What is the recommended imaging protocol for diagnosing renal stones, specifically using a CT (Computed Tomography) scan without contrast?

CT Scan for Renal Stone Detection Without Contrast

Non-contrast CT (NCCT) of the abdomen and pelvis is the reference standard imaging modality for diagnosing renal stones, with a sensitivity of 97% and specificity of 95-100%, and should be performed using low-dose protocols (<3 mSv) to minimize radiation exposure while maintaining diagnostic accuracy. 1

Why NCCT is the Gold Standard

Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even small stones (as small as 1 mm) without IV contrast. 1, 2 The key advantages include:

• Rapid acquisition with high spatial resolution and multiplanar reformation capabilities 1
• Detection of secondary signs of urolithiasis including periureteral inflammation, perinephric inflammation, and ureteral dilatation 1
• Accurate assessment of stone size and location, which predicts spontaneous passage rates and need for intervention 1, 2
• Larger and more proximal stones have higher intervention rates, making accurate characterization critical for management planning 1

Optimal Technical Protocol

Use low-dose CT protocols (<3 mSv) as standard practice, which maintain 97% sensitivity and 95% specificity while significantly reducing radiation exposure. 1, 2 This is particularly important for young patients and those requiring serial imaging. 1

Technical optimization for accurate measurements:

• Use thin (1-1.5 mm) axial slice images rather than thick (5 mm) coronal maximum intensity projections 1
• View images on bone window settings 1, 2
• Utilize coronal reformations 1, 2
• Apply magnified views for precise stone measurement 1, 2

Important Caveats and Limitations

Sensitivity decreases for very small stones (<3 mm), and further dose reduction can worsen detection of small stones. 1, 3 Specifically:

• For stones ≥3 mm, low-dose CT maintains 87% sensitivity and 100% specificity 4
• Uric acid stones, particularly 1 mm fragments, may require higher energy settings (>15 mAs) for reliable detection 3
• NCCT detects fewer total calculi compared to endoscopy (5.9 vs 9.2 stones per kidney), though clinically significant stones are reliably identified 1

When Contrast CT Has Already Been Performed

If a patient has already undergone contrast-enhanced CT, stones ≥6 mm can still be detected with approximately 98% accuracy, making repeat NCCT unnecessary for larger stones. 1, 2 However:

• Contrast-enhanced CT has only 81% overall sensitivity for all stone sizes, with 95% sensitivity for stones ≥3 mm 1, 2
• The phase of enhancement (corticomedullary vs nephrographic) does not affect stone detection rates 1

Alternatives and When to Avoid NCCT

Ultrasound should be reserved for pregnant patients where radiation must be avoided, but has significantly lower sensitivity (45-57% for renal calculi, 61% for ureteral stones). 1, 5 Additional limitations of ultrasound include:

• Tendency to overestimate stone size, particularly for stones ≤5 mm 1
• Poor detection of ureteral stones unless obstruction is present 1
• Should not be relied upon as the sole modality for definitive stone characterization 2, 6

Plain radiography (KUB) detects only 72% of large stones (>5 mm) in the proximal ureter and 29% overall, making it inadequate for initial diagnosis. 1 KUB is most useful for tracking known radiopaque stones during follow-up. 1, 5

Common Pitfalls to Avoid

• Do not add IV contrast for stone evaluation—there is no documented benefit of contrast-enhanced CT over NCCT for urolithiasis diagnosis, and contrast may obscure small stones 1, 6
• Do not use standard-dose protocols when low-dose protocols provide equivalent diagnostic accuracy with significantly less radiation 1, 2
• Do not rely on ultrasound alone for definitive stone diagnosis in non-pregnant patients, as it has poor sensitivity and significantly overestimates stone size 1, 6