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1 Department of Radiology, Centre Hospitalier Universitaire de Charleroi,
Hôpital Civil de Charleroi, 92 blvd. Janson, B-6000-Charleroi,
Belgium.
2 Statistical Unit, Institut de Recherche Interdisciplinaire en Biologie Humaine
et Nucléaire, Université Libre de Bruxelles, Brussels,
Belgium.
3 Department of Radiology, Hôpital Erasme, Université Libre de
Bruxelles, Brussels, Belgium.
Received December 28, 2001;
accepted after revision June 13, 2002.
Address correspondence to D. Tack.
Abstract
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SUBJECTS AND METHODS. The study included 106 patients who underwent unenhanced MDCT with 4 x 2.5 mm collimation, 120 kVp, 30 mAs, and, if necessary, additional focused acquisitions at 60 or 120 mAs on areas with an equivocal ureteral stone or with significant image noise. The effective radiation dose was computer-simulated with software based on the Monte Carlo model and International Commission on Radiological Protection recommendations. CT scans were archived and independently reviewed by three radiologists during two interpretation sessions on a workstation with three dimensions functions. Intraobserver and interobserver agreements were calculated with the kappa statistics. Accuracy for detection of ureteral stone on low-dose MDCT was calculated by comparison with combined clinical (stone passage), surgical (stone retrieval, extracorporeal shock wave lithotripsy), biologic (urinalysis, urine culture), and other imaging (excretory urography, standard-dose MDCT, follow-up sonography, and abdominal radiography) findings or by evidence for an alternative diagnosis.
RESULTS. Ureteral stones were present in 38 (36%) of 106 patients. Thirty-six of 38 ureteral stones were detected by low-dose MDCT. From reviewer to reviewer, the number of true-positive, false-positive, true-negative, and false-negative findings ranged, respectively, from 34 to 36, 1 to 4, 64 to 68, and 2 to 4. The corresponding sensitivity, specificity, and accuracy ranged from 89.5% to 94.7%, from 94.1% to 100%, and from 93.4% to 98.1%, respectively. The intraobserver and interobserver agreements were excellent, with kappa values ranging from 0.87 to 0.98. In 13 patients, an alternative diagnosis explaining the patient's symptoms was proposed by all reviewers using images obtained at 30 mAs. No additional or alternative diagnosis was found at standard dose. At 30 mAs, the mean effective dose was 1.2 mSv in men and 1.9 mSv in women. Additional acquisitions at 60 mAs, all focused on the lower pelvis, were acquired in 20 patients, but the corresponding images were needed by the reviewers for only six of them. The acquisitions at 60 mAs were responsible for an additional mean effective dose of 0.5 in men and 0.8 mSv in women.
CONCLUSION. Our study shows that low-dose unenhanced MDCT is appropriate for the diagnosis of ureteral stones, and that it provides excellent intraobserver and interobserver agreements and does not obscure alternative diagnoses.
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The aim of this study was to determine the intraobserver and interobserver agreements, sensitivity, specificity, predictive values, and accuracy of unenhanced MDCT obtained with a low radiation dose based on 30 mAs acquisition—corresponding to an effective-dose level within the range of the lowest previously reported dose delivered with helical single-detector CT [6].
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40
kg/m2). In the whole study group, the mean BMI was 26.2
kg/m2 (range, 18.0-48.7 kg/m2). The study protocol was
approved by our institutional review board. Informed consent was obtained from
all patients and, for those who were adolescents, from their parents.
CT Examinations
Images were obtained using a commercially available MDCT scanner (Somatom
Plus Volume Zoom; Siemens Medical Systems, Erlangen, Germany). Patients were
examined while in a supine position, and none received contrast material. A
52-cm scout image was first obtained at 120 kVp and 35 mA, followed by a
helical scan with simultaneous acquisition of 4 x 2.5-mm collimations at
120 kVp and 30 effective mAs. Effective mAs represent constant mAs when
increasing the pitch, owing to the automatic tube current adaptation
[14]. Table feed was 15 mm per
0.5-sec scanner rotation (30 mm/sec). These parameters result in a pitch
(table feed per scanner rotation divided by the collimation of one detector
row) of 6:1, equivalent to a pitch of 1.5:1 as defined by Silverman et al.
[15]. All examinations were
performed from the upper pole of the kidneys to the symphysis pubis. From the
raw data, 3-mm-thick sections were reconstructed with a 2-mm increment. If the
staff radiologist who conducted the examination considered that the image
quality was insufficient because of excessive noise, a second
acquisition—focused on the artifactual zone—was obtained at 60 mAs
and reconstructed with the same parameters as the first acquisition. If this
second set of images was also unsatisfactory, a third acquisition was obtained
at 120 mAs.
Effective Dose Calculations
The effective dose was computer simulated with a commercially available
software installed on a personal computer (WinDose; Institut für
Medizinische Physik, Universität, Erlangen, Germany)
[16]. This software does not
require phantom measurements. Inputs corresponding to CT parameters, the
patient's gender, and the scanned region, as represented on a graph of the
Monte-Carlo phantom model, were given to the program, which took into account
the multidetector nature of the CT examinations. The height of the scanned
region was calculated by the difference in table position between the first
and the last image. The effective dose was then computed according to the
Monte-Carlo simulations for anthropomorphic phantoms as recommended by Zankl
et al. [17]. The conversion
factors for CT used in this study were generated according to recommendations
appropriate for our scanner unit (ImpactMC)
[18]. The calculated effective
doses were expressed following the International Commission on Radiological
Protection recommendations (ICRP60). CT parameters determining the radiation
exposure were verified both at the beginning and at the end of the study. We
also used this software to calculate the effective dose delivered by
previously reported CT protocols
[2,
3,
6,
9,
10,11,12,
19,
20]. As stated by Kalender and
Schmidt [21], comparisons of
calculated radiation doses showed good agreement with previously reported
measurements, with a maximal difference of less than 15% for CT scanners
listed in the GSF Report 30/91
[17] as well as for other
scanners.
Image Analysis
The images were stored on compact disks and reviewed, for the purpose of
this study, on a clinical workstation (Wizard; Siemens Medical Systems) by two
board-certified radiologists (reviewers A and B) who had more than 10 years'
experience in interpreting abdominal CT scans and by one resident (reviewer C)
with 1 year of experience in radiology. These three reviewers were unaware of
the final results, the patient's BMI, and the side on which the pain was felt,
but they knew that the patient presented with acute lumbar or flank pain
suggestive of renal colic. Image interpretation was performed in two separate
and independent sessions with an interval between sessions of more than 30
days. The reviewers were asked to record the presence of an intraureteral
calcification as classified by Sourtzis et al.
[3]. The reviewers were also
asked to record the presence of the following indirect signs: renal
enlargement, dilatation of the excretory tract, perirenal or periureteral
stranding as proposed by Smith et al.
[22] and Varanelli et al.
[23]; and rim sign as defined
by Heneghan et al. [24]. When
a ureteral calcification was detected, its size and location were recorded;
any alternative diagnosis and unsuspected CT findings were also recorded.
Each reviewer analyzed the images obtained at 30 mAs and was allowed to use the three-dimensional functions of the workstation, including multiplanar reformation, curved reformation, and maximum intensity projection. The reviewer examined the images with the cine-viewing mode in the axial plane and in the coronal and sagittal reformations. If a reviewer found the images obtained at 30 mAs of insufficient quality, images obtained at 60 mAs were requested. If those images also were found to be of insufficient quality, a request was made for those obtained at 120 mAs.
Methods of Reference
The ureteral stone was considered to be definitely present if at least one
of the following criteria were fulfilled: surgical retrieval of the stone,
depiction of a ureteral stone by contrast-enhanced imaging studies (i.e.,
excretory urography, unenhanced or enhanced standard-dose CT, or both, within
24 hr), subsequent radiographs and sonograms showing evidence of calculus
migration, calculus excretion followed by relief of pain, macroscopic
hematuria, microscopic hematuria (minimum of two RBCs per high power field),
or positive dipstick urinalysis.
The ureteral stone was considered to be definitely absent if at least one of the following criteria was fulfilled: a negative microscopic or dipstick urinalysis and relief of pain with no treatment, depiction of absence of ureteral stone and obstruction by imaging studies (i.e., excretory urography, unenhanced or enhanced standard-dose CT, or both, within 24 hr); low-dose CT depiction of an alternative diagnosis and relief of pain after a specific treatment, a laboratory-based alternative diagnosis (e.g., urinary tract infection), or abdominal radiographs or sonograms obtained during the following days to look for an alternative diagnosis that showed no abnormal calcification or urinary tract dilatation.
Reviewer A acted as the main investigator. After interpreting the images, he reviewed all clinical data and calculated the results.
Statistical Analysis
The intraobserver and interobserver agreements were evaluated with the use
of kappa statistics. The 95% confidence intervals (CI) for the kappa
statistics were calculated, the null hypothesis of no agreement between
observers was tested, and the associated p values were calculated
[25]. All kappa values were
interpreted according to suggestions from the literature
[26]: a kappa value of less
than 0.20 indicated poor agreement; 0.21-0.40, fair agreement; 0.41-0.60,
moderate agreement; 0.61-0.80, good agreement; and 0.81-1.00, excellent
agreement. Sensitivity, specificity, negative predictive value, positive
predictive value, and accuracy (proportion of the number of concordant
observations among all) were also calculated. The McNemar test was used to
compare the performances (estimated by the proportion of correct observations
compared with the gold standard) between two assessments. Statistical
significance for all tests was set at a p value of less than 0.05.
All statistical analyses were performed with the StatXact 3 statistical
software (Cytel, Cambridge, MA).
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Sixty-eight of 106 patients (27 males and 41 females) were classified as definitely having no ureteral stone on the basis of one or more of the following results: negative microscopic or dipstick urinalysis findings and relief of pain with no treatment in 28 patients, depiction of absence of a ureteral stone and obstruction by contrast-enhanced imaging studies (excretory urography in five patients, unenhanced and enhanced standard-dose MDCT in 21 patients), CT depiction of an alternative diagnosis and relief of pain after a specific treatment in 11 patients (ovarian cysts in three patients, acute diverticulitis in three patients, abdominal wall herniation in two patients, lower lobe pneumonitis in one patient, pancreatitis in one patient, and pyelonephritis in one patient), biologic findings indicating a urinary infection in 13 patients, and abdominal radiographs or sonograms performed during the following days that showed no abnormal calcification or urinary tract dilatation in 10 patients.
Intraobserver agreements as determined by using the kappa statistics were (mean ± SD) 0.98 ± 0.02 (95% CI, 0.94-1.00) for reviewer A, 0.96 ± 0.03 (95% CI, 0.90-1.00) for reviewer B, and 0.90 ± 0.04 (95% CI, 0.81-0.98) for reviewer C (resident), with all p values lower than 0.001. The lowest interobserver agreement was found between reviewers B and C, with a kappa value of 0.88 ± 0.04 (95% CI, 0.81-0.98), whereas the highest interobserver agreement was found between reviewers A and B, with a kappa value of 0.98 ± 0.02 (95% CI, 0.94-1.00) and all p values lower than 0.001. Thus, both intraobserver and interobserver agreements were excellent.
The three reviewers identified a ureteral stone on the CT scans of 36 patients. The number of true-positive, false-positive, true-negative, and false-negative findings, and the sensitivity, specificity, predictive values, and accuracy of both interpretation sessions of the three reviewers are listed in Table 1. In two patients classified as definitely having a ureteral stone on the basis of a positive urinalysis, none of the three reviewers reported a stone. These findings were probably not false-positives because the stone likely had been recently passed. Nevertheless, they were classified as false-negative because no alternative diagnosis was shown during the follow-up period.
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Among the detected ureteral stones, 21 were left-sided and 15 were right-sided, nine were located in the upper third of the ureter, nine in the middle third of the ureter, and 16 close to or in the ureterovesical junction. The mean diameter of the stone was 4 mm (range, 2-9 mm). One patient had bilateral ureteral stones. In addition, renal calculi were seen in 17 patients. Two women in whom a ureteral stone in the distal ureter had been spontaneously excreted each had a simultaneous urinary infection shown by urine culture.
To evaluate possible differences in performances between the resident (reviewer C) and the board certified radiologists (reviewers A and B), we compared the number of correct and false diagnoses recorded by reviewer A and reviewer C in their first interpretation session. The number of correct diagnoses in 106 patients was higher for reviewer A (n = 104) than for reviewer C (n = 99). Nevertheless, this trend toward a difference in the number of false diagnoses did not reach statistical significance (p = 0.063, McNemar test). To evaluate a possible learning effect for the resident, we compared the number of correct and false diagnoses recorded during both of the resident's sessions. The number of correct diagnoses in 106 patients was higher during the second interpretation session (n = 102) than during the first one (n = 99). Nevertheless, this trend toward an increase in the number of correct diagnoses did not reach statistical significance (p = 0.375, McNemar test).
The staff radiologist who conducted the examination obtained supplementary, focused MDCT acquisitions at 60 mAs in 20 patients (eight males and 12 females), none of whom were underweight. These acquisitions were obtained in three (6%) of 49 normal-weight patients, in eight (22%) of 37 overweight patients, in seven (44%) of 16 obese patients, and in two extremely obese patients (100%). All three reviewers asked for these corresponding images for only six of the 20 patients (in one overweight, three obese, and two extremely obese patients). An illustrative case is shown in Figure 1A,1B,1C. Three of these six patients definitely had a ureteral stone. The remaining 14 of 20 supplementary acquisitions obtained at 60 mAs were not needed by any reviewer. An illustrative case with an unnecessary acquisition at 60 mAs is shown in Figure 2A,2B,2C. One patient also had an additional focused CT acquisition at 120 mAs, but the three reviewers did not need these images. No image at 60 mAs that had not been acquired by the staff radiologist who conducted the examination was requested by any of the three reviewers. In all patients in whom a supplementary acquisition was requested, the area with an equivocal ureteral stone or with significant image noise was the lower pelvis, at the level of the hips. Among the six patients in whom the 60-mAs images were requested by the reviewers, five patients had a BMI higher than 351. The diagnosis was reached at 30 mAs in only one of the six patients with a BMI higher than 35 kg/m2. Figure 3 shows an example of multiplanar reformation images revealing the presence of a ureteral stone in an obese patient imaged at 30 mAs.
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The mean height of the scanned region at 30 mAs was 31 cm (range, 20-36 cm). At 30 mAs, the calculated mean effective radiation dose was 1.2 mSv (range, 0.8-1.5 mSv) in men and 1.9 mSv (range, 1.5-2.3) in women. The mean height of the scanned region at 60mAs was 11 cm (range, 7-15 mSv). The corresponding effective radiation dose was 0.5 mSv (range, 0.4-0.7) in men and 0.8 mSv (range, 0.6-1.1) in women. For the only patient who underwent scanning at 120 mAs, an extremely obese woman (BMI = 48.7 35 kg/m2), the height of the scanned region was 31 cm, and the corresponding effective dose was 7.6 mSv.
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Because standard-dose helical CT has the highest accuracy in the assessment of ureteral stones, this technique is probably the most appropriate method of reference for comparative studies [1, 11, 20]. Performing both standard-dose and low-dose MDCT in all patients at the same time would therefore have been ideal in this regard but would have also resulted in an excessive radiation dose in a study group including young patients. We thus used various criteria validated by previous studies to establish the definite diagnosis [2, 3, 9, 22]. Probably this is not the most appropriate method of reference and, as a consequence, the clinicians may have ordered additional diagnostic tests when they doubted the proposed diagnosis made on the low-dose MDCT examinations. Thus, in our study, one third of the patients with or without a ureteral stone had excretory urography or standard-dose CT prompted by clinicians in the immediate follow-up period. With this study design, we also had two patients with a history consistent with recently passed stones who were classified as having false-negative findings on low-dose MDCT on the basis of positive findings on urinalysis—a method that is known to have a positive predictive value of only 76% [30]. These patients would probably have been classified as true-negative if standard-dose CT had been considered as the method of reference. If we consider these two patients as true-positive cases, the accuracy would range, among reviewers, from 95% to 100%.
One of the major advantages of CT is to provide alternative and supplementary diagnoses. In a retrospective analysis of 1000 patients with suspected renal colic who underwent standard-dose helical CT, Katz et al. [4] reported 101 examinations revealing alternative or supplementary diagnoses. We found 13 patients (12%) with an alternative diagnosis and 17 patients (16%) with renal stones. The spectrum of these diagnoses was similar to those reported by Katz et al. [4]. Twenty-one of 55 patients without ureteral stone in whom no alternative diagnosis was suggested on low-dose CT underwent standard-dose CT. Among them, no additional alternative diagnosis was revealed, even in patients admitted to the emergency department the night before. As previously shown by Liu et al. [9] and by Diel et al. [10] in single-detector CT studies, an alternative diagnosis can be accurately identified with low radiation doses, despite increased pitch, volume-averaging artifacts, reduced mAs, and subsequent increased image noise.
In 14 of 20 patients in whom a focused acquisition was obtained at 60 mAs, these corresponding images were not needed by any of the reviewers. The staff radiologist who conducted the examination had to be absolutely sure that no ureteral stone could be missed and had to decide whether to request an additional acquisition with a higher dose within a few seconds in order not to disturb the day-to-day organization of our CT department. On the other hand, the reviewers involved in our study had no time constraints for the image analysis. All six examinations considered to be of insufficient quality when obtained at 30 mAs by the three reviewers had severe artifacts in the lower pelvis at the level of the ureteropelvic junction. To obtain high quality images in this region is critical because approximately half of ureteral stones are located in the distal ureter [1, 11]. In this particular anatomic region, which is not more than 10 cm in height, the tube current should be higher than that used in the flanks. Both the image quality and the radiation dose delivered to the patient should benefit from an on-line tube-current modulation dependant on the rotation angle of the X-ray beam. However, because images obtained at 60 mAs were needed by the reviewers in only one of 36 overweight and three of 16 obese patients, the low-dose acquisition obtained at 30 mAs should be sufficient except in extremely obese individuals. On the other hand, because the images obtained at 30 mAs did not permit a diagnosis of ureteral stone in five of six patients with a BMI higher than 35 kg/m2, 60 mAs should primarily be used in these patients. Our results differ from those of Hamm et al. [6], who performed low-dose single-detector CT with an equivalent dose level. These authors propose a BMI threshold of 30 kg/m2 for excluding patients from the low-dose technique. Our results suggest an intermediate dose acquisition at 60 mAs for these patients.
The mean effective dose delivered with MDCT at 30 mAs (1.2 mSv in men and 1.9 in women) is within the range of a three-film excretory urography examination (1.5 mSv) [5] and is lower than that of six-film excretory urography (2.5 mSv) [31]. In addition, we have recalculated the effective dose delivered in various protocols described in previous studies using single-detector helical CT scanners [3, 9, 10, 19, 21] and in two studies using a MDCT scanner [11, 12] (Table 2). The radiation dose delivered with our protocol is from one third to one fourth of that delivered with single-detector helical CT protocols, and as low as one ninth of that of a more standard MDCT technique. This radiation dose is similar to that from the study recently reported by Hamm et al. [6], which used a single-detector CT technique with 5-mm collimation, 70 mAs, and a pitch of 2. Other studies using single-detector CT reduced radiation dose by increasing the table feed by rotation, reaching a pitch of 2-3 [9, 10]. In these studies, mAs were maintained at high levels and the resulting effective doses were still relatively high, as listed in Table 2. Recent studies using MDCT were based on thin collimations ranging from 1 to 3 mm but with mAs settings similar to those previously used with single-detector CT, leading to an effective radiation dose increased by a factor of 2-3, as shown in Table 2 [11, 12]. However, when using MDCT scanners, the radiation dose can be dramatically reduced by decreasing the mAs. Factors permitting the dose reduction with these scanners are solid-state detectors more sensitive than previously used helium gas detectors, rotation time reduced to 0.5 sec, and easily used controls of the mAs settings directly available on the screen of the CT unit. In addition, with the new concept of effective milliampere-second (i.e., automatic adapted tube current when varying the pitch), the duration of acquisition can be adapted to the patient's capability of breath-holding, with the radiation dose remaining constant [14].
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As used in our study, the computer simulation of the effective dose has several limitations. Rainbow and Liu [32] recently pointed out that calculation of the effective dose provides an examination-specific estimate based on many assumptions and is not directly applicable to any individual because patient-specific estimates depend also on patient size and anatomy. In dose calculations based on the Monte Carlo model, the considered distance in a standard trunk between the upper pole of the kidneys and the lower aspect of the bladder is 40 cm [17, 23]. In our study group, the height of the scanned region was measured in each patient and the radiation dose calculated accordingly. Neither our program nor the Monte Carlo model take into account the weight or the abdominal diameter of the patient. As shown by Huda et al. [33] with phantoms of various sizes scanned with constant kilovoltage and milliamperes-second, the energy imparted on CT increases with the patient's size, but the corresponding effective radiation dose is higher in smaller phantoms than in bigger ones. Because pelvic organs responsible for a significant part of the effective dose (i.e., the bladder, colon, and gonads) are close to the center of the pelvis, the effective dose should be lower in obese patients than in underweight ones. In other words, an increase in milliamperes-second settings in patients with a high BMI may not result in an increased effective radiation dose.
Contrary to the suggestion by Haaga [34], we did not classify the patients according to their abdominal diameter but rather to their BMI for several reasons. First, the BMI is a practical measure of the body size because it does not require reference tables of ideal weight and is available before the CT examination. Second, with MDCT, four contiguous axial slices are acquired simultaneously, whereas only one is necessary to measure the abdominal diameter, the other slices resulting in useless radiation exposure. Third, the maximum abdominal diameter is usually close to the umbilicus, much more cranial than the lower pelvis where most artifacts related to the hip structures are seen.
In conclusion, our study suggests that, in the assessment of ureteral stones, using low-dose unenhanced MDCT scans obtained with an effective dose not higher than that of a three-film radiographic examination or of the lowest dose of single-detector CT results in excellent intraobserver and interobserver agreements and an accuracy equivalent to that of standard-dose single-detector CT.
Acknowledgments
We thank Christian Delcour, Ingrid Perlot, and Michel Vanhaeverbeek for
their support and advice. We also thank all the CT technicians for their kind
help in archiving the CT data.
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