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Radiation Effective Doses to Patients Undergoing Abdominal CT Examinations

¹ Department of Radiology, University of Florida, Gainesville.

	Abstract

TOP Abstract Introduction MATERIALS AND METHODS RESULTS DISCUSSION References

 
PURPOSE: To determine the radiation effective dose to adult and pediatric patients undergoing abdominal computed tomographic (CT) examinations.

MATERIALS AND METHODS: Technique factors were obtained for three groups of randomly selected patients undergoing abdominal CT examinations: 31 children aged 10 years or younger; 32 young adults aged 11–18 years; and 36 adults older than 18 years. The radiographic techniques, together with the measured cross sections of patients, were used to estimate the total energy imparted to each patient. Each value of energy imparted was subsequently converted into the corresponding effective dose to the patient, taking into account the mass of the patient.

RESULTS: All abdominal CT examinations were performed at 120 kVp with a section thickness of approximately 7 mm for all sizes of patients. The mean number of CT sections increased from 22.0 for children to 31.5 for adults, and the mean quantity of x radiation in milliampere-seconds increased from 220 mAs for children to 290 mAs for adults. The mean values (± SD) of energy imparted were 72.1 mJ ± 24.4 for children, 183.5 mJ ± 44.8 for young adults, and 234.7 mJ ± 89.4 for adults. The corresponding mean values of patient effective dose were 6.1 mSv ± 1.4 for children, 4.4 mSv ± 1.0 for young adults, and 3.9 mSv ± 1.1 for adults.

CONCLUSION: Values of energy imparted to patients undergoing abdominal CT examinations were a factor of three higher in adults than in children, but the corresponding patient effective doses were 50% higher in children than in adults.

Index terms: Computed tomography (CT), in infants and children, 70.1211 • Computed tomography (CT), radiation exposure, 70.1211 • Dosimetry • Radiations, exposure to patients and personnel, 70.47 • Radiations, measurement, 70.47

	Introduction

TOP Abstract Introduction MATERIALS AND METHODS RESULTS DISCUSSION References

 
The effective dose is a radiation descriptor that may be used to characterize radiation exposures to patients undergoing computed tomographic (CT) examinations, where radiation levels are well below threshold doses required to induce deterministic effects. (The effective dose E, defined in publication 60 of the International Commission on Radiological Protection [1], and the effective dose equivalent H_E, defined in publication 26 of the International Commission of Radiological Protection [2], are conceptually identical but use different organ-weighting factors; E and H_E are interchangeable in this article.) The magnitude of the effective dose is related to the stochastic radiation risks of cancer induction and the production of genetic effects.

National and international organizations are using the effective dose to quantify exposures of patients to radiation in diagnostic radiology (3–5). Availability of radiation doses to patients during CT permits a direct comparison to be made of the radiation hazards of CT scanning with alternative diagnostic procedures that also use ionizing radiation (6). Additional uses of CT radiation doses to patients include (a) optimization of CT examinations with respect to the radiation risk to the patient and (b) submission to an institutional review board for inclusion in informed consent forms when volunteers are irradiated for research purposes (7).

Effective radiation doses to patients from CT examinations are high compared with those from other types of diagnostic radiographic examinations (5,6). In the United Kingdom, for example, CT examinations have been estimated to account for about 2.4% of all radiographic examinations (8) but account for about 20% of the annual collective dose from medical x rays (9). Studying the radiation exposures of infants and children who undergo CT examinations is also interesting because their radiosensitivity is higher than that of adults (1,10). To date, however, to our knowledge, studies of radiation doses to pediatric patients undergoing abdominal CT examinations have investigated only individual organ doses (11–16).

To our knowledge, no study has been performed that has investigated effective doses to individual patients by taking into account the individual technique factors, as well as the physical size of the patients undergoing these CT procedures. A comparison of the selected technique factors and the corresponding patient doses will help to determine whether these CT radiation doses to patients are as low as reasonably achievable, as required by the International Commission on Radiological Protection (17).

In this study, radiation doses to patients from routine CT abdominal examinations were calculated with a method recently developed for determining effective doses to patients ranging from the newborn to the adult (18,19). Mean CT doses were determined for three groups of patients: children aged 10 years or younger, young adults aged 11–18 years, and adults older than 18 years. The technique factors used to perform these CT examinations were also documented as a function of the size of the patient.

	MATERIALS AND METHODS

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For a given patient and a constant x-ray tube potential, the value of the effective dose from an abdominal CT examination depends on the tube current (in milliamperes), the scanning time (in seconds), the section thickness (T), and the total number of sections (N). These four factors were obtained for 99 randomly selected patients undergoing abdominal examinations on CT scanners (HiSpeed Advantage; GE Medical Systems, Milwaukee, Wis) at our institution during 1996 and 1997. All abdominal CT examinations were performed at 120 kVp. Included in this sample of patients were 31 children aged 10 years or younger, 32 young adults aged 11–18 years, and 36 adults older than 18 years.

The effective dose (E) to a patient of mass M who absorbs _p joules of energy during an abdominal CT examination is as follows (19):

For a patient undergoing CT with N contiguous CT sections, the imparted energy (_p) to the patient is given by

For a section with a thickness T, the directly irradiated mass of the water phantom simulating the patient, M_p, is given by

	RESULTS

TOP Abstract Introduction MATERIALS AND METHODS RESULTS DISCUSSION References

 
Table 1 summarizes the key demographic and CT technique factors for the three age groups in this study. Data for each parameter in Table 1 correspond to the mean value for the specified age group, together with the corresponding SD. Figure 1 shows how the equivalent water radius r varied with the mass of the patient, where the line is a least-squares fit with a second-order polynomial. Section thickness (approximately 7 mm) was relatively independent of the size of the patient, but the mean total number of CT sections required to cover the abdomen increased from 22.0 for children to 31.5 for adults.

View larger version (15K): [in this window] [in a new window]   Figure 1. Graph of the radius r of a mass-equivalent cylinder of water (see Equation [4] in text) versus mass M of patient. The quadratic equation was obtained by a least-squares fit linking the size of the patient (radius) to the mass of the patient, with a computed coefficient of determination (r2 = 0.935).

View larger version (14K): [in this window] [in a new window]   Figure 2. Graph of the selected technique factor (quantity of x radiation in milliampere-seconds) versus mass of patient, with all the CT examinations performed at 120 kVp. The low value of the coefficient of determination (r2 = 0.346) shows that little account is taken of the size of the patient undergoing the abdominal CT examination.

View larger version (25K): [in this window] [in a new window]   Figure 3. Graphs of the mass of patient versus (top) mean section dose, (middle) energy imparted, and (bottom) effective dose. Horizontal axis on each graph represents the mass of patient in kilograms. Energy imparted generally increases with increasing mass of patient, whereas the values of mean section dose and effective dose generally decrease with increasing mass of patient. Note that for all three dose parameters, there is a large amount of scatter about the mean values, which reflects the large variation in the selected quantity of x radiation in milliampere-seconds.

	DISCUSSION

TOP Abstract Introduction MATERIALS AND METHODS RESULTS DISCUSSION References

Because of the increased radiation risk in young children, it is important that radiographic technique factors for pediatric patients be carefully evaluated to ensure that these doses are as low as reasonably achievable. Because infant doses will be directly proportional to the selected quantity of x radiation in milliampere-seconds, minimizing this value will reduce doses to patients. The small size of newborn patients, however, should also permit reduction of the x-ray tube potential, which will also markedly reduce doses to patients. For a HiSpeed Advantage CT scanner, for example, reducing the x-ray tube potential from 120 kVp to 80 kVp would reduce the dose to the patient by 65% for a constant quantity of x radiation in milliampere-seconds (19).

Figure 2 shows that there is a very poor correlation between the selected technique factor and the size (mass) of the patient. The poor correlation between technique factor and the size of the patient is in part caused by the absence of any indicator of the mean exposure level for a given patient examination on current commercial CT scanners. A mean exposure level could be readily determined from the projection radiographs that are obtained before each axial CT examination and would permit the adjustment of radiographic techniques to ensure that the mean detector intensity was approximately constant for all patients.

The provision of such a radiation exposure meter on commercial CT scanners would also likely result in adjustment of x-ray tube potentials, given the limited range of milliampere stations available on current CT scanners. Education of radiologists and technologists about the specific factors that affect radiation doses to patients, as well as the steps that could be taken to minimize such doses, would also help to ensure that CT examinations explicitly take into account the size of the patient.

The results of this study demonstrate that the size of an average adult is equivalent to a cylinder of water 29 cm in diameter. It is interesting to compare this average patient size with the standard acrylic CT phantom used for simulating patients. Body CT dosimetry is normally performed by using an acrylic cylinder that has a 32-cm diameter (20). Because the acrylic material has a density of 1.19 g/cm³, this acrylic phantom corresponds to a water phantom with a diameter of approximately 35 cm by Equation (4). The difference between the standard CT dosimetric phantom and the average adult abdomen thus corresponds to a difference in x-ray path length of approximately 6 cm of water. Patient doses, expressed either in terms of mean section dose or the CT dose index (20), will generally increase as the size of the patient is reduced because of the lower patient mass and reduced x-ray beam attenuation (19).

The dosimetric importance of the large difference between a typical adult abdomen and the CT dosimetric phantom may be estimated by considering the corresponding differences in the primary x-ray beam transmission. The effective energy for a representative CT spectrum may be taken to be approximately 60 keV for a hardened CT spectrum with a half-value layer on the order of 10 mm of aluminum (21). Because the mass attenuation coefficient for tissue (muscle) is 0.2 cm²/g at 60 keV (22), the primary x-ray transmission for the CT dosimetric phantom will only be approximately 30% of the corresponding transmission through an average adult abdomen.

For a given choice of radiographic technique factors, CT doses (ie, D_m and CT dose index) will thus be higher in an average adult patient than those doses indicated by any measurements made with an adult CT dosimetric phantom (ie, 32-cm-diameter acrylic phantom). For example, at 120 kVp with a GE HiSpeed Advantage CT scanner, the estimated reduction in the value of D_m as the size of the water phantom increased from 29 to 35 cm in diameter is estimated to be approximately 30% (19).

Children and adult patients differ in the diameter of the water phantom by approximately 13 cm, which corresponds to a difference in primary x-ray beam transmission of a factor of approximately 13. This difference in primary x-ray beam transmission may be compared with the selected quantity of x radiation in milliampere-seconds for children, which was only approximately 20% lower than the mean adult quantity of x radiation in milliampere-seconds.

It is reasonable to expect the CT detector dose to be approximately constant for any size of patient because this will maintain a constant level of image mottle on the CT scans (23). With the assumption that the technique factors for adults are optimal, this implies that the quantity of x radiation in milliampere-seconds for children could be greatly reduced with no detrimental effect on the resultant quality of the scan. For any given patient, the effective dose is directly proportional to the quantity of x radiation in milliampere-seconds.

Reducing the x-ray tube potential may also be used to reduce the effective dose (see previous discussion) to infant and pediatric patients who undergo CT examinations. In this context, the use of a single kilovolt peak in CT is in marked contrast to current practice in screen-film radiography, where any increase in the size of the patient generally requires an increase in both the kilovolt peak and the quantity of x radiation in milliampere-seconds used for the radiographic examination.

Doses to patients at abdominal CT are at the upper end of the patient doses encountered in diagnostic radiology. Doses to patients from abdominal CT examinations are comparable to those in nuclear medicine (2–10 mSv), barium enema examinations (3–7 mSv), and excretory urography (2.5–5.0 mSv) and are markedly higher than those associated with chest radiography (0.02–0.05 mSv), skull examinations (0.1–0.2 mSv), or abdominal radiographic examinations (0.5–1.5 mSv) (6).

It is also interesting to compare doses to patients from abdominal CT with those from natural background radiation. In North America, inhabitants receive annual doses of approximately 3 mSv from natural background radiation. Mean doses from the natural background include cosmic radiation (approximately 0.28 mSv), terrestrial radioactivity (approximately 0.28 mSv), internal radionuclides (approximately 0.39 mSv), and exposure from inhaled radon daughters (approximately 2 mSv) (24).

Current radiation protection guidelines in North America are 1 mSv/y for members of the public and 50 mSv/y for radiation workers, although the doses to the latter are likely to be reduced in the foreseeable future to 20 mSv/y (25). Note, however, that comparisons of patient CT doses with natural background radiation and regulatory dose limits are made for educational purposes and not for justifying any exposure of patients. Patients who are exposed to radiation during an abdominal CT examination may be expected to benefit directly from the resultant diagnostic information, and the exposure must be justified solely in terms of the expected benefit to the patient.

The energy imparted to a patient (and the corresponding effective dose) for any given x-ray tube potential will be directly proportional to the selected x-ray tube current (ie, milliamperes), scanning time, section thickness, and the total number of sections in the CT examination. The changes in the dose to the patient as a result of changes in abdominal scanning protocols, such as obtaining additional scans before, during, and after the administration of contrast agents, may be readily estimated by taking into account the quantity of x radiation in milliampere-seconds, the section thickness, and the total number of sections scanned.

Obtaining three sets of scans over the same region at the same quantity of x radiation in milliampere-seconds, as in a "multiphase" abdominal CT examination, will generally triple the energy imparted and the corresponding effective dose. Addition of a series of 10 (7-mm) bolus-tracking scans at 60 mA, however, would increase the effective dose from the average adult abdominal examination (Table 1) by less than 10%.

A helical examination for the same abdominal region will result in values of energy imparted that are similar to those from a conventional CT examination, provided the technique factors (ie, kilovolt peak and quantity of x radiation in milliampere-seconds) are kept the same, together with the same section thickness and a pitch ratio of 1:1. Increasing the pitch ratio to 2:1 would normally halve the energy imparted, as well as the corresponding effective dose to the patient.

Any changes in CT technique factors must take into account the anticipated changes in the quality of the scans, as well as the dose to the patient. Changing the quantity of x radiation in milliampere-seconds and the x-ray tube potential will generally affect the contrast of the resultant scan and the level of mottle (noise) on the scan. The quality of the scans may be quantified by specifying a signal-to-noise ratio of the CT scan, and any optimization process would need to consider how the signal-to-noise ratio and the dose to the patient varied with CT technique factors for any specified imaging task. In general, adjustment of the quantity of x radiation in milliampere-seconds will affect the level of mottle on the scan, whereas changes in the x-ray tube potential will affect both the contrast and the mottle on the scan.

Recent studies have been performed to investigate the relationship between the choice of CT technique factors and the corresponding quality of the scan (26,27) by looking at how mottle changes with the level of milliampere-seconds and what level is deemed to be acceptable. Because reducing the x-ray tube potential can markedly affect the dose to the patient and can increase contrast on the scan, it would be particularly useful to investigate how this parameter affects image quality at abdominal CT scanning.

	Acknowledgments

 
The authors gratefully acknowledge useful discussions on patient dosimetry with James V. Atherton, PhD, and Zhenxue Jing, PhD. Expert editorial assistance was provided by Linda Waters-Funk.

	Footnotes

 
Address reprint requests to W.H., Department of Radiology, SUNY Health Science Center at Syracuse, 750 E Adams St, Syracuse, NY 13210.

From the 1997 RSNA scientific assembly.

Author contributions: Guarantors of integrity of entire study, W.H., D.E.W.; study concepts and design, W.H., D.E.W.; definition of intellectual content, W.H., D.E.W.; literature research, W.H.; data acquisition, D.E.W., A.L.L.; data analysis, D.E.W., W.H.; statistical analysis, W.H.; manuscript preparation, W.H.; manuscript editing, W.H., D.E.W.; manuscript review, W.H., D.E.W., P.J.M., A.L.L.

Received March 23, 1998; revision requested June 19, 1998; revision received September 17, 1998; accepted October 14, 1998.

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