British Journal of Radiology 74 (2001),920-925 © 2001 The British Institute of Radiology
Neonatal chest and abdominal radiation dosimetry: a comparison of two radiographic techniques
N F Jones, BSc(Hons)1,
T W Palarm, PGD, BSc(Hons), DCR2 and
I S Negus, BSc(Hons), MSc3
1Directorate of Clinical Imaging, Royal Cornwall Hospitals NHS Trust (Treliske), Truro, Cornwall TR1 3LJ, 2School of Radiography, Faculty of Health and Social Care, University of the West of England, Bristol BS16 1DD and 3Department of Medical Physics and Biomedical Engineering, Plymouth Hospitals NHS Trust, Derriford Hospital, Plymouth, Devon PL6 8DH, UK
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Abstract
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Radiographs of the chest and the abdomen are the most commonly requested diagnostic X-ray examinations undertaken in neonatal intensive care units. Frequently, for a single child, both radiographs are requested simultaneously. These images can be obtained either as two separate exposures (one of the chest and one of the abdomen), or as a single exposure to include both anatomical regions on one film. This study compared the effective dose imparted as a result of each technique. A neonatal anthropomorphic phantom was designed and constructed, and each radiographic technique was simulated. Entrance surface dose (ESD) and dose–area product (DAP) were measured and estimates of effective dose were made from the DAP values. The mean effective dose for the separate exposure technique was estimated to be 37.3 µSv compared with 35.5 µSv for the combined exposure technique. However, observed variations in field size gave rise to uncertainties in DAP and thus the effective doses estimated from it. Hence, no significant difference in effective dose was observed between the radiographic techniques. The observed coefficient of variation in field size (16% for a 2.5 kg neonate) demonstrates that good standards of radiographic practice are more important than choice of technique.
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Introduction
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Infants born prematurely, with a gestational age as low as 23 weeks, are now surviving due to the continuing advancement of neonatal intensive care practices [1]. Frequent and accurate diagnostic radiographs are important in the initial assessment and subsequent monitoring of these children. Consequently, some infants may require multiple radiographic examinations throughout their neonatal care. However, radiation exposure in the first 10 years of life may have an attributable lifetime risk three to four times greater than that after the age of 30 years [2]. Furthermore, due to the long life expectancy of children compared with adults, there is a greater period for the potential expression of the delayed effects of radiation [3]. Therefore, methods of reducing the radiation dose to children as a consequence of diagnostic radiography are of paramount importance. The results of a survey of five London hospitals cited chest (63.7%) and abdominal (11.2%) examinations to be the most frequently performed radiological procedures on children up to the age of 1 year [4]. For a single child it is not uncommon for both chest and abdominal radiographs to be requested simultaneously, for example to localize an umbilical artery catheter inter alia. However, the results of a small survey conducted at the outset of this study revealed that there was no general agreement between hospitals as to whether these images should be obtained as two separate exposures (one of the chest and one of the abdomen), or as a single exposure to include both anatomical regions on one film. Furthermore, there appears to be a paucity of literature relating directly to this subject area. The aim of this study was to compare the effective doses imparted as a result of each technique.
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Materials and methods
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Design and construction of the anthropomorphic phantom
As there appeared to be no commercially available neonatal phantom, it was necessary to design and construct a phantom that would absorb and scatter X-ray photons in a similar manner to a neonatal human subject. In order for a material to simulate photon interaction properties, it must have a similar mass attenuation coefficient to human tissues [5]. 25% of the mass of a neonate is composed of skeletal muscle, which thus forms a larger part of the body mass than any other soft tissue [6]. At the X-ray tube potentials commonly used in diagnostic radiography, polymethylmethacrylate (Perspex) has a similar mass attenuation coefficient to human skeletal muscle [5] and is therefore suited to the construction of anthropomorphic phantoms. The anteroposterior thickness of the trunk was derived from its approximate relationship with mass [7] to avoid the myriad problems associated with obtaining measurements from human subjects. The mean mass of neonates admitted to the neonatal unit at Derriford Hospital, Plymouth between November 1997 and October 1998 was found to be 2.5 kg, which corresponds to a trunk thickness of 7.96 cm. The length and width of the trunk of a 2.5 kg neonate were estimated by reviewing 65 chest and 42 abdominal radiographs obtained during the same time period and calculating the mean dimensions.
The phantom (Figure 1
) was then constructed from eight sheets of Perspex, 21.8 cm in length, 13 cm wide and 1 cm thick. To simulate the photon scattering properties due to the presence of air in neonatal lungs, two right-angled triangular sections were removed from the Perspex block. The resulting cavities contained 88 ml of air, which is typical of that present in the lungs of a 2.5 kg neonate at peak inspiration [8].
Experimental design and procedure
The phantom was placed inside a Neocare air-controlled incubator (mark II), model 141 (Vickers Medical, Basingstoke, UK), typical of that used in neonatal intensive care units. Each technique was simulated using an AMX-4 mobile X-ray unit (GE Medical Systems, Milwaukee, WI). This unit has a medium frequency generator and a tube target angle of 15°. Total X-ray tube filtration was determined from half-value layer measurements to be 3 mm of aluminium equivalent. X-ray tube output constancy was measured using a Radcal 9010 dosemeter with a 6 cm3 ionization chamber (Radcal Corporation, Monrovia, CA) and a multifunction meter model 240 (Gammex RMI, Middleton, WI). All readings were well within 10% of those recorded when the X-ray unit was commissioned and were thus considered to be acceptable [9]. The exposure parameters (Table 1) for each radiographic technique were typical of those used at this hospital for a 2.5 kg neonate, at a focus-to-film distance of 105 cm, using a Fuji AC1 computed radiography system (Fuji Photo Film Co. Ltd., Tokyo, Japan). The X-ray tube potential used in this experiment was within the range recommended by the Commission of the European Communities [2]. The X-ray field areas were determined by viewing the same radiographs as used to determine the phantom trunk size, and the mean values were recorded (Table 1). Standard deviation values were also calculated.
The X-ray field area (Table 1
) for each technique was plotted on paper and placed inside the incubator, and the X-ray beam was collimated to the plotted field. The phantom was placed on each plotted field, in turn, so as to simulate each of the radiographic examinations (chest, abdomen, combined chest and abdomen). Once the phantom was in position, strips of lead (2 mm thick) were placed on the lid of the incubator, as routinely used in clinical practice, to simulate "shadow shielding" of the head, gonads and humeri, etc. (Figures 2a–c).
Dose measurements
Consideration was given to the use of thermoluminescent dosemeters as a means of recording entrance surface dose (ESD). However, using standardized reading regimens, the minimum detectable dose is approximately 100 µGy, which renders this method unsuitable for neonatal dosimetry [10]. The decision was therefore taken to measure ESD using a 90 x 5–60 (60 cm3) ionization chamber connected to a 9010 electrometer (Radcal Corporation, Monrovia, CA). Calibration of this dosemeter was traceable to the Physikalische-Technische Bundensanstalt primary standard. The overall uncertainty in the calibration does not exceed ±4% at the 95% confidence level. The ionization chamber was placed on the surface of the phantom to coincide with the central ray of the X-ray beam for each radiographic technique. 10 X-ray exposures [11] were performed for each of the examinations (chest, abdomen, combined chest and abdomen), and mean values for ESD were calculated.
Dose–area product (DAP) values were also recorded for the same three examinations, as this is increasingly used for patient dose measurement [11] and allows comparison with other studies. A Diamentor chamber (PTW, Freiburg, Germany) was attached to the X-ray tube head and calibrated using the Radcal dosemeter. Due to the low DAP values, 10 consecutive X-ray exposures were carried out for each of the examinations. The DAP for a single exposure was calculated from the total DAP for the 10 exposures.
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Results
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ESD measurements
The mean ESD for each examination is shown in Figure 3, with error bars indicating the ±4% uncertainty in the calibration of the dosemeter. For the separate exposure technique, the ESD was 56.7 µGy for the chest examination and 73.6 µGy for the abdomen. For the combined exposure technique, which includes both anatomical regions on one film, the ESD was 71.5 µGy. Standard deviations were also calculated.
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Figure 3. Mean entrance surface doses for chest, abdomen and combined chest and abdomen examinations. Error bars indicate ±4% error in calibration of ionization chamber. Standard deviations in parentheses.
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DAP measurements
The mean DAP for each examination is shown in Figure 4, with error bars indicating the ±4% uncertainty in the calibration of the dosemeter. For the separate exposure technique, the DAP was 8.3 mGy cm2 for the chest examination and 11.5 mGy cm2 for the abdominal examination. For the combined exposure technique the DAP was 18.7 mGy cm2.
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Figure 4. Dose–area product for chest, abdomen and combined chest and abdomen examinations. Error bars indicate ±4% error in calibration of ionization chamber.
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Effective dose estimates
The DAP results were then used to estimate the effective dose (ED) for each technique. This method was used in preference to ESD because it takes account of variations in field size. The coefficients for neonates published in NRPB Report R279 [4] were used. These are published at 10 kVp intervals, so linear interpolation was made [12] between 60 kVp and 70 kVp to provide coefficients for 62 kVp, which was the tube potential used in this study. For the separate exposure technique, the mean ED was 15.4 µSv for the chest examination and 21.9 µSv for the abdomen, giving an overall mean ED of 37.3 µSv for this technique (Table 2). NRPB Report R279 does not include data for the combined chest and abdominal examination. Therefore, to estimate ED for this technique, the mean DAP (18.7 cGy cm2) was multiplied by the mean of the chest and abdominal coefficients. This gave an estimated ED of 35.5 µSv for the combined exposure technique (Table 2).
Estimation of risk
The major concern regarding the use of ionizing radiation on the newborn is the risk of inducing a fatal childhood cancer [3]. Since the majority of neonates in intensive care are pre-term, fetal risk factors were used to estimate the risk of exposure to ionizing radiation [13]. However, there is some uncertainty as to which fetal risk factor is the most appropriate. The overall risk of inducing a fatal cancer in the first decade of life, following pre-natal exposure, has been estimated to be between 2.8 x 10-2 Sv-1 and 13 x 10-2 Sv-1 [3]. The latter figure was used to calculate the worse case scenario and multiplied by the mean ED for each technique. The results suggest that the chest examination and the abdominal examination result in a risk of inducing 2.0 and 2.8 fatalities per million examinations, respectively (Table 2). This gives a combined risk of 4.8 for the separate exposure technique, whereas for the combined exposure technique there is a slightly lower risk of 4.6.
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Discussion
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ESD measurements
The low standard deviation associated with the ESD measurements for each radiographic technique would suggest acceptable levels of reliability. The accuracy of the ESD measurements was also considered to be acceptable (uncertainty in ionization chamber calibration ±4%). Recorded ESDs for the chest (56.7 µGy), abdomen (73.6 µGy) and the combined exposure technique (71.5 µGy) compare favourably with some other neonatal dosimetry studies [14–17]. However, other recent studies have reported considerably lower ESD measurements for each anatomical area. Wraith et al [7] recorded ESD measurements of 36 µGy for the chest examination, 38 µGy for the abdomen and 31 µGy for the combined exposure technique. Using the same model of mobile X-ray machine as in this experiment, McParland et al [13] reported ESD measurements of 16 µGy, 15 µGy and 18 µGy, respectively. Lowe et al [18] reported median chest doses as low as 6 µGy. In addition, by applying the relevant normalization factor as described by Hart et al [19], using our data we obtain an ESD for a standard neonatal chest of 61 µGy compared with the provisional reference dose of 50 µGy. This disparity in dose may be explained in part by the lower mean mass of the infants studied by Wraith et al [7] and McParland et al [13], and the lower "back-scatter" factor associated with smaller X-ray field sizes. However, the main reason the above studies were able to demonstrate such low ESDs was because of the low mAs values employed. Comparable mAs settings might also be achieved at this hospital if exposure factors were to be further optimized, either through the use of a computed radiography system or a conventional film–screen system.
DAP measurements
The DAP values measured in this experiment are likely to be overestimated because the true X-ray field size is reduced by the use of "shadow shielding". However, our measurements of 8.3 mGy cm2 for the chest, 11.5 mGy cm2 for the abdomen and 18.7 mGy cm2 for the combined technique compare favourably with published figures for neonates. Hufton et al [15] reported a mean DAP value of 11.7 mGy cm2 for abdominal radiographs using the same model of computed radiography system used in this study. Wraith et al [7] reported values for chest, abdomen and combined chest and abdomen of 12.3 mGy cm2, 16.0 mGy cm2 and 12.8 mGy cm2, respectively. Their value for the combined technique is relatively low owing to a lower mean patient weight for this examination.
Other recent studies quote mean DAP values of7–15 mGy cm2 for chests and 19–29 mGy cm2 for abdomens [15, 20, 21]. Factors relating to variations in patient age (up to 1 year), radiographic equipment and technique may account for this range. These studies do not make reference to the combined chest and abdominal technique.
ED estimates
The mean estimated ED is 37.3 µSv for the separate exposure technique and 35.5 µSv for the combined, a difference of 5%. These figures would appear to suggest that the combined exposure technique imparts a lower ED than the alternative technique. The higher dose estimate for the separate exposure technique is explained by the area of overlap in X-ray fields between the chest and abdomen where this region is irradiated twice. Although the combined exposure technique appears to impart a lower dose than the separate exposure technique, there are various uncertainties inherent in the estimation of ED. The NRPB coefficients [4] used to estimate these doses were calculated using X-ray field sizes larger than in this study, such as would be used for a 3.47 kg neonate. However, this is a systematic error and applies equally to both techniques. Furthermore, because there are no published coefficients for the combined exposure technique, individual coefficients for the chest and abdomen were employed, although the validity of this method remains untested. It is anticipated that the largest source of error is in the calculation of the mean field sizes. The samples of radiographs used to calculate these field sizes showed coefficients of variation of between 9% and 16% in area.
Estimation of risk
Our results would suggest that radiography of the chest and abdomen, using either of the techniques, would carry a risk of inducing a fatal childhood cancer of between 4.6 and 4.8 per million examinations. These estimates are similar to the figures published by Fletcher et al [16] and Chapple et al [14] of 3.57 and 5.2 per million, respectively. Our figures for risk are derived from estimates of ED and are therefore subject to the same uncertainties discussed above.
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Conclusion
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The results obtained from this study are comparable with other published data, which suggest that the neonatal anthropomorphic phantom adequately simulates the X-ray absorption and scattering properties of a 2.5 kg neonate. However, further validation is required.
It would appear that there is no significant difference in ED between the two radiographic techniques, because the estimated error in the calculation of field size greatly exceeds the difference in ED. Therefore, other factors must influence the choice of radiographic technique employed. The separate exposure technique may increase the risk of chilling and cross-infection. However, this technique is reported to yield images of higher diagnostic quality [22, 23]. These more "immediate" risks may be substantially reduced by strict adherence to exemplary working practices by radiographers and neonatal nurses.
The observed variation in field size (coefficient of variation 16% for a 2.5 kg neonate) demonstrates that good standards of radiographic practice are more important than choice of technique.
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Acknowledgments
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We wish to convey our sincere thanks to the radiographers who assisted with this study, and in particular to Rachel Heseltine and Faith Constantine. Thanks are also due to the following staff at the University of the West of England: Ian Parsons, Jane Wathen, Ken Holmes, Sally Waddington and Mollie Gilchrist.
Received for publication January 12, 2001.
Revision received May 4, 2001.
Accepted for publication June 4, 2001.
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Abstract
Introduction
