CT Imaging for Mild Unilateral Hydronephrosis Without Visualized Stone or Mass
For mild left hydronephrosis/pelviectasis without an obstructing stone or mass on ultrasound, CT urography (CTU) without and with IV contrast is the preferred imaging modality, as it provides comprehensive morphological and functional evaluation of the entire genitourinary tract to identify both intrinsic and extrinsic causes of obstruction. 1
Primary Recommendation: CTU With Contrast
CTU without and with IV contrast is the American College of Radiology's recommended comprehensive evaluation for asymptomatic unilateral hydronephrosis of unknown cause, providing near-complete assessment of the entire genitourinary tract including both morphological and functional information 2, 1
The CTU protocol includes three critical phases: an unenhanced phase to detect calculi that ultrasound may have missed (sensitivity 97%), a nephrographic phase to assess renal parenchyma, and an excretory phase (acquired at least 5 minutes post-contrast) to evaluate collecting system anatomy 1, 3
IV contrast is particularly helpful for assessing pelvic masses, extrinsic compression, and soft tissue causes of obstruction that would be missed on non-contrast imaging 2
Alternative Option: Non-Contrast CT (Limited Utility)
Non-contrast CT abdomen and pelvis may be considered if urolithiasis remains the primary concern despite negative ultrasound, as CT has 97% sensitivity for detecting stones that ultrasound can miss 1, 3
However, non-contrast CT has significant limitations in this clinical scenario: it cannot adequately evaluate for pelvic masses, extrinsic compression, vascular abnormalities, or soft tissue pathology that may be causing the hydronephrosis 2, 4
Non-contrast CT can produce false-positive findings, as peripelvic cysts and other conditions can mimic hydronephrosis without contrast enhancement to clarify anatomy 5
Clinical Context Matters
Since your ultrasound already failed to visualize an obstructing stone or mass, the diagnostic question has shifted from "is there a stone?" to "what is causing this hydronephrosis?" - a question that requires contrast enhancement to answer 1
The differential diagnosis for unilateral hydronephrosis without visible stone includes: ureteropelvic junction obstruction, extrinsic compression from masses or vessels, retroperitoneal fibrosis, urothelial lesions, and functional obstruction 1, 6
Important Caveats
Do not order CT with and without contrast (dual-phase) as a routine abdomen/pelvis CT - this is different from CTU and provides limited evidence for hydronephrosis evaluation 2
If the patient has contraindications to IV contrast (renal impairment, severe contrast allergy), MR urography (MRU) with IV gadolinium contrast is the preferred alternative, providing excellent soft tissue characterization 1
If CTU shows persistent hydronephrosis but no clear anatomic cause, diuretic renography (MAG3 scan) should be the next step to determine if true functional obstruction exists versus non-obstructive dilation 1
Follow-Up Strategy
If CTU identifies a specific cause, direct management at that etiology 1
If no cause is identified and hydronephrosis remains mild with normal renal function, repeat ultrasound in 3-6 months to assess for progression 1
Any progression of hydronephrosis warrants urologic consultation 1
Progressive dilation can lead to irreversible nephron loss even when initially asymptomatic, making timely diagnosis essential 6