Dilation of renal collecting (pelvicalyceal) system ± ureteral dilation from intrinsic or extrinsic cause

Renal collecting system dilation, pelvicalyceal dilatation, pelvocaliectasis

Dilated intercommunicating fluid-filled anechoic channels (renal calyces and pelvis) in kidney

Severity of hydronephrosis depends on degree and duration of obstruction

Presence of internal echoes within dilated collecting system may represent underlying infection/pyonephrosis

Acute hydronephrosis: RI > 0.70 or RI 0.08-0.10 higher than normal contralateral side in unilateral obstruction

Antenatal US: Renal pelvis AP diameter ≥ 4 mm prior to 20-week gestation

Fetal renal pelvis diameter ≥ 7 mm at 20-28 weeks or ≥ 10 mm beyond 28-week gestation requires postnatal follow-up

CT has high sensitivity in evaluating site and etiology of obstruction (intrinsic [stone] or extrinsic)

CT may show striated nephrogram &/or urothelial enhancement in superadded infection (pyelonephritis)

CT urogram: Useful in nonstone etiology (urothelial neoplasm, necrosed/sloughed papillae, clot)

MAG 3/DTPA scan: Central photopenic area at vascular phase, tracer accumulation within hydronephrotic collecting system with delayed drainage

Parapelvic cyst

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  Parenchymal cysts that extend into renal pelvis

Peripelvic cyst

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  Ovoid lymphocysts in pelvic hilum, usually multiple

Extrarenal pelvis

Prominent renal vasculature

Autosomal dominant polycystic kidney

Chronic obstruction: Loss of renal parenchyma and function

Evaluate with color Doppler to exclude prominent renal vessels in hilum that could mimic hydronephrosis

Look for debris in collecting system, which could indicate pyonephrosis, urologic emergency

Evaluate kidneys after voiding, as mild hydronephrosis may be transient with full bladder

Look for bladder jets, which may help delineate complete from partial obstruction

Look at ureterovesical junction (UVJ) with color Doppler for subtle twinkling artifact, which may indicate distal ureteral or UVJ stone as etiology of hydronephrosis