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1 Department of Radiology, Federal University of São Paulo School of
Medicine, Rua Napoleão de Barros 800, São Paulo CEP 04024-002,
Brazil.
2 Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical
School, 330 Brookline Ave., Boston, MA 02215.
3 Department of Radiology, Hospital do Rim e Hipertensão UNIFESP, Rua
Borges Lagoa 960, São Paulo CEP 04038-002, Brazil.
4 Department of Urology, Federal University of São Paulo School of
Medicine, Rua Napoleão de Barros 715/2, São Paulo CEP 04024-002,
Brazil.
5 Department of Radiology, University of Alabama at Birmingham, 619 S 19th St.,
Birmingham, AL 35233.
Received August 18, 2003;
accepted after revision November 10, 2003.
Address correspondence to S. Faintuch
(sfaintuc{at}bidmc.harvard.edu).
Abstract
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MATERIALS AND METHODS. Consecutive unenhanced helical CT scans of patients referred for acute unilateral renal colic were retrospectively reviewed. Patients with CT evidence of other urinary system diseases were excluded. Included scans (n = 145) were assessed for ureteral stone and secondary signs of obstruction such as unilateral collecting system or ureteral dilatation, perinephric stranding, and periureteral edema. Renal attenuation in Hounsfield units was measured in the upper, middle, and lower portions of the parenchyma, and a mean value was determined for each kidney.
RESULTS. Ureteral stones were present in 76 patients. Renal attenuation on the side with lithiasis was lower than on the opposite kidney: 27.2 ± 3.9 H vs 32.6 ± 3.4 H (p < 0.001). Attenuation differences between kidneys were higher for patients with ureterolithiasis: 5.4 ± 3.2 H (range, –3.3 to 13.0 H) versus 1.2 ± 1.0 H (range, 0–4.7 H) (p < 0.001). An attenuation difference between kidneys greater than or equal to 5.0 H had 61% sensitivity, 100% specificity, 100% positive predictive value, 69% negative predictive value, and 79% accuracy for diagnosis of ureteral lithiasis.
CONCLUSION. Attenuation difference between kidneys greater than or equal to 5.0 H was a valuable sign and had diagnostic performance similar to other secondary signs of obstructive ureterolithiasis. Furthermore, attenuation difference had the advantage of being an objective, measurement-based indicator.
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Attenuation values were systematically measured with an elliptic region of
interest (ROI) in the upper, middle, and lower portions of renal parenchyma
(corticomedullary) (soft-tissue window: center, 20 H; width, 300 H). In the
upper pole, the ROI was placed in the first section that depicted the
collecting system. In the middle portion, the measurement was made at the
level of the hilum and, in the lower pole, the ROI was placed in the last
section in which the collecting system could be identified. All measurements
were made with a similar-sized ROI (
40 mm2) in the posterior
region of the kidney parenchyma (Fig.
2A,
2B,
2C). Whenever a rib was close
to the assigned region, the ROI was moved to avoid measurement bias. A mean
attenuation value (in Hounsfield units) was calculated for each kidney from
the three measurements.
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The difference between the mean attenuation values for each kidney was calculated as follows: unobstructed kidney minus obstructed kidney (in patients with ureteral stone) or higher attenuation kidney minus lower attenuation kidney (when no ureteral stone was detected).
One hundred forty-five examinations were suitable for this study. Three patients were excluded from the analysis because of the presence of multiple secondary signs of obstruction (collecting system dilatation, perinephric stranding, periureteral edema, and attenuation differences between kidneys) with no direct identification of calculus. These findings, probably explained by recent calculus passage, would represent a confounding variable in data analysis (attenuation cutoff point determination), because of the adopted reference standard.
Statistical analysis was performed using the chi-square test (frequency of secondary signs) and the t test (comparison of attenuation values). Confidence intervals (95% CI) were calculated using a normal approximation to the binomial distribution.
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Attenuation differences between kidneys, in the same individual, were higher for patients with ureterolithiasis: 5.4 ± 3.2 H; range, –3.3 to 13.0 H (n = 76), compared to patients with no lithiasis: 1.2 ± 1.0 H; range, 0–4.7 H (n = 66) (p < 0.001).
The cutoff point for the diagnostic attenuation difference between kidneys was determined as 5.0 H, on the basis of the range of difference values observed for patients without lithiasis (0–4.7 H; n = 66). None of those patients had a difference value greater than 5.0 H. An attenuation difference between kidneys equal to or greater than 5.0 H was present in 46 patients (61%) with ureteral lithiasis. This attenuation difference between kidneys equal to or greater than 5.0 H had 61% sensitivity (95% CI, 50–72%), 100% specificity (100–100%), 100% positive predictive value (100–100%), 69% negative predictive value (59–79%), and 79% accuracy (70–88%) in the diagnosis of ureteral lithiasis.
Attenuation difference between kidneys equal to or greater than 5.0 H was more strongly associated with other secondary signs, such as collecting system dilatation (p < 0.01), perinephric stranding (p < 0.001), and periureteral edema (p < 0.05), than attenuation values less than 5.0 H in patients with ureterolithiasis.
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In this study, unequivocal stone characterization within the lumen of the
ureter was adopted as the reference standard for ureterolithiasis. Cases were
sorted, on the basis of this standard, for interpretation of attenuation
measurement results. Of the 145 scans suitable for this investigation, three
showed multiple unilateral secondary signs of obstructive urolithiasis
(collecting system dilatation, perinephric stranding, periureteral edema, and
attenuation difference 
5.0 H) in the absence of visible calculus. It was
decided that these three patients were unsuitable for inclusion in the
analysis because they had strong evidence of ureteral stone disease, probably
explained by recent stone passage, but did not conform to the adopted
standard. On the other hand, their inclusion in the nonlithiasis group would
be a source of bias in the attenuation cutoff point determination (all three
had attenuation difference between kidneys 5.0 H).
Several secondary signs of obstructive urolithiasis have been proposed as having value to resolve the issue of recent stone passage, as well as the issue of lack of visualization of a stone due to small size, low attenuation, respiratory movement, and volume averaging. The role of these signs is to help diagnose patients with acute flank pain with inconclusive evidence of ureteral stone. These signs, however, are not universally present and may vary in degree of severity from patient to patient [13].
Renal attenuation measurements (attenuation difference between the kidneys) as an adjunct sign of obstructive urolithiasis can provide additional diagnostic benefit to other secondary signs. The attenuation differences between kidneys are objective, measurement-based parameters, eliminating the subjectivity associated with other signs.
The choice of attenuation difference values rather than absolute attenuation values was made to allow better reproducibility with different CT scanners and calibration settings.
Investigators who previously described this sign had small samples (24 and 26 patients with ureterolithiasis, respectively) and have not determined a reliable diagnostic cutoff point for attenuation values nor the sign's sensitivity and specificity. Georgiades et al. [14] emphasized the subjective evaluation of the attenuation difference, which is the visual identification of a low-attenuating kidney parenchyma (pale kidney sign). Regarding the attenuation measurements, of 26 patients with ureterolithiasis, only one did not show lower attenuation values in the obstructed kidney (5%).
Using our data, we found it to be evident that attenuation difference
values between kidneys are much more variable than originally suggested. Also,
attenuation difference values between kidneys high enough to be useful for
diagnostic classification ( 5.0 H) are not highly prevalent (61%). This
finding may be a result of the fact that not every ureteral stone produces
high-grade obstruction at the time of presentation. Only highly obstructive
stones might lead to an attenuation difference value above the cutoff point,
just as only obstructive stones lead to other secondary signs
[10,
13]. A common pathophysiology,
represented by edema, hyperemia, and increased lymphatic flow, may explain
both low parenchymal attenuation and other signs such as perinephric stranding
[13,
16].
This hypothesis is confirmed by the increased association of difference values greater than or equal to 5.0 H with other secondary signs, when compared to values less than 5.0 H, in patients with lithiasis. The frequency of all secondary signs increases as duration of flank pain increases [13].
Patients with no CT evidence of ureteral lithiasis were considered the control group of this study. This choice is justified by the main purpose of the proposed sign, which is to exclude ureterolithiasis. One might hypothesize that an attenuation difference between kidneys 5.0 H or greater could also be found in asymptomatic subjects or in patients with other abdominal diseases. This is a potential limitation of our study. To address this question, the inclusion of asymptomatic subjects or patients with nonspecific symptoms or nonurinary abdominal diseases as an additional control group would have been useful.
Another potential limitation of this study is its retrospective design. A prospective design would have allowed us to address other interesting parameters, such as the relationship between the attenuation difference development and the duration of pain, or the possible association between the magnitude of the attenuation difference (i.e., obstruction) and the clinical outcome (spontaneous calculus elimination or necessity for intervention).
In conclusion, attenuation difference between kidneys 5.0 H or greater had high specificity (100%), positive predictive value (100%), and accuracy (79%) in the diagnosis of ureterolithiasis. Its diagnostic performance was similar to that of other secondary signs of obstructive ureterolithiasis. Attenuation difference had the additional advantage of being the only objective, measurement-based indicator.
Acknowledgments
We thank Andrea P. Scaciota, Donna H. Wolfe, and Michael E. Larson for
image art and processing. We acknowledge the help of Robson F. Q. Fanton and
Sabrina A. Bravo in the retrieval of CT scans included in this study.
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