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Original Research |
1 Department of Radiology, Haukeland University Hospital, Jonas Lies vei 65,
Bergen 5053, Norway.
2 University of Bergen, Bergen, Norway.
3 Section for Radiology, Department of Surgical Sciences, University of Bergen,
Bergen, Norway.
Received May 19, 2006;
accepted after revision October 4, 2006.
Address correspondence to E. N. Eikefjord
(eli.eikefjord{at}hib.no).
Abstract
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SUBJECTS AND METHODS. One hundred nineteen patients with acute flank pain were included. All patients were examined using both MDCT and excretory urography. CT involved one acquisition from the upper kidney margin to the symphysis pubis. The only protocol variation was in the tube current (mAs), which was made according to patient body mass. The excretory urography protocol consisted of three images, with more when supplementary images were needed. Effective radiation doses were computer-simulated using dosimetry programs for CT and conventional radiography, based on Norwegian Radiological Protection Board dose data sets. Mean and SDs of measured patient doses were calculated and compared. Further analyses of dose variations in body mass categories (body mass index) were conducted, as were analyses concerning the number of images taken.
RESULTS. The mean effective doses were 7.7 mSv with MDCT and 3.63 mSv with excretory urography. The effective dose varied both in and between techniques but could be predicted. Radiation risk decreased significantly with increased patient weight.
CONCLUSION. The average effective dose with MDCT was more than double that with excretory urography. However, the appropriate dose could be strongly predicted by the patient's body mass index and by procedure. An optimum low-dose protocol should be considered before initiating unenhanced MDCT for ureteral colic in order to minimize the radiation-induced cancer risk and to secure adequate image quality.
Keywords: dosimetry • excretory urography • MDCT • radiation dose • radiography • urinary tract
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When considering the implementation of a new radiologic procedure, radiation dose should be an important issue and should accord strictly to the principles recommended by the International Commission on Radiological Protection (ICRP) [4]. One fundamental principle is optimization, emphasizing that all radiation exposure should be as low as possible when choosing equipment and procedures for a certain medical approach. Furthermore, in January 2006, the European Union authorities introduced new regulations regarding the documentation of individual patient radiation doses [6]. To meet this regulation, an evaluation of the radiation dose is necessary to understand which technical and procedural factors affect it, both within and between the imaging procedures. With this knowledge we can choose the most appropriate examination alternative for a given clinical indication.
The primary aim of this study was to use commercially available software to evaluate effective radiation doses between different radiologic examination procedures. The second aim was to explore the relationship between technical and practical factors that could affect the effective radiation dose, both during and between the chosen imaging procedures.
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Patients were carefully informed of the increased radiation doses involved in undergoing two examinations. All 119 patients gave their informed consent. This article describes part of a larger study primarily regarding diagnostic accuracy, for which we obtained the approval of our regional ethics committee review board. In addition to radiation dose as accounted for in this article, image quality and cost-effectiveness were assessed for all patients. All the patients' medical treatments were according to standard hospital procedures, independent of the study.
Procedures
All patients were examined with MDCT, followed, preferably within 1 hour,
by excretory urography, using our standard examination protocols and
procedures. No bowel preparation was given.
CT—All CT examinations were performed on a 4-MDCT scanner (LightSpeed Plus, GE Healthcare) with scanning parameters of 120 kV; 250 mA; rotation time, 0.8 seconds; detector configuration, 3 x 3.75 mm; table speed, 22.5 mm (pitch of 1.5); and slice thickness, 5 mm. One acquisition was acquired from the upper kidney margin to the symphysis pubis. No oral or IV contrast agent was used. The radiographers subjectively adjusted the tube current (mAs) according to the patient's abdominal size.
Excretory urography—Excretory urography examinations were performed on a digital radiography system (Canon CXDI-11, Medical Systems Division of Canon U.S.A.) with standard parameters of 70 kVp, an automatic mAs device, 3-mm Al filtration, and standard film-focus distance (FFD) of 110 cm. Fifty milliliters of Omnipaque (iohexol, Amersham Health), 350 mg I/mL, was injected IV. The excretory urography protocol consisted of the standard acquisition of three anteroposterior images—that is, one of the abdomen before the injection of the contrast medium, one of the kidneys 5 minutes after the injection, and the third of the abdomen 15 minutes after the injection. Supplementary images were taken when needed, so the total number of images could vary for different examinations.
Data Collection
At each radiologic examination, data were recorded on a registration form
by the performing radiographer. With excretory urography, the following data
from each exposure were obtained: projection, tube current value, incident
radiation (dose-area product [DAP]), and the total number of images taken for
the whole examination. Filtration (in mm Al) and kVp were fixed parameters.
DAP (in milligray per square centimeter [mGy · cm2]) is
defined as the absorbed dose to air averaged over the area of the X-ray beam
in a plane perpendicular to the beam axis, multiplied by the area of the beam
in the same plane [7]. With
excretory urography, mAs and DAP values were read from the console. The DAP
was measured with the special large-area ionization chamber (DAP meter)
attached to the diaphragm housing of the X-ray tube. The DAP meter was
calibrated as described previously
[8].
With CT, data from technical parameters (mAs) and dose estimates
(dose-length product [DLP]) were obtained. DLP is defined by the equation
 |
where CTDIvol is the average dose (mGy) in the central region of a multiple-scan examination. Other relevant dose-influencing scanning parameters (kV, rotation time, detector configuration, and table speed) were constant, according to standard protocol.
The body mass index (BMI) (weight [kg] / height squared [cm]) was obtained
for each patient to analyze effective dose variations relative to body size.
The BMIs are characterized with values in the range of < 18.5 to > 30
and can be divided into four categories: underweight (< 18.5), normal
weight (18.5-24.9), overweight (25-29.9), and obese (
30)
[9].
Evaluation of Effective Dose
In this study, software specially developed for each of the X-ray
techniques was used to estimate effective doses (in millisieverts). Effective
dose is a risk estimate defined by the equation
[4]:
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where wT is a tissue-weighting factor that takes into account that different organs have variable sensitivity to radiation, and HT is the given equivalent dose (Sv) to the respective organ. The ICRP publication 60 specifies 12 tissues with well-established sensitivities for these effects [4]. With X-ray examinations, the equivalent dose is the same as the absorbed dose (Gy). Absorbed dose can be defined by the ratio E / m, where E is the energy absorbed by the medium due to a beam of ionizing radiation being directed at a small mass, m [10]. The effective dose is defined as the sum, over specified tissues, of the products of the equivalent dose in a tissue and the tissue-weighting factor for that tissue [11]. The testes and ovaries are the most radiosensitive tissues because they have the highest tissue-weighting factor [4].
Radiation risk is usually related to the effective dose, using the tissue-weighting factors recommended by ICRP [4, 11]. Radiation risk is defined as the stochastic effect associated with the lifetime probability of a cancer fatality in the general population and the hereditary effects on the next generation. The ICRP has defined the total probability coefficient for serious stochastic effects as 7.3% per Sv of effective dose. This represents an average value for the whole population, whereas the individual risk is age-specific and sex-specific [7].
The software used in this study offered a computerized mathematic model based on adult anatomy for the calculations of effective doses. With excretory urography, the software XDOSE (National Radiological Protection Board [NRPB], United Kingdom) developed by John Le Heron was used [12]. This software estimates organ doses using the Monte Carlo technique for user-specific combinations of 68 radiographic projections as determined by NRPB SR262 [12]. In our study, data from 108 patients were used to determine the mean exposure factors (DAP), projection type, and projection frequency per examination. When these data were entered on the software spreadsheet, the effective dose (mSv) for each examination could be calculated.
With CT, ImPACT CT Patient Dosimetry Calculator software (St. George's Hospital, London) was used [13]. This software makes use of the NRPB Monte Carlo dose data sets produced in report SR250 [14]. SR250 provides normalized organ dose data for irradiation of a mathematic phantom by a range of CT scanners. Scanning parameters for each examination were entered on the spreadsheet: scanner model—manufacturer, scanner, kV, and scanning region; scan range—starting and ending positions and patient sex; and acquisition parameters—mAs, rotation time, collimation, slice width, and pitch.
The mathematic effective dose estimations were made on the basis of the entered data. In addition, simulated (software) estimates for DLP were registered for each patient to be compared with our console measurements for validation.
Statistical Analysis
Mean and SD were calculated for all patient dose estimates (mSv, mAs, DLP,
and DAP) and BMIs for both MDCT and excretory urography. Pearson correlation
coefficients (r) analyses were done to explore relationships between
dose estimates and BMIs. These analyses were also performed between dose
estimates and number of images. A one-way between-group analysis of variance
(F test) using a post hoc test (Tukey HSD [Honestly Significant Difference]
test) was conducted to explore the impact of BMI on levels of effective doses.
All statistical analyses were performed using SPSS software (version 13,
SPSS).
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Table 1 presents mean dose data and SDs for excretory urography (mAs, DAP) and for MDCT (mAs, DLP). With MDCT, the mean effective dose for both sexes was more than double (x 2.1) compared with excretory urography doses, x 2.02 for men and x 2.6 for women. The mean effective dose with excretory urography corresponds to 0.9 years of radiation, on the basis that Norwegians are exposed to about 4 mSv per person per year from all types of radiation [15]. Consequently, the increased effective dose after MDCT as compared with excretory urography corresponds to 1 (total, 1.9 years) and 1.5 (total, 2.4 years) extra years of radiation exposure for men and women, respectively.
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The effective doses varied considerably more with excretory urography than with MDCT. Both the smallest (0.5 mSv) and largest (17 mSv) effective doses were observed at excretory urography, compared with the range of 5-14 mSv at MDCT. These variations were due to two important factors: the number of images taken with excretory urography, and the difference in mAs selection and range between excretory urography and MDCT.
The average number of images taken per patient with excretory urography was five, varying from three to 11. The relationship between the number of images and the effective dose was statistically significant (r = 0.45, p > 0.001). This means that the number of images explained 20.3% (r2) of the variance in effective doses. Our usual standard of three anteroposterior images was taken in only 25% of the examinations. On average, an effective dose of 0.7 mSv was used for each excretory urography image. This means that the effective dose level (5-14 mSv) with MDCT was reached if seven or more excretory urography images were taken per patient, in accord with our standard protocols. Between seven and 11 images were taken in nearly 15% of our patient group.
The average mAs selection range (Table 1) was considerably more narrow with CT (185 mAs; SD, 40.6 mAs) than with excretory urography (74.3 mAs; SD, 40.1 mAs), reflecting the differences in effective dose ranges. With excretory urography and MDCT, the correlation coefficients (r) between mAs and effective dose were 0.78 and 0.83, respectively, at the p < 0.01 level. This means that mAs explained 61% and 69% (r2) of the variance in effective doses within each technique. Because other technical variables were constant for both excretory urography and MDCT, the important predictor was body mass (as measured by BMI). With both excretory urography and MDCT, mAs increased proportionally to BMI, as illustrated in Figure 1.
The effective doses were strongly predicted by BMI with both excretory urography (r = 0.7) and MDCT (r = 0.5) at the p < 0.01 level (Table 2). With excretory urography, the mean effective dose approximately doubled for each weight category: 1.8 mSv (SD = 1.0 mSv), 4.1 mSv (SD = 2.3 mSv), and 7.4 mSv (SD = 4.5 mSv) for normal weight, overweight, and obese, respectively. Consequently, CT as a replacement for excretory urography causes a radiation risk that is quadrupled for patients who are of normal weight, doubled for patients who are overweight, and about equal for patients who are obese. Figure 2 illustrates this; we can see that the mean effective dose with CT compared with excretory urography equalizes as patient mass increases. The variance between the categories (underweight has been omitted because there was only one patient in this category) corresponded to a statistically significant difference (F test[2, 104] = 30.5) at the p < 0.01 level. Despite a significant positive correlation between MDCT effective dose and BMI, its variability was less evident than the variability at excretory urography: 7.2 mSv (SD = 1.3 mSv), 7.8 mSv (SD = 1.8 mSv), and 9.6 mSv (SD = 2.7 mSv) for the respective categories. However, the variance was statistically significant at MDCT in all categories (F test[2, 112] = 10.7) at the p < 0.05 level.
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A good-sized patient data set (n = 119) was used in this dose estimation study because the main study required this for the analysis of diagnostic accuracy. We were thus able to perform extensive exploration of variability in effective doses related to patient mass.
By choosing only computerized mathematic models (software) for the calculations of effective doses, we were not able to make direct measurements of individual absorbed doses (e.g., entrance skin dose [ESD]). A common means of estimating absorbed doses is the use of thermoluminescent dosimeter (TLD) chips. These could have been used on our patients, where effective doses could have been estimated. TLD measurements will generally be regarded as more exact for individual measurements, thereby more sensitive to small dose variations than are mathematic models. However, TLD chips would be considerably more complex and impracticable to use in a large study population.
Valid estimations of effective dose were achieved by using XDOSE and ImPACT CT Patient Dosimetry Calculator. Both software packages are internationally recognized and also recommended in reports written by the Norwegian Radiological Protection Authority [16]. Effective dose is actually the only comparative unit between techniques, making other units less relevant. On the other hand, we were unable to validate direct measured (TLD) and calculated dose estimates (software). XDOSE did not distinguish between effective doses for men and for women, as the ImPACT Patient CT Dosimetry Calculator does. This can be regarded as a methodologic shortcoming rather than a statistical bias in our data. The effective doses calculated with MDCT showed a difference between sexes of 20%. It is likely that an equivalent effective dose difference exists with excretory urography as well.
Overall, the average effective dose estimates and the dose difference between excretory urography and MDCT found in our study correspond to those previously reported. Typical reported effective doses for patients with acute flank pain range from 4.7-10 and 1.5-3.5 mSv in standard unenhanced CT and excretory urography examinations, respectively [17-20]. This implies that patients are usually exposed to a 2-5 times higher radiation dose after unenhanced CT as compared with excretory urography. However, it should be kept in mind that all reported doses are specific for each individual institution (such as scanner or examination protocol), thus complicating direct comparisons.
We found a significantly wide dose range of effective doses with both MDCT and excretory urography, mainly influenced by body size. In patients of normal weight, the effective dose with MDCT was increased by a factor of 4. In comparison, those patients who were overweight and obese received an effective dose that was increased by factors of 1.9 and 1.3, respectively (Table 2). Consequently, patients of normal weight were exposed to a significantly higher radiation risk with MDCT than with excretory urography when compared with the other weight categories. Patients of normal weight represented nearly half of the study population of patients with acute flank pain.
To our knowledge, no reports on acute flank pain have explored the relationship between radiation risk and BMI. A report published in 2004 by Nawfel et al. [21] compared CT urography (three acquisitions) and excretory urography, considering patient size as the anteroposterior thickness. Their results showed that the CT effective dose was reduced with increasing anteroposterior thickness. The CT examinations in their study were performed with a relatively constant tube current. Theoretically, this would involve a constant effective dose, according to our software. The method they used to calculate effective dose contributed to a significant variation in this, because effective dose was corrected for patient size. This shows that different methods for estimating effective dose might have a major influence on these types of analyses, and must be taken into consideration when comparing results from different studies.
We found that the mAs with both excretory urography and CT varied considerably according to BMI. With excretory urography, an exponential variation in BMI might be expected from the automatic exposure control system (photo timing). However, with MDCT, the mAs varied as a consequence of subjective considerations. This was possibly caused by operator attempts to avoid an increased noise level for patients with high BMI.
In recent years, several studies have focused on and evaluated low-dose kidney MDCT protocols, which has led to the use of radiation doses comparable to those used with excretory urography [5, 17, 22, 23]. All studies concluded that a considerable dose reduction was achievable, and at the same time the image quality was adequate. A recent study of a low-dose kidney protocol discussed whether obesity is a factor that necessitates an increase in radiation dose in MDCT [22]. Those authors suggest that the increased noise in images taken of obese patients can be explained at least partly by the presence of intraabdominal fat. Both too much fat and too little fat were the main reasons given in a subjective evaluation of difficulty in radiologic interpretation, because intraabdominal fatty tissue separates the structures of interest and thereby compensates for increased noise [22]. If this is the case, it may indicate that our mAs adjustments for overweight and obese patients might not be necessary, a concept that ought to be considered when perfecting our procedures.
In our study, the number of images taken and the mAs selection were important in regulating the effective dose. Analysis showed that the excretory urography effective dose level reached MDCT dose levels when seven or more images were taken. This should be kept in mind because this number of images was taken in 15% of our study population. Our results indicate that the effective dose using MDCT as compared with excretory urography for patients with acute flank pain is not of an absolute proportion but is highly influenced by adjustable factors.
In conclusion, MDCT gives, on the average, an effective dose that is over double that with excretory urography (x2 and x2.6 for men and women), using our standard protocols. However, the risk is strongly predicted by patient characteristics (BMI) and procedure. Using our protocols, overweight patients and patients in whom seven or more excretory urography images are taken will become exposed to a radiation risk comparable to that with MDCT. However, this should not justify the considerably increased radiation risk for patients of normal weight or for patients in whom fewer than seven excretory urography images are taken. Following the ICRP principle of optimization, it is quite important to adapt technical parameters on the basis of clinical indication. We cannot justify the replacement of excretory urography with MDCT in the framework of this principle. Previous studies also support that the radiation risk of MDCT can be made equal to that of excretory urography by adjusting exposure protocols. Other factors, such as BMI, should also be taken into consideration because BMI too is a risk predictor. Overall, unenhanced optimized low-dose CT usually will be the most appropriate study for acute flank pain.
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