KEY FACTS
Imaging
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Dilated renal pelvis and calyces ± dilated ureter
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Distended bladder may cause functional obstruction or reflux resulting in hydronephrosis
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Low-level echoes within lumen suggest pus (pyonephrosis) or blood (hemonephrosis)
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Highly echogenic shadowing intraluminal structures represent stones, twinkling artifact on color Doppler
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Urothelial thickening suggests infection or rejection
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Ultrasound is sensitive and specific for hydronephrosis but may be limited for site of obstruction
Top Differential Diagnoses
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Nonobstructive dilatation, early postoperative edema
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Functional obstruction from overdistended bladder
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Prominent hilar vessels
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Renal sinus cysts (more common in native kidneys)
Pathology
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Causes include ischemic stricture, rejection, clot, calculus, extrinsic compression, infection, and tumor
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Reflux, infection, and decreased ureteral tone may cause nonobstructive dilatation
Clinical Issues
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Ureteral obstruction occurs in 3-6% of renal allografts
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Most common in first 6 months after transplantation
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Over 90% of strictures at ureterovesical anastomosis and distal 1/3 of ureter
Scanning Tips
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Look for transition zone and cause for obstruction
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Use color Doppler to distinguish hilar vessels from dilated renal pelvis and to distinguish clot or debris from solid tumor
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Use color Doppler to look for ureteral jet
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If bladder is distended, rescan with empty bladder
in a renal transplant. The ureter 
is dilated secondary to obstruction by a fluid collection 
, which wrapped around the ureter.
