Figure 5a. Renal sinus cysts in a 53-year-old asymptomatic man. (a) Axial contrast-enhanced CT scan obtained during the nephrographic phase shows low-attenuation cystic lesions (arrows) in the left renal sinus; this finding could be misinterpreted as hydronephrosis. (b) Axial contrast-enhanced CT scan obtained during the excretory phase shows the cysts (arrows) in the renal sinus, an appearance different from that of hydronephrosis. The enhanced calices are stretched and attenuated but not obstructed by the cysts.