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Medical Physics |
1 From the Department of Radiology, Indiana University School of Medicine, 550 N University Blvd, Indianapolis, IN 46202 (H.T.W.M., S.G.J.); Department of Radiology, University of California-Davis Medical Center, Sacramento (J.M.B.); Department of Obstetrics and Gynecology, St Vincent’s Medical Center, Indianapolis, Ind (H.L.B.); Department of Obstetrics and Gynecology, University of Tennessee College of Medicine, Memphis (W.C.M.); and Division of Pulmonary Medicine and Critical Care, New York Methodist Hospital, Brooklyn, NY (G.T.L.). Received September 24, 2001; revision requested November 27; revision received December 17; accepted January 22, 2002. Address correspondence to H.T.W.M. (e-mail: hwinermu@iupui.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Maternal-fetal geometries were determined in 23 pregnant women with varying body mass index and fetal gestational age. Monte Carlo techniques were used to estimate the dose that would be received by each fetus from CT scanning performed with the following parameters: 120 kVp; 100 mA; scanning time, 1 second per section; collimation, 2.5 mm; pitch of 1. Craniocaudal extent of the scan was 11 cm, with the most caudal section edge being 5 mm inferior to the xiphoid process.
RESULTS: For helical CT, estimated mean fetal doses in micrograys at varying gestational ages were as follows: 3.3–20.2 µGy, first trimester; 7.9–76.7 µGy, second trimester; and 51.3–130.8 µGy, third trimester. These values were all less than mean fetal doses reported with scintigraphy, with 37-74 MBq of macroaggregates of human serum albumin labeled with technetium 99m. If 200 mAs (pitch of 1.8) was used, the mean fetal doses were still less than those with scintigraphy.
CONCLUSION: The average fetal radiation dose with helical CT is less than that with ventilation-perfusion lung scanning during all trimesters.
© RSNA, 2002
Index terms: Embolism, pulmonary, 60.72 • Pregnancy, 85.131, 85.47 • Radiations, exposure to patients and personnel • Thorax, CT, 60.1211, 60.12115
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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The incidence of pulmonary embolism (PE) depends on whether deep venous thrombosis (DVT) has been treated adequately. Up to 24% of patients with untreated DVT develop PE, with a mortality rate of approximately 15% (5). Because venous thromboembolism is potentially preventable and treatable, early and accurate diagnosis and treatment are mandatory (6).
Several studies have provided guidelines for investigating PEs in pregnant patients who are suspected of having them. These guidelines attempt to balance diagnostic efficacy and minimization of fetal exposure to ionizing radiation (6–8). Ventilation-perfusion (V-P) lung scanning is still considered to be the primary diagnostic tool for PE in pregnant women (9,10). By using 37–74 MBq of macroaggregates of human serum albumin labeled with technetium 99m, the fetal dose from lung scanning is approximately 100–370 µGy, a relatively low exposure for the fetus (7,8).
Although helical CT is being used more and more to diagnose PE, there are questions about the safety of its use during pregnancy. The purpose of this study was to calculate the mean fetal radiation dose from helical chest CT by using maternal-fetal geometries obtained from healthy pregnant women and to compare the calculated CT doses with the reported fetal doses with scintigraphy.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Transverse dimensions were measured at the widest point of the uterus. The top of the uterus, or fundal height, was marked, and the distance from the xiphoid process to the fundus was measured. Measurements in patients with more than 13 weeks gestation were performed with standard techniques. The top of the fundus of the uterus was again marked by using transabdominal US and measured down to the pubic symphysis and up to the xiphoid process. The widest uterine measurement was obtained by marking the maximum outer width on both sides of the umbilicus by using US and then was measured from mark to mark. Geometric assumptions for the Monte Carlo studies are discussed later.
The following CT protocol was used for calculations: 120 kVp, 100 mAs, 2.5-mm section interval, 11-cm craniocaudal distance, pitch of 1. This distance is generally sufficient to extend from just inferior to the xiphoid process to the aortic arch. With this protocol, the mean fetal doses were calculated by using Monte Carlo techniques in the 23 study patients.
Monte Carlo Calculations
The circumference in each of the 23 women was measured physically, but for the Monte Carlo studies we also needed to obtain an estimate of the relative shape of a patient at the xiphoid process. Assuming an ellipsoidal cross section, we computed the eccentricity from a number of CT scans. CT scans that were acquired as a part of clinical care in 21 consecutive nonpregnant women (mean age, 50.6 years; SD, 17.8) were evaluated. Coronal and sagittal diameters at the level of the xiphoid process were determined by using quantitative measurement software (E-Film; E-Film, Toronto, Ontario, Canada). This analysis of existing patient data was performed with a protocol approved by the institutional review board at University of California-Davis Medical Center, Sacramento, with waiver of informed consent.
An elliptical cross section was assumed, and the eccentricity of each patient at the level of the xiphoid process was calculated on the basis of image-based measurements of the thickness and width of the patient. The mean eccentricity and the median eccentricity were 0.68 and 0.66, respectively (Fig 1). Since the mean age of this group was substantially higher than that of the study group, the eccentricity was evaluated as a function of patient age by using linear regression. Only a slight dependence was found, and, therefore, the mean eccentricity value was used for all 23 pregnant women modeled in the Monte Carlo calculations. Although the eccentricity measurements were obtained in nonpregnant women, the shape of a woman at the level of the thoracic cavity changes only slightly during pregnancy, and, thus, we thought that the eccentricity measurement determined in this group would be reasonably reflective of the pregnant patient.
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Previously validated simple investigational environment for radiology research applications, or SIERRA, Monte Carlo techniques were used for this study (11,12), and additional validation efforts were also performed. The Monte Carlo code propagated 22 million x-ray photons per patient. Photoelectric, Compton, and Rayleigh scattering interactions were modeled with the energy range of 1–150 keV. Energy deposition was determined in water-equivalent ellipsoid (mathematic) phantoms by using the geometries of each of the 23 study patients. The same eccentricity (mean, 0.68) was assumed for each patient.
The x-ray output data of a commercially available CT scanner (CT/i; GE Medical Systems, Milwaukee, Wis) were used to relate milliampere seconds to photon fluence. The normalized output (milliroentgens per milliampere seconds at isocenter versus kilovolt peak) of a scanner was measured by using an exposure meter (MDH 1015; Radcal, Monrovia, Calif) and a 3-cm3 CT pencil chamber. By using a physical scale drawing of an actual beam-shaping filter. The projection thickness of an actual beam-shaping filter was measured on a scale drawing with a ruler and protractor as a function of fan angle, and the thickness was computer fit as a function of angle. The filter composition was synthetic resin (Teflon) (C2F4), with a density of 2.2 g/cm3. The x-ray attenuation of the beam-shaping filter was used to modify the photon distribution in the plane of the fan angle. A source-to-isocenter distance of 63 cm was used. The x-ray spectrum was generated by using a spectral model (13), and a half-value layer of 8.2 mm of aluminum was achieved by filtering the native spectrum with 8.0 mm of aluminum. This half-value layer matched that measured with a clinical scanner at 120 kVp at University of California-Davis Medical Center.
A Monte Carlo simulation was performed in each patient by using the measurements for each maternal-fetal geometry. The axial diameter and craniocaudal length were used to simulate the fetus as a right cylinder at the center of the ellipse that was used to define the mother’s geometry (Fig 1). The energy deposited in this cylindric simulated fetus of water-equivalent unit-density material was used for calculating the dose. The assumption of a cylindric shape for the fetus facilitates the Monte Carlo computation of dose to this region.
Differences in shape between a cylinder and the actual fetus have only a small effect on the fetal dose calculation, as long as the cylinder dimensions simulate the bounds of the fetus. The US-based anatomic measurements were performed to accurately determine these distances. In each woman, the measured circumference at the xiphoid process was used to determine the dimensions of each elliptic semiaxis (values a and b in Fig 1). In addition to calculation of the mean fetal dose, determination of the maximum fetal dose was performed in a separate series of Monte Carlo experiments. The maximum fetal dose was estimated by means of calculating the dose to the uppermost 1 cm of the fetus (the portion closest to the scanned volume).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Fetal Dose Assessment
Maternal and fetal measurements are presented in Table 1. Eight patients were in the first trimester, nine were in the second trimester, and six were in the third trimester. Mean maternal age was 31 years (SD, 6.7).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Numerous studies have been published in which the researchers describe the value of helical CT for PE diagnosis, and findings in most show that helical CT is an accurate tool for diagnosis of PE in main, lobar, and segmental pulmonary arteries. For emboli in these sites, helical CT is approximately 90% sensitive (range, 60%–100%) and 90% specific (range, 80%–100%) (18). Helical CT also has been reported (19) to demonstrate excellent interobserver agreement. However, helical CT is less accurate for imaging peripheral emboli in subsegmental vessels.
When all pulmonary vessels are included, the sensitivity and specificity of helical CT for the diagnosis of PE range from 53%–100% and 75%–100%, respectively (18). Poor contrast opacification, motion artifacts, or technical factors may cause 5%–10% of CT scans to be nondiagnostic (18). In addition, for diagnosis of PE, administration of intravenous contrast material is necessary. Although intravenous administration of nonionic contrast material during pregnancy is performed with other imaging studies (eg, head CT), and studies in pregnant animals have shown no evidence of harm to the fetus caused by nonionic contrast media (20), to our knowledge, no adequate and well-controlled studies have been performed in pregnant women.
Scintigraphy is the primary screening study in the assessment of PE in pregnant patients (9). However, V-P scans are usually not definitive in the diagnosis of acute PE; the presence or absence of PE is inconclusive in up to 80% of these scans (21). Most radiologists categorize abnormal lung scans into three classes according to the probability—low, intermediate, or high—that the finding is a PE. Even for a scan with a high probability, the sensitivity is only 41% (21). Thus, most patients with PE will have scans with intermediate or low probability. Moreover, prior episodes of PE may cause a false-positive result (21).
Magnetic resonance (MR) imaging is an alternative to V-P scanning and helical CT because the fetus is not exposed to ionizing radiation or intravenous contrast material. Moreover, the sensitivity (90%) and specificity (77%) of MR are similar to those of helical CT for the diagnosis of PE (22). However, for this application, long acquisition times are needed, as well as respiratory and cardiac gating, and even then spatial resolution is relatively poor. Availability of this MR protocol is limited as well.
Because clinicians are reluctant to order additional imaging tests in pregnant women following an inconclusive V-P scan, we believe that it is important for them to recognize that helical CT is not only safe during pregnancy but also accurate for the diagnosis of PE in main, lobar, and segmental pulmonary arteries. Accurate diagnosis is critical, because there is substantial risk of morbidity to both mother and fetus from treatment. The recommended therapy for DVT and PE during pregnancy is intravenously administered heparin for 5–10 days, followed by subcutaneously administered heparin for the remainder of the pregnancy (4).
Postpartum therapy includes combined administration of heparin and warfarin initially, followed by administration of warfarin alone for 6 weeks or until at least 3 months of anticoagulation therapy have been completed (4). DVT prophylaxis must be considered during subsequent pregnancies, as the incidence of recurrent PE during each subsequent pregnancy is 4%–15% (23). Furthermore, a history of PE may preclude the future use of oral contraceptives or hormonal replacement therapy (24).
Although it is desirable to limit fetal radiation exposure, a review of the literature suggests that in utero exposure of up to 50,000 µGy results in a negligible increase in the risk of childhood cancer (4,7). With careful use of available procedures, a diagnosis of PE can be made with exposures of less than 5,000 µGy. For example, even the combination of chest radiography (10 µGy), V-P scanning (370 µGy), helical CT scanning (131 µGy), and pulmonary angiography with a brachial approach (500 µGy) exposes the fetus to approximately 1,000 µGy. This dose is less than that received by the fetus from background radiation (eg, cosmic rays, radon, potassium 40) during the 9 months of pregnancy (1,150–2,550 µGy) (25). By comparison, an exposure of at least 100,000 µGy is necessary before pregnancy termination is considered (26).
In summary, findings in this study show that the average fetal radiation dose with helical CT is less than that with V-P lung scanning during all trimesters. Pregnancy should not preclude use of helical CT for the diagnosis of PE.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, H.T.W.M.; study concepts, G.T.L.; study design, H.T.W.M., J.M.B.; literature research, H.T.W.M., J.M.B.; clinical studies, H.L.B.; experimental studies, J.M.B.; data acquisition, H.L.B., J.M.B.; data analysis/interpretation, H.T.W.M., J.M.B., S.G.J.; statistical analysis, J.M.B.; manuscript preparation, S.G.J., H.T.W.M.; manuscript definition of intellectual content, H.T.W.M., J.M.B.; manuscript editing, J.M.B., H.T.W.M., S.G.J.; manuscript revision/ review, S.G.J., H.T.W.M., W.C.M.; manuscript final version approval, H.T.W.M.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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) with the physically measured results (
) and the results derived by means of the EGS4 Monte Carlo calculations (
) as reported by Caon et al (14). The x axis refers to the location along a diameter within a cylindric phantom, with the position of x = 0 cm being at the center. A, Moderate agreement is seen with no bow tie filter. B, Excellent agreement between the SIERRA-derived results and the measured data of Caon et al (14) is seen with the bow tie filter in place. The bow tie filter was used in the development of the pediatric dose values in this study.
