In the emergency room setting, noncontrast helical abdominal/pelvic CT has become the examination of choice in the evaluation of flank pain and obstructive anuria (Niall et al. 2002; Shokeir et al. 2002, 2004; Coli-stro et al. 2002). Introduced by Smith and colleagues in 1995, noncontrast CT is quick, relatively easy to interpret, and obviates risks associated with the use of contrast media (Smith et al. 1999). Noncontrast CT is the gold standard in the detection of urinary calculi with an associated sensitivity of more than 95 % and an associated specificity greater than 98% (Fig. 10.2) (Ruk-ker et al. 2004). When upper urinary tract obstruction is related to stone disease, CT can provide a wealth of diagnostic information. In addition, when stone disease does not exist, noncontrast CT can provide an accurate first glance to the underlying etiology of upper urinary obstruction (Rucker et al. 2004). In comparison to KUB with abdominal US, Shokeir and co-workers noted that noncontrast CT had a superior sensitivity (94% vs 58%) and diagnostic accuracy (95% vs 77 %) for patients with obstructive anuria, respectively (Shokeir et al. 2002). Also, noncontrast CT can provide incidentally discovered findings and nonurologic sources of flank pain. Indeed, up to one-third of unen-hanced CT scans performed for flank pain may reveal unsuspected findings unrelated to stone disease that are contributing to the patient's symptomatology (Ruk-ker et al. 2004; Ather et al. 2005). When flank pain is unrelated to stone disease, the most common alternative diagnoses with unenhanced CT include adnexal masses, pyelonephritis, appendicitis, and diverticulitis. In cases where additional diagnostic information is needed on the basis of the noncontrast examination, additional CT phases can be easily performed after the administration of contrast. For instance, CT is the imaging modality of choice to evaluate associated inflam-
Fig. 10.3. Appearance of ureteral obstruction related to TCC on excretory urogram
Fig. 10.4. Corresponding appearance of obstructive urothelial carcinoma of the ureter on contrast-enhanced CT urogram
Fig. 10.3. Appearance of ureteral obstruction related to TCC on excretory urogram matory lesions of the kidney and for the evaluation of renal trauma (Kawashima et al. 1997, 2001). Although less commonly obtained in the urgent evaluation of upper urinary tract obstruction, techniques of CT urography are also increasingly favored as diagnostic tests for evaluating hematuria and urothelial carcinoma (Figs. 10.3 and 10.4) (Kawashima et al. 2004).
Both noncontrast and contrast-enhanced CT can reveal characteristic signs associated with the diagnosis of upper urinary tract obstruction. Stone disease, essentially regardless of composition, is easily recognized on noncontrast CT because the attenuation of stones is higher than that of soft tissue. The possible exception to this rule would be human immunodeficiency virus
Fig. 10.4. Corresponding appearance of obstructive urothelial carcinoma of the ureter on contrast-enhanced CT urogram patients with indinavir stones and select patients with completely uncalcified matrix stones. The diagnosis of stone disease on noncontrast CT is simplified by the appearance of proximal ureteral dilation, hydronephro-sis, and the possibility of stranding in the perinephric fat (Colistro et al. 2002). Another secondary sign of acute obstruction recently described by Ozer and associates is the difference in renal parenchyma density on noncontrast CT. In a study of 49 patients with obstructing ureteral stones, a Hounsfield unit (HU) decrease in the ipsilateral kidney was observed in all cases. When a parenchyma decrease of more than 5 HU was observed on the obstructed side, the sensitivity, specificity, positive predictive value, and accuracy of acute obstructing stone on the ipsilateral side were 89.1%, 100%, 100%, and 93.4%, respectively (Ozer et al. 2004).
Concurrent findings of pyelonephritis or abscess can also be suggested on noncontrast CT by nephrome-galy and changes in the renal parenchyma. It is especially important to recognize the presence of air in the collecting system or renal parenchyma, as this may suggest a diagnosis of emphysematous pyelonephritis. With advanced transaxial imaging, our ability to diagnose air in the urinary tract has increased. On comput erized tomography, some gas in the collecting system can be observed after instrumentation or among patients that have had a ureterosigmoidostomy urinary diversion, and this is not worrisome. On the other hand, gas in the renal parenchyma on computerized tomography or gas in the collecting system on a less advanced imaging test (i.e., plain abdominal radiograph) can represent the more serious problem of emphysematous pyelonephritis. It is important to correlate finding of gas in the collecting system to other clinical findings for the patient. In some instances, pelvic phlebo-liths may cloud the accurate diagnosis of upper urinary tract obstruction. A soft-tissue rim sign (i.e., soft tissue ring around the calcification) favors the diagnosis of stone on noncontrast CT (Colistro et al. 2002). Upper tract obstruction related to UPJ obstruction can often show associated hydronephrosis on noncontrast CT. Clues to a diagnosis of upper tract obstruction related to a retroperitoneal process (e.g., retroperitoneal fibro-sis, adenopathy, primary retroperitoneal malignancy, metastatic retroperitoneal malignancy, aneurysm, etc.) are also initially deciphered or frequently diagnosed on noncontrast CT.
When the noncontrast CT shows associated abnormalities without presence of stone disease or when unrelated abnormalities are encountered that require further characterization, contrast administration should be strongly considered in patients with acceptable renal function. Use of contrast-enhanced CT in the setting of upper urinary tract obstruction provides the same diagnostic signs as seen on EXU; however, the anatomic details of the parenchyma and the ability to diagnose associated findings such as forniceal rupture are superior. Contrast-enhanced CT can provide superior diagnostic information when obstruction is related to urothelial carcinoma (Kawashima et al. 2004). Nonetheless, the presence of obstruction related to ur-othelial carcinoma is relatively rare in the kidney and UPJ. Among five presentation patterns described by Lowe and Roylance, the presence of urothelial carcinoma associated with long-standing UPJ obstruction and atrophy was relatively uncommon (13% incidence), as was the presence of urothelial carcinoma in the setting of hydronephrosis, renal enlargement, and acute UPJ obstruction (6% incidence). More commonly, urothelial carcinoma located at or above the UPJ will be associated with irregular filling defects of the renal pelvis or calices (61 % incidence) or infundibular amputations (20% incidence) (Lowe and Roylance 1976). Upper urinary tract obstruction related to urothelial carcinoma is more commonly associated with tumors in a ureteral location. Among the presentations of ureteral tumors, the presence of concurrent nonfunctioning kidneys with high-grade obstruction was seen in 46% and hy-dronephrosis with or without hydroureter was the presenting finding in 34% of cases. In the remaining 20 %
of cases, ureteral tumors were observed on standard ur-ographic or cross-sectional imaging as ureteral filling defects or as irregular narrowed segments ofthe ureteral lumen, respectively (Kawashima et al. 2004).
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