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1 All authors: Department of Radiology, University of Michigan Hospital, 1500 E. Medical Center Dr., Ann Arbor, MI 48109-0030.
Received November 10, 1999;
accepted after revision January 10, 2000.
Address correspondence to J. F. Platt.
Abstract
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MATERIALS AND METHODS. Over a 6-month period, patients presenting to the emergency department with acute flank pain were examined with standard abdominal radiography and unenhanced helical CT, which included CT scout radiography. In 60 patients in whom a diagnosis of ureteral calculus was made, CT scout radiographs and abdominal radiographs were examined by two interpreters who assessed whether stones could be visualized. All CT scout radiographs were viewed on a workstation using optimized window settings.
RESULTS. CT scout radiography and abdominal radiography revealed 28 (47%) and 36 (60%) of 60 ureteral calculi, respectively. All ureteral calculi that appeared on CT scout radiography also appeared on abdominal radiography. However, eight calculi that were visible on abdominal radiography were not visible on CT scout radiography. CT scout radiography and abdominal radiography revealed 28% and 46% of 39 calculi less than or equal to 3 mm in diameter, respectively. For 21 calculi larger than 3 mm, the sensitivity of CT scout radiography and abdominal radiography was 81% and 86%, respectively.
CONCLUSION. Abdominal radiography is more sensitive than CT scout radiography in revealing ureteral calculi; however, some calculi revealed on unenhanced helical CT cannot be seen on either abdominal radiography or CT scout radiography. Ureteral calculi not visible on either study can only be followed, when necessary, with unenhanced helical CT.
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Abdominal radiography can be used for following ureteral calculi diagnosed on CT; however, calculi are not revealed on radiography in a considerable number of patients. In a recent study of 178 patients with acute flank pain, Levine et al. [9] reported that the accuracy of radiography to reveal ureteral calculi was relatively low at 40% and only improved to 59% when read in conjunction with helical CT.
It would be very helpful if the CT scout radiograph obtained immediately before the axial CT scans could be used to determine the presence or absence of ureteral calculi; after that, patients could be followed up serially with abdominal radiographs. Therefore, a patient undergoing helical CT for suspected ureteral calculi would not have to undergo additional abdominal radiography after a ureteral calculus was diagnosed. Chu et al. [10] examined the sensitivity of the CT scout image for revealing ureteral calculi. In that study, the scout radiographs revealed ureteral calculi in only 49% of patients who had visible calculi on their axial CT images. Although this suggests that the CT scout radiograph is not as sensitive as the abdominal radiograph, the authors did not directly compare the sensitivity of CT scout radiography with that of conventional abdominal radiography.
To our knowledge, no prior published data have directly compared the sensitivity of abdominal radiography with that of CT scout radiography in the same cohort of patients. Therefore, our study was designed to compare these sensitivities in a group of patients who presented to the emergency department with acute renal colic.
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The anteroposterior CT scout image was obtained from the level of the xyphoid process to the level of the lesser trochanter before the axial CT sections on each patient and photographed to occupy an entire film panel. CT scout radiography was performed using 120-140 kVp and 80 mA. Axial CT images were then obtained from a level above the upper poles of the kidneys to the level of the base of the bladder. CT was performed on either a LightSpeed QXI scanner or a HiSpeed scanner (General Electric Medical Systems, Milwaukee, WI). Two helical series were obtained (one breath-hold per series): the first, from above the kidneys to the iliac crests, and the second, from 2 cm above the iliac crest to below the symphysis pubis. Slice thickness was 5 mm for images obtained from the upper poles of the kidneys to the iliac crests and 3 mm for images obtained from the level of the iliac crests to the base of the bladder. All images were reconstructed at contiguous intervals. The axial CT images were obtained using 120-180 kVp and between 180 and 200 mA. These parameters were adjusted according to patient body habitus.
Using previously published data [2, 3, 5], 60 patients were identified as meeting CT criteria for ureteral calculi and were included in the study. The axial CT images, CT scout radiographs, and abdominal radiographs obtained on these 60 patients were retrospectively reviewed during the same review session. Decisions were made by the consensus interpretation of two radiologists. The images were analyzed for calculus identification and size and location of each calculus. Calculi were scored as visible or not visible for each of the two techniques (abdominal radiography and CT scout radiography). Calculus size was measured on the axial CT images (maximum transverse diameter), and calculus location was scored as being in the proximal, mid, or distal ureter or at the ureterovesical junction. The CT scout radiographs were viewed on a workstation using optimized window settings individualized for each patient. The Fisher's exact test was used to compare the sensitivity of CT scout radiography with that of abdominal radiography. A p value of less than 0.05 was considered significant.
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Detection Versus Size
Table 1 shows the
relationship of calculus size to visibility on abdominal radiography and CT
scout radiography. When calculus size was correlated with visibility, for
calculi larger than 3 mm, the sensitivity for detection improved to 86% for
abdominal radiography and 81% for CT scout radiography. Most calculi not
revealed on either method were smaller than 3 mm (range, 1-6 mm). Mean size of
calculi revealed on CT scout radiography was 3.7 ± 1.4 mm compared with
2.3 ± 1.1 mm for those not visible on CT scout radiography. Similarly,
mean calculus size on abdominal radiography was 3.5 ± 1.45 mm compared
with 2.2 ± 1.2 mm for those not visible. We identified 15
intermediate-size (4-6 mm) calculi in our study. Twelve were seen on abdominal
and CT scout radiography, whereas two were not seen on either technique. One
calculi was seen on abdominal radiography but not on CT scout radiography.
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Detection Versus Location
For abdominal radiography, the sensitivity for calculus detection based on
calculus location was 67% for the proximal ureter, 86% for the mid ureter, 57%
for the distal ureter, and 52% for the ureterovesical junction compared with
44%, 57%, 48%, and 43% for CT scout radiography at the same locations,
respectively (Table 2).
Abdominal radiography was superior to CT scout radiography for revealing
calculi located in the proximal and the mid ureter; however, both techniques
were relatively poor in revealing calculi in the distal ureter and at the
ureterovesical junction.
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In most patients, ureteral calculi will pass spontaneously, and follow-up imaging may not be required; however, some patients with small or intermediate-size calculi (measuring up to 6 mm in diameter) will need follow-up imaging to monitor the progress of their calculi and determine the need for intervention. If the calculus does not change location, if symptoms worsen, or if superimposed infection develops, then prompt intervention is usually required. On the other hand, a conservative "watch and wait" plan may be more suited for calculi that show progression along the course of the ureter on serial imaging studies [12,13,14,15].
The treatment of some patients with ureteral calculi may require serial imaging studies. This has been traditionally performed with abdominal radiography. Previous studies have shown that the sensitivity for revealing calculi on radiography was as high as 90% [16, 17]. However, in a recent study, Levine et al. [9] showed that the sensitivity of abdominal radiography for revealing ureteral calculi is considerably lower. In this study, only 40% of CT-diagnosed ureteral calculi were revealed on abdominal radiography when the radiographs were interpreted independently of CT scans. However, the detection rate improved to 59% if abdominal radiographs were interpreted in conjunction with axial CT scans. The results from our study, which showed an overall 60% sensitivity of conventional radiography for revealing ureteral calculi of all sizes, are similar to those reported by Levine et al.
CT scout radiography is also capable of revealing renal and ureteral calculi. In the only published study regarding the sensitivity of CT radiography for revealing ureteral calculi, Chu et al. [10] found that the sensitivity of CT scout radiographs obtained at the same time as unenhanced CT scans was 49% in 215 patients. These results are similar to those of our study, in which CT scout radiography had an overall sensitivity of 47% for revealing ureteral calculi in 60 consecutive patients.
Although direct comparison of the results of Levine et al. [9] and Chu et al. [10] suggests that conventional radiography is superior to CT scout radiography, our study's direct comparison of the two techniques confirms this fact.
As suggested by Chu et al. [10], the reason for the lower sensitivity of CT scout radiography may be based on the fact that calculus detection is dependent on kilovoltage settings. These authors routinely used settings between 120 and 140 kVp for obtaining CT scout radiographs (as did we in our study). Lower kilovoltage settings might have improved detection because of the higher percentage of photoelectric reactions that result from X-ray interactions with calcium-containing stones.
Chu et al. [10] did not attempt to correlate calculus size with detection. When we performed such an analysis, we found a significant correlation between calculus size and sensitivity for abdominal radiography and CT scout radiography. Sensitivity improved from 47% to 81% (CT scout radiography) and from 60% to 86% (abdominal radiography) for revealing calculi larger than 3 mm in comparison with calculi 3 mm or smaller.
Although previous studies have shown that calculus detection is not influenced by its location [9, 10], in our study, a higher percentage of proximal ureteral calculi was revealed on abdominal radiographs than on CT scout radiographs. The greatest difference in sensitivity was noted for proximal and mid ureteral calculi.
In summary, our study found abdominal radiography to be superior to CT scout radiography for revealing calculi. For this reason, we recommend that abdominal radiography be obtained immediately after renal stone CT (if the CT scout radiograph does not reveal the calculus) to determine whether a calculus can be followed with conventional radiography. In a patient whose CT-revealed calculus cannot be identified on abdominal radiography, serial imaging follow-up, if indicated, must be performed with repeated unenhanced helical CT (Fig. 3A,3B,3C).
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Accordingly, we recommend that a statement be dictated in every CT report as to whether calculi can be seen on CT scout radiography. This would ensure that follow-up images are obtained with the appropriate technique. Additionally, a statement could be made in the CT report about stone location on the CT scout radiograph relative to anatomic landmarks, such as osseous structures.
In the future, comparison of the CT scout radiograph obtained using a more optimized technique with abdominal radiographs may be useful to determine whether an optimized CT scout radiograph can approach the sensitivity of radiography.
Acknowledgments
We thank Julie L. Lucas for help and support in the preparation of this
manuscript.
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