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Patient Radiation Dose at CT Urography and Conventional Urography¹

¹ From the Departments of Radiology (R.D.N., P.F.J., S.G.S.) and Health Physics and Radiopharmacology (A.R.S.), Brigham and Women’s Hospital, 75 Francis St, Boston, MA 02115. Received February 7, 2003; revision requested April 23; final revision received November 7; accepted November 20. Address correspondence to R.D.N. (e-mail: nawfel@bwh.harvard.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
PURPOSE: To measure and compare patient radiation dose from computed tomographic (CT) urography and conventional urography and to compare these doses with dose estimates determined from phantom measurements.

MATERIALS AND METHODS: Patient skin doses were determined by placing a thermoluminescent dosimeter (TLD) strip (six TLD chips) on the abdomen of eight patients examined with CT urography and 11 patients examined with conventional urography. CT urography group consisted of two women and six men (mean age, 55.5 years), and conventional urography group consisted of six women and five men (mean age, 58.9 years). CT urography protocol included three volumetric acquisitions of the abdomen and pelvis. Conventional urography protocol consisted of acquisition of several images involving full nephrotomography and oblique projections. Mean and SD of measured patient doses were compared with corresponding calculated doses and with dose measured on a Lucite pelvic-torso phantom. Correlation coefficient (R²) was calculated to compare measured and calculated skin doses for conventional urography examination, and two-tailed P value significance test was used to evaluate variation in effective dose with patient size. Radiation risk was calculated from effective dose estimates.

RESULTS: Mean patient skin doses for CT urography measured with TLD strips and calculated from phantom data (CT dose index) were 56.3 mGy ± 11.5 and 54.6 mGy ± 4.1, respectively. Mean patient skin doses for conventional urography measured with TLD strips and calculated as entrance skin dose were 151 mGy ± 90 and 145 mGy ± 76, respectively. Correlation coefficient between measured and calculated skin doses for conventional urography examinations was 0.95. Mean effective dose estimates for CT urography and conventional urography were 14.8 mSv ± 90.0 and 9.7 mSv ± 3.0, respectively. Mean effective doses estimated for the pelvic-torso phantom were 15.9 mSv (CT urography) and 7.8 mSv (conventional urography).

CONCLUSION: Standard protocol for CT urography led to higher mean effective dose, approximately 1.5 times the radiation risk for conventional urography. Patient dose estimates should be taken into consideration when imaging protocols are established for CT urography.

© RSNA, 2004

Index terms: Computed tomography (CT), radiation exposure, 80.122, 80.1211 • Dosimetry, 80.122, 80.1211 • Radiations, exposure to patients and personnel, 80.122, 80.1211 • Radiations, measurement, 80.122, 80.1211 • Urography, 80.122, 80.1211

	INTRODUCTION

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Computed tomography (CT) has been used extensively for examination of the kidneys and urinary tract (1–11). With the advent of multi–detector row CT technology, three-dimensional and multiplanar imaging of the entire urinary tract have allowed acquisition of conventional urogram–like images during the excretory phase. As a result, CT urography with a multi–detector row CT scanner has been suggested as a replacement for conventional urography (1,10,11). At our institution, CT urography is performed with three acquisitions. Three-dimensional and multiplanar reformatted imaging necessitate that these acquisitions are obtained with thin collimation to maintain sufficient spatial resolution. Image acquisition time and limitations on x-ray tube heat capacity are reduced with multi–detector row CT. Because of the repeated acquisitions during CT urography, however, the patient may receive a radiation dose that is as much as three times that froma typical abdominal CT examination. Concern has been raised in recent years regarding the radiation dose from CT examinations in general, especially multi–detector row CT, and the increasing number of multiple-acquisition examinations performed (12–17). Furthermore, since scanning is performed in the region of the pelvis during CT urography, the dose to the gonads may be especially substantial.

The purpose of this study was to measure and compare patient radiation doses from CT urography and conventional urography and to compare those doses with dose estimates determined from phantom measurements.

	MATERIALS AND METHODS

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Study Group
Doses were measured at the entrance skin surface of patients and at the surface of a pelvic-torso phantom. Hereafter, these doses will be referred to as skin dose and surface dose, respectively. Patient skin doses were determined by placing a thermoluminescent dosimeter (TLD) strip consisting of six TLD chips on the abdomen of eight patients undergoing CT urography and 11 different patients undergoing conventional urography. We intended to select patients consecutively for each examination, but this was not possible because of the limited availability of technologists to place the TLD strips on the patients. The CT urography group consisted of two women and six men (mean age, 55.5 years; range, 25–75 years). The mean age and age range for the women were 70.5 years and 66–75 years, respectively, and those for the men were 50.5 years and 25–68 years, respectively. The conventional urography group consisted of six women and five men (mean age, 58.9 years; range, 32–78 years). The mean age and age range for the women were 56.2 years and 32–75 years, respectively, and those for the men were 62.2 years and 41–78 years, respectively. There were no statistically significant differences in age between the CT urography and conventional urography groups (P = .804) or between male and female patients (P = .836, Mann-Whitney significance test). Our institutional review board approved this study. Informed consent was not required.

The dose measured with the TLD strip approximates the dose at the skin surface since the strip was placed on the patient’s clothing, which was slightly elevated from the surface. TLD chips (TLD 100; Harshaw Bircon, Solon, Ohio) were 3 x 3 x 1 mm and were arranged in a linear pattern adjacent to each other (Fig 1). This arrangement was used to obtain a mean dose over the length of the TLD strip and to eliminate variation in dose that might be associated with gaps between the TLD chips. The TLD chips were processed and analyzed (Landauer, Glenwood, Ill). Analysis included appropriate calibration and correction factors for beam qualities used in our study. Final dose values were reported in milliroentgen-equivalent man units (mrem). For this study, an individual patient’s skin dose was specified as the mean dose of the six TLD chips used for that patient. Hereafter, skin doses measured with TLD strips will be referred to as measured doses.

View larger version (63K): [in this window] [in a new window] [Download PPT slide]   Figure 1. TLD array used in patient dose measurements.

Effective doses were calculated and used to compare radiation risk between CT urography and conventional urography examinations. Radiation risk is defined herein as the stochastic effect associated with the lifetime probability of a cancer fatality to the general population. It is estimated with the linear nonthreshold model risk coefficient of 5 x 10^–2 per gray (21). These estimates do not account for patient age or sex. The percentage risk (PR) is given by the following equation:

where ED is effective dose (in millisieverts).

In addition, to corroborate patient results, the same dose assessment that was performed with patients (measurements and calculations) was performed with a pelvic-torso phantom (21-cm anteroposterior thickness) for both CT urography and conventional urography protocols. A phantom was used so that the dose for both CT urography and conventional urography could be measured on the same object. The protocol for CT urography phantom measurements is given in Table 1. The CT dose index for this pelvic-torso phantom was calculated on the basis of dosimetric data obtained from a 20-cm-diameter acrylic dosimetric phantom because it more closely approximated the size of the pelvic phantom. The protocol for conventional urography phantom measurements is given in Table 2. The exposure series for conventional urography included 11 images.

CT Urography
CT urography examinations were performed with a multi–detector row scanner (Siemens Somatom Plus 4 Volume Zoom; Siemens Medical Systems, Forchheim, Germany). The protocol consisted of three volumetric acquisitions over the abdomen and pelvis (Table 3). For the eight patients undergoing CT urography, the TLD strip (Fig 1) was placed on the patient surface by one technologist, approximately 10 cm inferior to the xiphoid along the midline parallel to the z axis. Patient thickness (anteroposterior) was measured on CT images as the transverse diameter at the level of the umbilicus by using the CT system software.

Conventional Urography
Conventional urography examinations were performed with a computed radiography system (Konica, Tokyo, Japan). Overhead x-ray units (GE Medical Systems, Milwaukee, Wis) were used that included tomographic capabilities (three-phase 12-pulse generator; half-value layer, 3.1 mm of aluminum at 80 kVp). Measured skin doses were determined from TLD data collected during patient examinations. The conventional urography protocol consisted of an exposure series including full nephrotomography and oblique projections (Table 4). For the 11 patients undergoing conventional urography, a TLD strip was placed by one technologist on the patient surface at the same location that was used at CT urography. The average number of images obtained per patient was 11.6. Patient thickness (anteroposterior, lateral, oblique) was measured with a caliper and recorded to the nearest 0.5 cm.

Effective Dose
Effective doses were calculated for each radiographic projection at conventional urography by using the patient entrance skin dose, radiation field size, an effective dose conversion factor, and a normalized energy-imparted conversion factor for an abdominal radiographic projection (24). For all conventional urography images, the actual radiation field at skin entrance was determined by reviewing patient abdominal images and correcting the receptor field size (35.6 x 43.2 cm or 35.6 x 35.6 cm) for any additional collimation that may have been used by the technologist. Effective doses were used to estimate the radiation risk for conventional urography.

Statistical Analysis
The mean and SD were calculated for all patient doses, energy-imparted values, and anteroposterior thickness measurements for both CT urography and conventional urography. This was done to compare doses between CT urography and conventional urography, as well as to determine variations in patient doses between the two groups. The mean and SD of the phantom-measured doses (TLD) for both examination protocols were also calculated to compare with the patient data.

Linear regression was performed on the data in Figure 2 by fitting a straight line through the origin. The R² value was used to measure the strength of the relationship between measured and calculated skin doses for conventional urography patient examinations. Linear regression analysis was performed on the data in Figure 3, and the two-tailed P values, which were determined with the F test, were used to test that the slopes of the curves (effective dose vs size) were different from zero.

View larger version (28K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Correlation between calculated and measured skin doses for conventional urography examinations. Strong correlation (R2 = 0.95) between calculated and measured doses substantiates use of calculations to predict patient skin dose over this range of approximately one order of magnitude. Measured skin dose is the mean for six TLD chips for each patient.

View larger version (32K): [in this window] [in a new window] [Download PPT slide]   Figure 3. Effective dose versus patient thickness (AP) for CT urography (CTU) and conventional urography (IVU) examinations. Effective dose at CT urography decreased by approximately a factor of 2 as patient thickness increased by a comparable amount (thickness range, 18.5-40.0 cm), while that at conventional urography increased approximately threefold as patient thickness doubled (thickness range, 15-30 cm). Relationship between effective dose and patient size was statistically significant for both CT urography (P = .001) and conventional urography (P = .004) examinations. These findings support the notion that radiation risk is increased for smaller patients during CT urography and technique factors should be adjusted for patient size.

	RESULTS

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Dose and Patient Size
The measured and calculated mean skin doses at CT urography—56.3 mGy ± 11.5 and 54.6 mGy ± 4.1, respectively—were comparable. The measured and calculated mean skin doses at conventional urography—151 mGy ± 90 and 145 mGy ± 76, respectively—were also comparable. There was good correlation (R² = 0.95) between measured and calculated skin doses for conventional urography, as demonstrated in Figure 2. The slope of the linear regression was 0.93 with 95% confidence limits of 0.853 and 1.01. Figure 4 illustrates considerable increase in the mean skin dose with increasing patient thickness for conventional urography compared with the variation for CT urography.

View larger version (28K): [in this window] [in a new window] [Download PPT slide]   Figure 4. Mean measured skin dose versus patient thickness (AP) for CT urography (CTU) and conventional urography (IVU) examinations. Increase in dose with increasing patient thickness at conventional urography reflects use of phototimed exposures to compensate for patient size. Relatively constant skin dose with increasing patient thickness at CT urography demonstrates fairly constant exposure technique used by technologists.

Radiation Risk
The equation was used to estimate the radiation risk for each patient group. The risk was estimated as 0.074% for CT urography compared with 0.048% for conventional urography.

	DISCUSSION

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When considering CT urography as a replacement for conventional urography, radiologists should examine the risks of CT urography as well as the benefits. One of these risks is radiation. Our results indicate that the effective dose for CT urography was 1.5 times greater than that for conventional urography, while the skin dose for conventional urography was 2.7 times greater than that for CT urography. For CT urography, calculated and measured mean skin doses were comparable. There was also good agreement between calculated and measured mean skin doses for conventional urography. Thus, calculated results could be used to estimate, on average, the patient dose from either examination when TLD data are not available. However, these results are specific for our institution and imaging protocols.

Compared with patient doses for conventional radiography, those for CT are higher during imaging of the same anatomic area. The dose for a CT scan is almost constant over the patient surface, while the dose for a radiograph decreases exponentially from beam entrance to beam exit. At our institution, the surface dose for one abdomen-pelvis CT scan is typically 19 mGy compared with 3.5 mGy for one KUB radiograph. These doses correspond to effective doses of 6.9 and 0.58 mSv for CT and conventional radiography, respectively, or a radiation risk 15 times greater for one CT scan than for one KUB radiograph. However, when several radiographs are required for a particular examination, the skin dose quickly approaches that for CT. With this increase in skin dose, there is an increase in radiation risk to the patient.

The CT urography doses in our study are comparable to CT doses reported elsewhere in the literature if we correct for technical factors and the number of CT acquisitions (14,25–27). CT doses measured during patient examinations can vary considerably compared with those reported from other institutions. Doses are reported as surface dose, dose along the central axis of the scanner, dose in air, or dose with reference to a CT dosimetric phantom. In addition, a specific CT examination (ie, abdominal CT) or CT protocol (ie, CT urography) may be performed with substantially different scanning techniques from one institution to another. Thus, it is important to specify which radiation dose quantity is being reported.

The small variation in measured CT urography skin dose with patient size most likely reflects the fact that technologists used a relatively constant exposure technique (compatible with the CT urography protocol) despite the fact that patient thickness more than doubled from smallest to largest (18.5–40.0 cm). The tube current–time product was increased (from 155 to 200 mAs) in only two patients, who both weighed more than 220 pounds (99.8 kg).

CT urography was performed with a relatively constant tube potential and tube current–time product, and this contributed to the significant variation in effective dose with patient size. While patient size did not appear to have a significant effect on the CT urography skin dose, the effective dose for CT urography was inversely related to patient thickness. The effective dose decreased by a factor of 2 as patient thickness doubled. This finding corresponds to increased radiation risk for smaller patients and supports the argument that CT technique factors should be adjusted for patient size (16,17,23). Presumably, if technique factors had been adjusted to account for patient size, small patients would have received a lower dose and large patients would have received a higher dose. However, the mean effective dose to the group might have been unchanged.

For CT urography, the size of the phantom was comparable to that of smaller patients. Thus, the effective dose to the phantom (15.9 mSv) was comparable to the effective dose for smaller patients, even though the phantom skin dose was an overestimation of the mean patient skin dose by a factor of 1.4. Consequently, phantoms used to directly estimate patient dose at CT should approximate patient size as much as possible.

For conventional urography, calculated and measured patient skin doses were similar. This finding allows accurate prediction of patient skin dose if the output exposure of the tube is known and technical factors of the examination are recorded. Three factors most likely contributed to the wide range of doses at conventional urography: the number of images acquired, exposure technique factors used by technologists, and patient size. The average number of images acquired in our study was 11.6, which is more than the total number of images acquired in other conventional urography studies (25,28). With all other exposure factors constant, patient skin dose will increase in proportion to the number of images acquired. Also, since phototiming was used, the exposure techniques were varied to compensate for patient size. The automatic exposure control system compensated for patient size and resulted in a corresponding increase in patient dose for larger patients. The conventional urography data follow an exponential variation with thickness, as might be expected from attenuation during phototimed exposures. Thus, patient size had a great effect on skin dose at conventional urography.

For conventional urography, the effective dose increased with patient size by approximately a factor of 3 as patient size doubled. The pelvic-torso phantom effective dose (7.8 mSv) corresponds to the lower portion of the curve for conventional urography in Figure 3. The phantom provided an adequate prediction of the mean effective dose despite the fact that it is closer in size to the small patients in the group.

The number of images acquired and technique factors also contribute to the variation in effective dose. Yakoumakis et al (29) report a mean effective dose for conventional urography of 3.0 mSv with an average of 9.3 images acquired, and Muller et al (28) report an average of 3.7 images acquired at conventional urography. Liu et al (25) report a conventional urography protocol consisting of acquisition of one posteroanterior image and four anteroposterior images. Each exposure was performed with a constant technique: 70 kVp and 64 mAs. In our study, the tube current–time product for the same projection varied by as much as a factor of 2 or 3.

The effective doses for CT urography and conventional urography were equal at a patient thickness of approximately 31 cm. This result implies that patients with less than 31-cm anteroposterior thickness would incur a greater radiation risk from CT urography than from conventional urography, and patients with more than 31-cm anteroposterior thickness would be at less risk from CT urography compared with conventional urography. This finding was a consequence specific to our patient groups and examination protocols and is not a result that generally applies when radiation dose is compared between CT and radiography examinations.

The radiation risk for CT urography (0.074%) was 1.5 times that for conventional urography (0.048%). The risk associated with the dose of 3.6 mSv from annual background radiation is 0.018%. The comparison of radiation risk for CT urography and conventional urography with that for background radiation was used in this study to provide a frame of reference and not to validate one examination over the other on the basis of the magnitude of the associated risk. Certainly, the health benefits of either examination should be considered against any radiation risk predicted by this model. Clinicians should use caution when they perform procedures where doses can quickly accumulate, especially in young patients or examinations that include the pelvic region.

The radiation dose for conventional urography would be decreased substantially if fewer images were acquired, as is done at some institutions (25,28). Also, doses can be reduced by carefully selecting technical factors (tube potential, tube current–time product) for a given patient size so that only a sufficient but not unnecessary level of radiation is used. Currently, automatic exposure control is not routinely used in CT technology in the same manner as it is used in projection radiography. Tube current modulation, presently available, may provide a means of reducing the dose to the patient as the patient is being scanned.

CT technologists can adjust protocols to reduce radiation dose by changing exposure technique factors, number of acquisitions, scan pitch, or scan length. As is true for the number of images acquired during conventional radiography, the dose for CT is proportional to the number of CT scans acquired. Our CT urography protocol consists of three CT acquisitions with different doses over the abdomen and pelvis. This may not be necessary for all patients. A modified version of the protocol might be considered for younger patients when radiation dose is a concern or when only specific diagnostic information is necessary. Modification could include reduction of the number of acquisitions, volume scanned, or both. Also, specific protocols adapted to the size of the patient may be useful in reducing dose (30). However, radiologist preference for image quality is a major contributing factor to the choice of technique. The consideration for younger patients would be to limit the exposure to the gonads while at the same time to not compromise the diagnostic quality of the examination.

Limitations of our study include potential errors in calculated conventional urography doses due to inappropriate backscatter factor, inaccurate measurement of patient thickness, or inaccurate recording of technical factors. Improper placement of the TLD strip on the patient may lead to less precision in measured data for both conventional urography and CT urography. Also, the CT urography group had a greater percentage of men (n = 6) than women (n = 2) than did the conventional urography group. Given that men are larger than women on average, this may have resulted in an overall lower effective dose for the CT urography group than would have been expected if there were an equal proportion of male and female patients in this group. The numbers of men (n = 6) and women (n = 5) were more evenly distributed in the conventional urography group.

In clinical practice, CT doses are often reported as the dose to a standard quality assurance phantom by using a CT dose descriptor, the CT dose index; however, the CT dose index may not provide an accurate estimation of the dose delivered to the patient during a specific CT examination. The dose from the topogram was estimated to be less than 4% (2.0/56.3 mSv) of the total dose for the CT urography examination and was not included in the measurements. The CT dose index skin doses reported in this study represent a maximum dose to the patient but are not necessarily an accurate estimate of the patient’s radiation risk. The skin dose does not account for the dose delivered to all irradiated organs, nor does it account for the radiosensitivity of each organ. Conversely, the effective dose includes the contribution from each irradiated organ to the total radiation risk.

In conclusion, we determined that at our institution the patient effective dose and therefore radiation risk for CT urography was 1.5 times greater than that for conventional urography. Also, our calculated doses predicted our measured doses reasonably well. The phantom used in our study was appropriate in size and can be used to provide a reference estimate of patient dose when patient data are not available. However, one must be aware of a possible overestimation or underestimation of dose, depending on the size of the patient.

CT urography performed with multi–detector row CT may eventually replace conventional urography. However, the increased radiation risk from this examination compared with conventional urography should be considered in the context of the amount of information that is necessary for the diagnostic task. Dose evaluations should be performed when examination protocols are established or modified. When protocols involving multiple scans are designed, an effort should be made to obtain as much diagnostic information as necessary with a sufficient but not unnecessary amount of radiation whenever possible.

	ACKNOWLEDGMENTS

 
The authors thank Walter Huda, PhD, for useful comments regarding patient dose assessment.

	FOOTNOTES

 
Abbreviations: KUB = kidney, ureter, bladder, TLD = thermoluminescent dosimeter

Author contributions: Guarantors of integrity of entire study, R.D.N., P.F.J., S.G.S.; study concepts, all authors; study design, R.D.N., P.F.J., S.G.S.; literature research, R.D.N., A.R.S., S.G.S.; clinical studies, all authors; experimental studies, R.D.N., P.F.J., S.G.S.; data acquisition, R.D.N., A.R.S.; data analysis/interpretation, all authors; statistical analysis, R.D.N.; manuscript preparation, definition of intellectual content, editing, revision/review, and final version approval, all authors

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