Peripelvic cysts: an impostor of sonographic hydronephrosis.

Static and real-time ultrasound examinations have proved clinically useful in the evaluation of renal obstruction. False-negative findings are rare. False-positive findings have been reported as being due to slightly dilated, nonobstructed urinary systems causing the ultrasonographic appearance of mild (grade I) hydronephrosis. The authors describe the ultrasonographic appearance of 16 false-positive cases that had suggested moderate (grade II) to severe (grade III) hydronephrosis in collecting systems subsequently shown to be completely unobstructed. This appearance was caused by peripelvic cysts. These cases emphasize the screening role of ultrasound and the need to employ additional diagnostic modalities to verify the presence of hydronephrosis and renal obstruction.