Work Up of Obstructive Uropathy

Kidney stones are one of the most common and painful disorders to affect humans. It is estimated that up to 10% of the U.S. population will have a kidney stone at some point in time. Traditionally, suspected stone disease has been imaged by plain KUB and IVP studies. Historically, 80- 90% of uroliths were felt visible with plain radiography, especially stones with variable combinations of calcium, phosphate, magnesium and oxalate crystalloids. However, radiopaque calculi may be obscured by the density of the skeleton and matrix, uric acid, xanthine, and to a lesser degree, cystine. Calculi are radiolucent and not visualized on plain films. Additionally, the KUB frequently demonstrates numerous incidental calcifications projected in the area of the urinary system, which can be confused with uroliths, such as phelboliths and vascular calcifications.

The KUB is a useful and inexpensive test; however, it is plagued both by false negatives and false positives. A recent study from the Yale Univ. School of Medicine reported that the KUB was able to detect less than half the ureteral calculi present in a large retrospective study of patients with known stones. If a KUB does demonstrates possible uroliths, or if the clinical suspicion of obstructive uropathy is strong, an IVP is often requested.

An IVP is a sensitive test for calculi; however, it is an imperfect standard which may not reliably image smaller calculi. Patients who have a history of renal insufficiency, multiple myeloma, prior contrast sensitivity or other significant medical allergies are at increased risk for intravenous contrast reactions and may not be good candidates for an IVP. Many patients with renal colic present at urgent care settings, where it is desirable to complete the workup in a timely manner. An IVP may be delayed for hours if the patient has eaten recently, and the examination itself may take hours to complete if high-grade obstruction is present. Finally, if the IVP is negative, further imaging studies may have to be preformed in order to evaluate other organ systems.

Spiral CT reconstructed AP view demonstrating a right ureteral calculus.

Ultrasound is often a helpful examination in an urgent care setting as it is fast, inexpensive, and allows imaging of the solid abdominal viscera and pelvic organs. It is the test of choice in pregnant women. Unfortunately, ultrasound is insensitive to ureteral calculi and smaller renal calculi. In the absence of hydronephrosis, ureteral calculi may be missed. Pregnant females also often have physiological dilation of the ureters, which can be asymmetric, simulating hydronephrosis. If US Doppler evaluation with resistive indices and color evaluation of ureteral jets is inconclusive in pregnant women, then further assessment may be needed with a limited IVP or a nuclear renal scan (which actually has a much lower fetal radiation dose than an IVP).

Our test of choice for suspected stone disease is spiral CT of the urinary system. This study requires no contrast and no patient preparation. The only relative contraindication is pregnancy. Because there is no expense for contrast agents and in view of the fact that the scan time is very fast (about 60 seconds) we have priced this CT study only 10% over the cost of an IVP.

Spiral CT reconstructed coronal demonstrating bilateral renal calculi and right ureter calculi. Same patient as figure 1.

Axial non-contrast CT demonstrates an incidentally discovered right renal cell carcinoma.

Noncontrast CT is a highly accurate and diagnostic examination with sensitivity of 97%, a specificity of 96%, and an accuracy of 97% for diagnosing ureteral stone disease. In the seminal article by Smith, the sensitivity of the spiral CT was actually found to be superior to that of the IVP. We routinely detect uroliths on CT that are occult on the comparison KUB. Even "radiolucent" calculi typically have enough density to be visualized on CT. We often find small "passed" stones lying dependently in the bladder, which would be obscured by contrast on the IVP, confirming the presence of stone disease. A ureteral stone is considered to be present if a calcific density is identified within the lumen of the bladder, with or without unilateral ureteral dilatation and stranding of the periureteral fat.

The initial scan, which we perform with 5mm collimation with 2.5mm reformations, is immediately reviewed by the radiologist at the computer workstation. If needed, an additional 2 by 1 mm finely collimated acquisition is obtained, or rarely IV contrast is given to evaluate ureteral dilatation secondary to blood clots, etc. or to further assess incidental renal masses. The presence of hydronephrosis is determined on noncontrast CT by direct visualization or by the presence of perinephric soft tissue stranding.

Direct imaging correlation between spiral CT, plain radiographs and nephrotomography using a phantom has proved that spiral CT is the most accurate method of measuring stone size. Additionally, CT is highly accurate in determining stone volume, with is not possible with the other tests. CT has also been calculated to have less direct technical cost for imaging patients after EWSL than plain radiography and US.

Another major advantage of spiral CT is that the study frequently demonstrates the cause of the patient's flank pain when no calculi are present. In one study, over 20% of patients had significant pathology documented in other organ systems. We have diagnosed a spectrum of disease in patients with suspected obstructive uropathy, including ruptured aneurysms, diverticulitis, appendicitis, pancreatitis, and adnexal pathology using noncontrast CT. All of these conditions would have been occult in IVP.

In patients with suspected renal masses, contrast enhanced CT is preferred. Nonetheless, we have detected even small renal cell carcinomas as an incidental finding in patients who present with concurrent ureteral stones. This CT study requires a spiral scanner with a high heat capacity tube, and is best performed with a computer workstation where multiplanar and 3-D images can be generated if needed. Plain radiographs add no immediate additional information after spiral CT and are not mandatory, although some urologists like to obtain them to see if the stone is dense enough to be followed by KUB.

REFERENCES:

Smith RC, Verga M, et al. Diagnosis of acute flank pain: Value of unenhanced helical CT. AJR 1996;166: 97-101

Sommer FG, Jeffery RB, et al. Detection of ureteral calculi in patients with suspected renal colic: Value of reformatted noncontrast helical CT. AJR 1995;165: 509-513

Katz DS, Lane ML, et al. Unenhanced helical CT of ureteral stones: Incidence of associated urinary tract findings. AJR 1996;166:1319-1322

Heneghan JP, Dalrymple NC, et al. Soft-tissue "rim" sign in the diagnosis of ureteral calculi with the use of unenhanced helical CT. Radiology 1997;202:709-711

Olcott EW, Sommer FG, et al. Accuracy of detection and measurement of renal calculi: In vitro comparison of three-dimensional spiral CT, radiography, and nephrotomography. Radiology 1997;204: 19-25

Remer EM, Herts BR, et al. Spiral noncontrast CT versus combined plain radiography and renal US after extracorporeal shock wave lithotripsy: Cost-identification analysis. Radiology 1997;204: 33-37

Levine JA, Neitlich J, et al. Ureteral calculi in patients with flank pain: Correlation of plain radiography with unenhanced helical CT. Radiology 1997;204: 27-31