Urinary Tract Obstruction

In most patients, ureteral obstruction results from an acute process with associated symptoms. Some controversy exists as to which imaging studies are best for investigating suspected ureteral obstruction. In most hospitals in the United States, non-contrast helical computed tomography (CT) is preferred because it is safe and extremely rapid, and the accuracy rate for detecting ureter-al stones, the most common cause of ureteral obstruction, exceeds that of other imaging studies. Other causes of acute abdominal pain, such as appendicitis, leaking aortic aneurysm, and diverticulitis, can also be readily diagnosed and occur in 13-19% of cases. Non-contrast helical CT has an overall accuracy of 97% for diagnosing ureteral stone disease [1-8]. This far exceeds the accuracy of intravenous urography (IVU) or ultrasonography (US) (See Table 1). Regardless of composition, virtually all ureteral stones have high attenuation values, making them readily detectable with CT. Nonmineralized matrix stones and some drug-related stones (due, for example, to protease inhibitors) may not be visible on CT images, but are rarely encountered.

The proper technique for performing noncontrast helical CT to detect ureteral stone disease using a helical scanner includes 5 mm collimation scanning from the top of the kidneys to the base of the bladder without intravenous or oral contrast material. Scans should be ob tained during a single breath hold or in clusters. A pitch of 1-1.5 is preferable. Using a 16 slice MDCT, collimation of 1.5 mm is appropriate. In order to reduce radiation dose, a variable mA is used for each slice, based on beam attenuation. For the obese patient, a fixed mA equal to his/her weight in pounds will usually suffice. The expected dose is 30-40 mSv. Review of the images in cine mode on a workstation facilitates continuous identification of the ureter and workflow. Three-dimensional (3D) reconstructions are usually not necessary.

In addition to direct visualization of the ureteral stone, secondary signs of ureteral obstruction include unilateral nephromegaly, perinephric stranding, hydronephrosis, and periureteral stranding. The combination of per-inephric stranding and unilateral hydronephrosis has a positive predictive value of 96% for the presence of stone disease. The absence of both of these signs has a negative predictive value of 93% for excluding stone disease. CT also gives information that determines therapy. Stones that are 5 mm or less in size, of smooth shape, and located within the distal third of the ureter are likely to pass spontaneously [9].

The major pitfall of noncontrast helical CT evaluation of the urinary tract for stone disease is the difficulty in distinguishing pelvic phleboliths from ureteral calculi. The presence of a tissue 'rim' sign usually indicates that the calcification is a stone rather than a phlebolith. Alternatively, the absence of the tissue rim sign, or pres

Table 1. Diagnostic performance for CT, US, and IVU for detection of ureteral stones

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