The IVU, or intravenous pyelogram, is the classic study for the evaluation of ureteral obstruction due to urolithiasis. During an IVU, contrast excreted by the kidney visibly flows in the urinary tract up to the level of obstruction. If the obstruction is partial, its severity may be estimated. However, the obstructing calculus is not always visualized. More recently, noncontrast helical CT has been applied to detect ureteral calculi directly in the setting of suspected obstruction. The size and location of the calculus, which indicate the likelihood of spontaneous passage, can be accurately determined. However, the degree of functional obstruction cannot be evaluated without contrast administration. Therefore, noncontrast CT is not helpful in evaluating a patient with known calculi who presents with new symptoms.
Renal ultrasound is often requested in the evaluation of suspected acute ureteral obstruction. In such a setting, ultrasound is usually not helpful because (1) hydronephrosis may be absent in early acute ureteral obstruction, (2) a dilated renal collecting system may be due to causes other than acute ureteral obstruction, and (3) the detection of calculi within the kidney does not necessarily imply obstruction by a more distal calculus.
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