Pediatric Imaging
Original Research
Follow-Up Study on Fetal CT Radiation Dose in Japan: Validating the Decrease in Radiation Dose
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OBJECTIVE. In 2011, we collected data on fetal CT radiation dose to determine the diagnostic reference level (DRL); however, continuous evaluation of the DRL is necessary. The hypothesis of this study is that the fetal CT radiation dose has decreased, and we predict a widespread use of iterative reconstruction (IR). We also predict that the national decrease in exposure is because of the DRL reported as a result of the previous national study.
MATERIALS AND METHODS. Various testing protocols from each site were summarized as part of the study results. The minimum, one-fourth (25th percentile), median, three-fourths (75th percentile), and maximum values were obtained for volume CT dose index (CTDIvol), dose-length product (DLP), and scan length of 120 fetal CT examinations. The trends for IR usage and tube voltage were also investigated.
RESULTS. Compared to the results of the 2011 study (n = 119), the minimum, 25th percentile, median, and 75th percentile values for CTDIvol and DLP have decreased for the tabulated results in 2015 (n = 120). The 75th percentile value for CTDIvol was 4.9 mGy, which is 43% of the previous value. IR was used in 70% of the sites. The radiation dose was significantly lower among groups that used IR.
CONCLUSION. Four years passed between our initial survey on DRL and the present follow-up survey, and it appears that the previous report sufficiently fulfilled its objective and role in contributing to the decrease in DRL observed in this follow-up study.
Keywords: CT, diagnostic reference level, fetus, prenatal diagnosis, skeletal dysplasia
According to an international classification issued in 2015 [1], there are 436 disease names, classified into 42 disease groups, and 364 responsible genes associated with genetic disease skeletal dysplasia. During the neonatal period, these diseases are found at a rate of two in 10,000 births, and half are lethal [2]. The recent widespread use of fetal ultrasound testing and improvement in diagnostic accuracy has led to the detection of short limbs and more cases of suspected skeletal dysplasia [3]. The advantage of ultrasound is that it can be used for pregnant women without involving x-ray irradiation. However, it is extremely difficult to detect, evaluate, and interpret the abnormal findings of skeletal structures using ultrasound, and a specific diagnosis cannot be made by the ultrasound operator without vast knowledge and experience in making such diagnoses. Unless the region that may contain the abnormalities is searched in detail during the scan, sufficient material for retrospective diagnosis cannot be obtained. This is the dis-advantage of ultrasound testing that is based on real-time diagnosis.
However, the effectiveness of prenatal 3D CT for detecting fetal skeletal dysplasia has been previously reported, and 93% of pathognomonic abnormal findings confirmed after birth with simple x-ray were diagnosable with fetal skeletal CT. It was reported that the diagnosis determined by ultrasound was revised for about 60% of the cases after fetal CT [4].
Prenatal diagnosis using CT provides visual and clinical information to facilitate planning for a birthing method and selection of treatment by the various people involved, including the parents, family members, obstetrician, and perinatal care staff. It also allows the parents of the fetus to prepare themselves psychologically. Although there are many advantages, the risk of radiation exposure to the fetus and mother cannot be avoided because CT uses x-rays.
In 2011, we surveyed medical institutions in Japan that conduct fetal CT and collected data on CT conditions and radiation exposure; that study was published in 2014 [5]. From the data, we reported the estimated diagnostic reference level (DRL). To our knowledge, that report was the first and only fetal CT DRL study in the world. We decided to investigate DRL again in 2015 because 3 years and 9 months had passed since the initial survey was conducted. Periodic assessment of DRL every few years at each medical institution or at the national level is recommended [6]. Since the first study was conducted, CT equipment has seen further progress in the number of detectors, and high-performance CT equipment with 64 or more detectors are almost in common use. In addition, there are some reports in the pediatric radiology field that show an increase in popularity for using an iterative reconstruction (IR) protocol to reduce the radiation dose [7].
The hypothesis of this study is that the fetal CT radiation dose has decreased throughout the country compared with the nationwide study conducted in Japan in 2011 [5], and we predict the widespread usage of fetal CT with IR. We also predict that the national decrease in exposure is because of the DRL reported as a result of the previous national study.
| Materials and Methods |   |
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This study was approved by the ethics board of the National Center for Child Health and Development and was conducted after approval of the institutional review board.
The 16 sites that were studied in the 2011 survey [5] are major facilities performing fetal CT in Japan and were included in this study. In addition, an Internet forum for fetal skeletal dysplasia based in Japan, the Japan Forum of Fetal Skeletal Dysplasia (with 53 registered users), was used to call for participation by medical institutions that did not participate in the previous study. Medical institutions that consulted the radiologist and obstetrician members of the forum were also selected as candidates. Furthermore, medical institutions that have presented results in past society meetings in Japan and overseas, or submitted journal papers, were sought and considered as candidates. As a result, 25 medical institutions were selected as study sites. These study sites were mailed a CD-ROM with the questionnaire sheet, together with a letter inquiring about their interest regarding participation. The following 22 medical institutions expressed interest: Hokkaido University Hospital; Aomori Prefectural Central Hospital; Miyagi Children's Hospital; Tohoku University Hospital; Yamagata University Hospital; Chiba Kaihin Municipal Hospital; Juntendo University Urayasu Hospital; National Center for Child Health and Development; Jikei University School of Medicine Hospital; Tokyo Women's Medical University Hospital; Fujita Health University Hospital; Nagara Medical Center; University Hospital, Kyoto Prefectural University of Medicine; Osaka Medical Center and Research Institute for Maternal and Child Health; Hyogo College of Medicine Hospital; Shikoku Medical Center for Children and Adults; Perinatal Medical Center, Ehime Prefectural Central Hospital; Kochi Health Sciences Center; Tokushima University Hospital; Hiroshima University Hospital; Perinatal Medical Center, Yamaguchi Prefectural Grand Medical Center; and Kurume University Hospital.
The first page of the work sheet was to be entered by the obstetrician of the hospital, and the remaining pages were to be entered by the radiologic technologist or radiologist. Eight questions were listed on the first page. First, what is the category of your hospital (choice of three options: university hospital, perinatal medical center, or regional core hospital)? Second, was specific written consent for fetal CT obtained? Third, how many fetal CT examinations have been conducted in the past 3 years and 9 months? Fourth, at what range of fetal stage (weeks) was CT conducted and what was the mean number of weeks? Fifth, on the basis of the 2011 study [5], have there been any changes to the protocol? Sixth, if the protocol has been changed, has the diagnostic ability decreased as a result of lowering the dose? Seventh, is sedation used on the fetus when conducting fetal CT? Finally, do you know the approximate fetal CT radiation dose for your hospital?
The CT survey from the second page onward had entry sheets for four manufacturers: Toshiba, GE Healthcare, Siemens Healthcare, and Philips Healthcare. The sheet for the manufacturer of the equipment used for testing at each site was also filled out. If multiple pieces of CT equipment were used at one site, we requested that each piece was entered into separate work sheets. Because parameters are different depending on the manufacturer, the work sheet entry items tailored for each CT manufacturer were chosen by a subgroup of seven radiologic technologists selected from sites that frequently perform fetal CT. The staff in charge of setting imaging protocols for each manufacturer at each of the four CT manufacturers was requested to check for the appropriateness of the chosen work sheet items, and the revised items were distributed. The content of the work sheet differs for each company, but basic items include tube voltage, tube current, scan time, pitch, scan FOV, scan length, volume CT dose index (CTDIvol), dose-length product (DLP), whether IR is used for scanning and 3D imaging, and the name and degree of IR protocol used. This questionnaire was prepared using Excel software (version 2013, Microsoft) and was distributed to each site.
The study started when the survey was sent on December 5, 2014, and results were collected by January 31, 2015. The study implementation period was 58 days.
The frequency of usage for each piece of CT equipment and the protocol was summarized from the survey results. The CT parameters used at the sites were also compared among the results using the same protocol. The trend in results for the protocol that used the lowest dose and the protocol that used the highest dose was studied.
The minimum, one-fourth (25th percentile), median, three-fourths (75th percentile), and maximum values were obtained for the CTDIvol, DLP, and scan length from the 139 fetal CT examinations collected in this 2015 study. The results were compared with the previous results in 2011 [5], and the changes over the 4 years were evaluated. In addition, the 75th percentile values of CTDIvol and DLP in this study were determined as the new DRL. The median value was compared for scan length.
Regarding usage of IR, the frequency of usage was studied and the difference in CTDIvol value was compared among cases with and without IR use.
Regarding tube voltage, the frequency of usage of 80, 100, and 120 kV was studied. The results were compared with those of the 2011 study [5]. In addition, the CTDIvol values for the frequently used voltages, 100 and 120 kV, were compared among the different protocols used. For statistical analysis of the data, an unpaired t test was used to test for significance (Excel version 2013, Microsoft).
| Results | |
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Category of your hospital—The breakdown of the 22 sites was 13 university hospitals (59%), followed by six perinatal centers (27%), and three regional core hospitals (14%). However, there were inconsistencies in CT dose data on two sites. These sites were excluded and a total of 20 sites were used for analysis.
Specific written consent for fetal CT— Sixteen sites (80%) obtained specific written consent for conducting fetal CT.
Number of fetal CT examinations conducted in the past 3 years and 9 months— The number of cases per site during this time period ranged from one to 24 and there were 139 cases in total.
The range of and mean gestational week when CT was conducted—The range was 17–36 weeks' gestation, and the mean (± SD) was 30.1 ± 3.1 weeks.
Knowledge of the previous study report (2011), and whether the protocol has been changed—Six sites changed their protocol on the basis of the 2011 DRL (30%). Seven sites (35%) each answered that the study results did not affect their protocol or they did not know about the 2011 DRL.
Whether diagnostic ability decreased because of protocol change—No sites that changed their protocol answered that diagnostic ability decreased by lowering the dose.
Use of sedation on fetus during fetal CT— Two sites (10%) used sedation when conducting fetal CT, but the remaining 18 sites (90%) did not use sedation. One site that used sedation answered “walking, use of Diazepam in some cases,” and the other site did not give a detailed response.
Knowledge of an approximate fetal CT radiation dose at own site—Obstetricians at 17 sites (85%) answered that they knew the approximate fetal CT radiation dose at their own site.
Table 1 summarizes the survey results about the 32 CT protocols used by the sites for each scanner manufacturer. The most common scanner manufacturer among the survey sites was Toshiba; however, no obvious difference of parameter setting was identified among CT vendors.
There was a wide range of radiation doses as measured by CTDIvol and DLP, and the lowest scanning condition was a CTDIvol of 0.5 mGy with the full IR method (Veo, GE Healthcare), even though full IR was used for only 10% of the protocols. This is somewhat surprising, because the maximum values of CTDIvol and DLP (27.5 mGy and 943 mGy·cm, respectively) were 50 times as large as the minimum settings (0.5 mGy and 18 mGy·cm, respectively). Approximately 68% of CT protocols were performed using a tube voltage of 120 kV.
Among the 139 fetal CT examinations performed at the study sites during the study period, inquiries were made to the staff in charge of setting imaging protocols for each manufacturer regarding incomplete data submitted from the sites to recover and ensure consistency in the data. However, inconsistencies in data on CTDIvol and DLP for 19 cases could still not be resolved. These data were excluded, and a total of 120 cases were used for analysis.
The comparison of values for CTDIvol, DLP, and scan length in 2011 and 2015 is shown in Table 2, and boxplots are shown in Figures 1–3. Compared with the study in 2011 (n = 119), the minimum, median, 75th percentile, and 25th percentile values of CTDIvol and DLP for the tabulated results in 2015 (n = 120) were lower, although some sites had a higher maximum value than that of the previous study. The 75th percentile value of CTDIvol that is the DRL for this study was 4.9 mGy, and this was 43% of the previous value of 11.3 mGy. During this period, a 57% reduction in the radiation dose was found (Fig. 1). Similar to CTDIvol, the 75th percentile value for DLP was 176.4 mGy·cm, and this was 46% (about half) of the previous value of 382.4 mGy·cm. The differences were statistically significant (p < 0.01) (Fig. 2). On the other hand, the scan range was almost the same as in the previous study, and although the data included over 100 cases, the median was different by only 6 mm, and the data were considered to be accurate. It was found that the decrease in DLP was not caused by a shortening of the length (Fig. 3).
 View larger version (16K) | Fig. 1 —Boxplot of volume CT dose index (CTDIvol) values in 2011 and 2015. Boxes denote ranges, lines in boxes denote medians, and vertical lines and whiskers denote 95% CIs. |
 View larger version (18K) | Fig. 2 —Boxplot of dose-length product (DLP) values in 2011 and 2015. Boxes denote ranges, lines in boxes denote medians, and vertical lines and whiskers denote 95% CIs. |
 View larger version (18K) | Fig. 3 —Boxplot of scan length values in 2011 and 2015. Boxes denote ranges, lines in boxes denote medians, and vertical lines and whiskers denote 95% CIs. |
In this study, 14 of 20 sites (70%) used IR. Among the 14 sites that used IR, six (43%) did not use the method at the beginning of the study period but started to incorporate it during the study period. As shown in Figure 4, a comparison of CTDIvol between CT with IR (n = 72) and CT without IR (n = 48) shows that it is lower in the group that uses IR than the group that does not use IR, although one site had set a significantly higher radiation dose than the others (section surrounded by the dashed line in Fig. 4). If the site surrounded by the dashed line is excluded, the CTDIvol for sites using IR compared with those that do not use IR is statistically significantly lower (p < 0.01). The group that uses IR had a significantly lower radiation dose setting compared with the group that did not (Table 3).
 View larger version (26K) | Fig. 4 —Comparison of median volume CT dose index (CTDIvol) between CT performed with and without iterative reconstruction (IR). If sites surrounded by dashed line are excluded, CTDIvol values for sites using IR are lower than for those that do not use IR, with statistically significant difference between groups (p < 0.01). |
Tube voltage was 120 kV in 52 cases, 100 kV in 59 cases, and 80 kV in nine cases, with 100 kV the most frequently used voltage. Six sites had the voltage fixed at 120 kV, whereas the other 14 sites used a tube voltage lower than 120 kV.
As with IR, comparison of CTDIvol values by tube current (120 vs 100 kV) showed that the median CTDIvol values for 120 and 100 kV were 3.3 and 2.5 mGy, respectively. As shown in Figure 5, the tube voltage was set at a significantly higher radiation dose in one site in the group that used IR (section surrounded by a dashed line in Fig. 5). In contrast to the results for IR, the results for tube current did not show a significant difference in CTDIvol even when the site surrounded with the dashed line was excluded.
 View larger version (25K) | Fig. 5 —Comparison of median volume CT dose index (CTDIvol) values at 120 and 100 kV. In contrast to results for iterative reconstruction, results for tube current did not show significant difference in CTDIvol even when sites surrounded by dashed line were excluded. |
| Discussion | |
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From the response to the questions answered by the obstetricians, the following situation was revealed. In Japan, 86% of fetal CT examinations were performed at university hospitals or public perinatal centers that act as the center for medical care in that region. It is assumed that obstetricians and gynecologists at local clinics who detect short limbs by fetal ultrasound send a referral to obstetricians or fetal examination specialists for retesting by ultrasound with higher accuracy, and then the case is indicated for CT if necessary.
In Japan, 80% of the sites obtained specific written consent for fetal CT with informed consent from the parents regarding fetal radiation exposure.
The mean timing for the CT was 30.1 ± 3.1 weeks' gestation. This is thought to be the result of a recommendation by the leading academic research group on fetal CT in Japan, the Japan Forum of Fetal Skeletal Dysplasia, which recommends testing at around 30 weeks. This recommendation is based on several reasons, including the fact that the effect of radiation on the CNS is less of a concern during the third trimester of pregnancy [8] and that depiction of the skeletal structure is clearer compared with the early stages of pregnancy because of fetal growth. The result was almost the same as the previous study [5] (mean, 30.2 ± 2.6 weeks) with essentially no change. It is understandable that the sites that perform CT are actually following the recommendations of the Japan Forum of Fetal Skeletal Dysplasia.
Thirteen sites (65%) were aware of the DRL results from the 2011 study [5], and six sites (30%) had changed their protocol on the basis of the 2011 DRL. On the other hand, seven sites (35%) answered that they were not aware of the study results. DRL was proposed in the 1980s to optimize simple x-ray radiation dose and it was introduced as a method for optimizing CT radiation dose in the 1990s [7, 9]. It is considered that there is high awareness of DRL in the sites performing fetal CT, and DRL is correctly functioning as a regulatory pressure on the entire community as intended. No sites responded that a lower dose resulting from protocol changes led to decreased diagnostic ability, and this further reinforces that these scanning conditions are appropriate.
It was revealed that 90% of sites did not sedate the fetus for fetal CT. In the previous survey, one site used pancuronium (Mioblock, MSD) to suppress fetal movement, but this response was not found in the current study. It is considered that sedation is not necessary for fetal CT.
Obstetricians at 17 sites (85%) knew the approximate fetal CT radiation dose of their site when they order the scan. According to a literature search, it has been reported in a past survey that only 16% of residents in a radiology department knew the radiation dose for abdominal CT [10]. The obstetricians at sites performing fetal CT may be aware of the dose because they are required to explain the dose to patients. This may be inaccurate because the specific number was not asked, but compared with past reports, obstetricians who are involved in fetal CT are considered to have a fairly high awareness of CT exposure.
This study has revealed that since the 2011 study [5], the DRL for fetal CT in Japan has been reduced by half, a result that was statistically significant. This may be due to three factors. One is the first-ever proposal of fetal CT DRL in Japan from the study results in 2011. The results were widely reported at fetal CT forums and research groups that were active in performing fetal CT. A total of 65% of the sites studied in this analysis answered that they were aware of the results, and, furthermore, 30% of sites responded that they had lowered their dose setting on the basis of the DRL of the previous study. This is the result of DRL fulfilling its correct function, and it could be evaluated as an extremely significant result for managing CT radiation exposure in Japan. It is considered that DRL should be studied every few years and, indeed, this study was conducted at the appropriate time. We should conduct another national survey in 3–4 years and confirm the decrease in DRL as a community.
The next factor is the decrease in dose due to widespread use of IR. In the previous 2011 study [5], only one site of 16 (6%) used IR, but in this study, there was a significant increase with 14 of 20 sites (70%) using IR. This is considered to be due to the recent recognition of IR in all CT scans, including those of children [11] and adults, and the experience of the scanner, who can easily enhance image quality by changing the console setting. Therefore, among the 14 sites that used IR, the fact that six sites (43%) did not use IR at the beginning of the study but introduced IR later during the study period is considered to be the result of recognizing IR's usefulness in fetal CT.
The site that performs scans with the lowest exposure (CTDIvol, 0.5 mGy) in this study does not use hybrid type IR but has introduced full IR (Veo, GE Healthcare) to aim for lower exposure. There are only a few reports on using full IR for fetal CT [12], but a decrease in fetal CT radiation dose would be possible if the full IR method becomes widespread, leading to a further decrease in DRL throughout Japan.
Similarly, this study found that many sites used low tube voltage. No significant difference in CTDIvol was found between different protocols using 120 or 100 kV, but the use of low tube voltage at these sites may be the result of awareness of lowering exposure and an adjustment to avoid an increase in unnecessary tube current. This is thought to be due to a change in techniques and awareness among the technicians in charge of CT machines in the clinical field.
In this way, radiation exposure of fetal CT has been optimized, but as shown in Figures 4 and 5, there are still sites with conditions leading to high radiation doses, and further education on DRL is required.
On the other hand, a limitation of this and previous studies was that only the radiation dose to set DRL was studied, and we did not collect data on image quality. Focusing on decreasing the dose may lead to a failure to maintain sufficient diagnostic quality for images. Goske et al. [13] have published their thoughts regarding the diagnostic reference range as a method to maintain both an appropriate radiation dose management for CT and diagnostic image quality. They suggest that the upper limit for radiation exposure should be the 75th percentile value and the lower limit should be the 25th percentile value to maintain image quality, and the appropriate scan condition should fall within this range. If this is applied to the results of this study, the appropriate dose for fetal CT would be a CTDIvol between 2.5 and 5 mGy (median, 3 mGy). In cases without full IR, this range for diagnostic reference range is considered to be appropriate.
A significant reduction in radiation exposure is possible if fetal CT is used only for observation of skeletal structure of the fetus, but it is necessary for each medical institution to consider the characteristics of IR for their facility's CT equipment and to perform a CT examination with sufficient quality for diagnosis.
In summary, since the previous DRL survey was conducted [5], the original objective and role for DRL that the International Commission on Radiological Protection [14] had proposed has been sufficiently reached. In addition, widespread use of IR and low tube voltage has helped in managing the radiation dose throughout Japan.
The objective of CT is to deliver an accurate diagnosis when skeletal dysplasia is suspected after careful fetal ultrasound testing and to determine the strategy for perinatal patient care. From this study, it was heartening to find that 85% of obstetricians who order CT scans were aware of the approximate CT radiation dose at their site.
We intend to conduct another survey in 4 years to confirm whether there is a further decrease in DRL. In addition, compared with 2011, there is concern that image quality may have been affected by the decrease in dose. Ideally, the next study would also include an evaluation of image quality.
| Acknowledgments | |
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We thank the following seven radiologic technologists and clinical researchers who were members of a subgroup that worked on the CT work sheet: Tsukasa Sasaki, Hokkaido University Hospital; Kiyoaki Sasaki, Miyagi Children's Hospital; Toshiya Nasada, Hyogo University of Health Sciences Hospital; Rumi Imai, National Center for Child Health and Development; Tetsuya Horiuchi, National Center for Child Health and Development; Masao Kiguchi, Hiroshima University Hospital; and Shinji Sakai, Kurume University Hospital. We thank the obstetricians, radiologists, and radiologic technologists at the following medical institutions for filling out the work sheet: Hokkaido University Hospital; Aomori Prefectural Central Hospital; Miyagi Children's Hospital; Tohoku University Hospital; Yamagata University Hospital; Chiba Kaihin Municipal Hospital; Juntendo University Urayasu Hospital; National Center for Child Health and Development; Jikei University Hospital; Tokyo Women's Medical University Hospital; Fujita Health University Hospital; Nagara Medical Center; University Hospital, Kyoto Prefectural University of Medicine; Osaka Medical Center and Research Institute for Maternal and Child Health; Hyogo College of Medicine Hospital; Shikoku Medical Center for Children and Adults; Perinatal Medical Center, Ehime Prefectural Central Hospital; Kochi Health Sciences Center; Tokushima University Hospital; Hiroshima University Hospital; Perinatal Medical Center, Yamaguchi Prefectural Grand Medical Center; and Kurume University Hospital. We thank the following people who work for each vendor and are in charge of CT applications: Takashi Ichibakase, GE Healthcare; Yukie Oosawa, Toshiba; Tetsuo Onishi, Siemans Healthcare; and Taisuke Fujioka, Philips Healthcare.
Based on a presentation at the 2015 annual meeting of the Japan Society of Perinatal and Neonatal Medicine, Hakata, Fukuoka, Japan.
Supported by grant H28-nanchitou(nan)-ippan-017 from Research on Rare and Intractable Diseases, Health and Labour Sciences Research Grants.
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