What is the difference between CT (Computed Tomography) urography and CT stonogram for diagnosing kidney stones and other urinary tract abnormalities?

CT Urography vs CT Stonogram (Non-Contrast CT)

For suspected kidney stones, non-contrast CT (CT stonogram) is the gold standard imaging modality with 97% sensitivity, while CT urography (with contrast and delayed excretory phases) adds no proven benefit for stone detection and may actually obscure stones in the collecting system. 1

Key Differences Between the Two Studies

CT Stonogram (Non-Contrast CT Abdomen/Pelvis)

• Single-phase acquisition without IV contrast 1
• Optimal for stone detection: Virtually all renal calculi appear radiopaque on CT, allowing accurate detection of even small (1 mm) stones 1
• Sensitivity: 97% and specificity: 95% for urolithiasis detection 1
• Provides critical stone characteristics: Size, location, density, and number—all essential for predicting spontaneous passage and planning management 1
• Lower radiation exposure: Low-dose protocols (<3 mSv) maintain 97% sensitivity while minimizing radiation 1, 2
• No contrast-related risks: Avoids allergic reactions and nephrotoxicity 3

CT Urography (CTU)

• Multi-phase study: Includes non-contrast, nephrographic (contrast-enhanced), and delayed excretory phases 1
• Primary indication: Evaluation of urothelial masses, hematuria workup, and anatomic abnormalities—NOT stone disease 4, 3
• Disadvantage for stones: Enhancing renal parenchyma during contrast phases may obscure stones within the collecting system 1
• Reduced sensitivity: Only 81% sensitive for all stones (versus 97% for non-contrast CT), though 95% sensitive for stones ≥3 mm 1, 2
• No documented benefit: There is no relevant literature showing additional excretory phase imaging improves stone detection over non-contrast CT alone 1

Clinical Decision Algorithm

When to Order Non-Contrast CT (CT Stonogram)

• Acute flank pain with suspected urolithiasis 1
• Known stone disease with acute symptoms 1
• Stone surveillance and treatment planning 1
• Any scenario where stone detection is the primary goal 2

When CT Urography Might Be Considered

• Hematuria workup requiring comprehensive urinary tract evaluation (though non-contrast CT is still preferred initially) 4, 3
• Suspected urothelial malignancy 4, 3
• Complex anatomic abnormalities requiring detailed urinary tract opacification 1
• When stone has already been excluded and alternative diagnoses are being pursued 1

Important Technical Considerations

Optimizing Stone Detection on Non-Contrast CT

• Use thin-slice (1-1.5 mm) axial images for maximum sensitivity 1
• Coronal reformations, bone window settings, and magnified views improve stone measurement accuracy 1
• Low-dose protocols should be standard to minimize radiation while maintaining diagnostic accuracy 1, 2

Common Pitfalls to Avoid

• Do not order CT with IV contrast as first-line for suspected stones: Contrast obscures collecting system calculi and reduces sensitivity from 97% to 81% 1, 2
• Recognize that CTU is not interchangeable with CT stonogram: They serve fundamentally different clinical purposes 1, 4
• If contrast CT was already performed: Stones ≥6 mm can still be detected with ~98% accuracy, so larger clinically significant stones won't be missed 1
• Small stones (<3 mm) are particularly vulnerable to being obscured by contrast: Sensitivity drops significantly for these 1

Secondary Findings and Added Value

Non-Contrast CT Provides

• Secondary signs of obstruction: Hydronephrosis, hydroureter, perinephric stranding, periureteral inflammation 1
• Stone characteristics predictive of passage: Proximal location and larger size predict need for intervention 1
• Alternative diagnoses: Can identify other causes of flank pain without contrast 5

CT Urography Provides

• Anatomic detail of collecting system: Better visualization of urothelial surfaces 4, 3
• Functional information: Degree of obstruction, delayed nephrogram 1
• Soft tissue characterization: Superior for masses and inflammatory processes 3

Evidence Quality Note

The American College of Radiology 2023 Appropriateness Criteria provide the highest-quality guideline evidence, establishing non-contrast CT as the reference standard for urolithiasis evaluation 1. This supersedes older studies comparing IVU to CT 6 and establishes clear superiority of non-contrast technique over contrast-enhanced protocols for stone detection 2.