledge of fetal dose and associated risk estimates is critical in assessing the risk–benefit ratio of pregnant women in the first trimester who present with clinical features of renal colic or acute abdominal pain typical of appendicitis. The major risks to the fetus with these low levels of exposure are neurologic and carcinogenic in etiology. Based on the protocols in our study, the fetal radiation dose is below the threshold dose thought to induce significant neurologic detriment. According to sources, including the ICRP (using data derived from atomic bomb survivors who were pregnant at the time of exposure), one would not expect significant neurologic impairment unless a dose of 10 cGy or more is delivered to the fetus [29, 30]. With our protocols, the maximum dose was 3.5 cGy.

The correlation between prenatal radiation exposure and carcinogenesis is less well established, and the doses delivered in some of the protocols used here theoretically could double the chance of developing childhood cancer [31]. Data collected from the Japanese atomic bomb survivors and medical radiographic studies show a trend toward increased risk of carcinogenesis after early gestational irradiation [32]. Conclusions as to the exact risk for developing childhood cancer from prenatal irradiation are limited because of uncertainty of the risk per unit dose. For radiation protection purposes, a linear no-threshold model has been used to determine that a relative risk for development of childhood cancer exists when the early-gestational-age fetus is exposed to low-dose radiation of the type and quantity seen with diagnostic radiographic examinations. This relative risk has been reported to be 2.7 for early gestational age [33]. In addition, the excess relative risk (ERR) of developing childhood cancer has been estimated to be approximately 0.28 at 1.0 mGy in the first trimester, 0.03 at 1.0 mGy in the third trimester [34], and overall 0.037 at 1.0 mGy [35] during pregnancy. Using these figures, one can estimate that the overall risk of childhood cancer for a fetus incurring 3.0 cGy of radiation exposure from appendix MDCT protocols is approximately 2 in 600, as opposed to approximately 1 in 600 [36] for the control general population in developed nations. Although the risk remains low, it is nevertheless twice that of background radiation. On the basis of data from this investigation, health care providers can conduct a more informed discussion with both patients and other providers (such as surgeons, obstretricians, and emergency physicians) regarding the potential risks of CT early in pregnancy. As was pointed out recently, this type of discussion in the emergency setting is often lacking [37].

Strategies to reduce radiation exposure to the fetus from MDCT can apply techniques used in other imaging protocols [38]. These include lowering the tube current, limiting coverage in the z-axis, increasing the helical pitch, and reducing the gantry cycle time.

Limitations of this study include obtaining data from a phantom simulating a single body habitus. We recognize that radiation doses vary depending on the size of the individual. In addition, exact dose will depend on imaging protocols used at different institutions. Further work in evaluating radiation exposure to the fetus in the second and third trimesters is of importance and has yet to be accurately performed because an anatomically correct phantom to mimic the changes of pregnancy during this gestational time does not yet exist.

In conclusion, the various options for imaging pulmonary embolus, appendicitis, and renal calculus should be considered before making recommendations, particularly in pregnant women. Sonography and MRI have been shown to be useful in this setting [1, 2, 39, 40]. However, MDCT has become increasingly available and increasingly used. Radiologists and referring physicians need to remember that radiation to the fetus may occur with CT and understand the potential risks as elucidated in our study. Radiation doses to the fetus from institutional MDCT protocols that could be used during pregnancy to image these entities are below the level thought to induce significant neurologic detriment to the fetus. The fetal risk for developing childhood cancer theoretically may double when imaging the mother for appendicitis. This risk is reduced when imaging for renal calculi or pulmonary embolus during pregnancy because the overall absorbed radiation doses are reduced. Imaging for pulmonary embolus during pregnancy can be performed with both MDCT and V/Q scanning, which will yield similar absorbed fetal doses during early gestation.

Acknowledgments
 
We thank Vladimir Varchena, of CIRS, Norfolk, VA, for providing a blueprint of the pelvis of the CIRS adult female phantom.

References

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