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Original Report |
1
Department of Radiology, Johns Hopkins Hospital, 600 N. Wolfe St., Baltimore,
MD 21287, and Johns Hopkins University School of Medicine, Baltimore, MD
21287.
2
Russell H. Morgan Department of Radiology and Radiological Sciences, Johns
Hopkins University School of Medicine, Baltimore, MD 21287.
3
Department of Radiology, Johns Hopkins Bayview Hospital, Baltimore, MD
21224.
Received August 25, 2000;
accepted after revision September 25, 2000.
Address correspondence to C. S. Georgiades.
Abstract
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CONCLUSION. In 95% of patients with acute renal obstruction, the affected kidney was less dense than the unobstructed kidney. When visually detected by radiologists using CT, this difference in density was at least two standard deviations above normal, making it a reliable secondary sign for acute obstruction.
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96%; specificity, 98%
[1,2,3,4])—does
not reveal a ureteral stone. The physician either assumes that the patient has
already passed the stone and so discharges the patient or initiates further
examination, imaging, or even admission while other possible causes of the
symptoms are considered (e.g., hernia or testicular or ovarian torsion). The
purpose of our research was to ascertain whether the visual detection of a
difference in renal parenchymal attenuation between the two kidneys on helical
CT scans can be a reliable secondary sign for obstruction. Although the
sensitivity and specificity of other secondary signs such as stranding,
hydronephrosis, and renal enlargement have been established
[1], to our knowledge the
density difference between the two kidneys has not been studied. This
attenuation difference is associated with edema in the obstructed kidney (a
so-called pale kidney) (Fig. 1)
and persists for a certain period of time after the obstruction has resolved.
The edema is caused by increased interstitial fluid resulting from hyperemia
and increased lymphatic pressure and flow
[4]. If, in a properly selected
population, the physician could rely on such a sign to conclude that the
patient indeed had passed a calculus, then patient risk and discomfort would
be minimized because unneeded additional examinations would be eliminated.
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All scans were obtained on a scanner using a helical sequence (Somatom Plus 4; Siemens Medical Systems, Iselin, NJ) with 3-mm-thick axial images from the adrenals to the base of the bladder. On each soft-tissue window image (center, 7 H; width, 470 H) and for each kidney, we calculated the parenchymal attenuation in Hounsfield units. We arrived at this calculation by averaging the densities of three random regions of interest, exercising care to include only renal parenchyma within the selected regions. Then we sought to quantify the minimum differences in attenuation between two kidneys that are possible to detect visually. To achieve this goal, we asked 10 radiologists to review one preselected image from each of 20 CT scans of patients with acute ureteral obstruction (randomly selected from our test group) and decide which, if either, kidney in each scan appeared less dense. The radiologists were unaware that the images were from cases diagnosed as ureteral obstruction. We correlated their findings with the actual attenuation difference between the kidneys.
We calculated the renal parenchymal attenuation of normal kidneys, using the same method as the one we used for the test group. To achieve this end, we reviewed the helical CT examinations of 20 additional patients seen in the emergency department who had no history or symptoms related to renal disease and whose CT scans were interpreted as showing normal findings (control group).
Finally, ureteral dilatation and perinephric stranding were quantified for each patient in our test group, and each patient was classified as having none, mild, or marked evidence of each of the two conditions. The findings were correlated with the percentage of attenuation difference between the kidneys.
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After applying the exclusion criteria to the 30 patients with positive findings on CT scans, we were left with 26 patients. Of these, 20 (77%) were men and six (23%) were women, yielding a male-female ratio of 10:3. The average age was 44 ± 15 years.
With the exception of one patient, the obstructed kidney in all patients was always less dense than the unobstructed kidney. The average difference in attenuation between the two kidneys was 14.5 H ± 10% (from 3% to -42%). The average (±SD) attenuation of the obstructed kidney was 32.2 ± 3.5 H, whereas that of the uninvolved kidney was 38.0 ± 3.1 H (Fig. 2). The latter attenuation figure matched that of our control group, which was calculated to be 38.1 ± 5.2 H. The standard deviation of the difference between the two normal kidneys was 2.56 H.
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Figure 3 illustrates the relationship between measured (x-axis) and subjective (y-axis) differences in renal parenchymal attenuation. Thus, when a radiologist confidently detects a difference in density between the two kidneys (the high values on the y-axis), the actual density difference is likely to be caused by a pathologic process. For example, when eight or more of the 10 radiologists detected decreased density in the obstructed kidney, the percentage of difference in attenuation (x-axis value) was at least two standard deviations above the expected difference for healthy kidneys. Thus, the probability that the difference did not reflect a pathologic process and was the result of chance was less than 5%. The pale-kidney sign was visually detected by the 10 reviewing radiologists in 16 of the 26 unenhanced helical CT scans. Figures 4, 5, and 6 show the attenuation difference as seen on unenhanced helical CT scans of three patients with various degrees of actual differences in attenuation. These patients are labeled as A, B, and C, respectively, in Figure 3.
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As we have shown, a quarter of the patients who presented with the classic symptoms of acute ureteral obstruction underwent additional workup because helical CT scans failed to show an obstructing ureteral calculus. As illustrated in Table 1, we divided these patients into two groups: those whose helical CT scans showed the pale-kidney sign and those whose scans did not. In patients who had scans that showed the pale-kidney sign, even in the absence of an obstructing calculus, additional testing to detect any abnormality other than ureteral obstruction produced negative findings. In patients who had scans that did not show the pale-kidney sign, none of the additional studies performed showed ureteral obstruction. These observations suggest a high specificity for the pale-kidney sign. This conclusion is further supported by the fact that all the patients whose additional studies showed obstruction were in the group who had scans that showed the pale-kidney sign.
Figure 7 shows the relationship between the percentage of difference in attenuation and hydronephrosis. The correlation coefficient (R) for these two secondary signs was calculated as 0.61. The relationship between the percentage of difference in attenuation and perinephric stranding is shown in Figure 8. These two signs were less strongly correlated, with a correlation factor of 0.34.
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The feared complication of ureteral obstruction is loss of renal function. When radiologists attempt to predict the seriousness of the obstruction, we believe that they should regard renal edema as the relevant variable because it indicates the kidneys' response to the obstruction. Hydronephrosis and hydroureter are phenomena that depend on the properties of the collecting system (elasticity, reactivity, and capacitance) and its interaction with the calculus. Thus, mildly increased collecting system pressure due to partial obstruction may cause marked dilatation of a flaccid ureter, whereas significantly increased intraureteric pressure may cause mild dilatation in a low-capacitance ureter. Increased ureteral pressure causes hyperemia and increased lymphatic pressure and flow [4], conditions that may result in diminished renal function if allowed to persist for more than 2 weeks [6]. The secondary sign of perinephric stranding was, in fact, inversely related to the need for a urologic procedure to relieve the obstruction [5].
In conclusion, renal edema can be diagnosed when a density difference between the two kidneys is seen on unenhanced CT scans or when the measured attenuation difference is more than 5 H (the equivalent of 2 SD of the density difference between two healthy kidneys). The pale-kidney sign can be used in conjunction with other secondary signs to increase the specificity for recently relieved obstructive ureterolithiasis. We believe the pale-kidney sign is more useful when seen in conjunction with hydronephrosis because neither pyelonephritis nor renal vein thrombosis causes this dyad. If pyelonephritis and renal vein thrombosis have been excluded by other tests, then the presence of the pale-kidney sign should be considered as a very specific sign for a passed calculus. Furthermore, this sign can be useful when the interpreting radiologist cannot reliably differentiate a distal ureteral calculus from a phlebolith, a situation that is not rare.
Though adequate for the general conclusions we have drawn in this article, our sample size did not allow assignment of specificity and sensitivity values to the pale-kidney sign. A larger test group is needed if we are to assign specific values to this sign.
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) and percentage of attenuation
difference between two kidneys (solid line and x). Correlation factor
for these two secondary signs is 0.61; x-axis shows amount of
attentuation in 26 patients with acute ureteral obstruction charted by
increasing attenuation difference.