CT Stonogram (Non-Contrast CT for Kidney Stones)
A CT stonogram does NOT require contrast—non-contrast CT is the gold standard imaging modality for detecting kidney stones, with 97% sensitivity and 95% specificity. 1, 2
Why Non-Contrast CT is Preferred
- Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even stones as small as 1 mm without IV contrast 1, 2
- Non-contrast CT (NCCT) is currently the reference standard for urolithiasis evaluation, superior to all other imaging modalities 1, 2
- Contrast actually reduces sensitivity for stone detection—contrast-enhanced CT has only 81% overall sensitivity (95% for stones ≥3 mm) compared to 97% for non-contrast CT 2, 3
- Low-dose protocols (<3 mSv) maintain diagnostic accuracy with 97% sensitivity and 95% specificity while minimizing radiation exposure 1, 2
When Contrast May Be Added (Not for Stone Detection)
While contrast is not needed for detecting stones, there are specific scenarios where contrast may be added after the non-contrast phase:
- To differentiate stone mimics: When non-contrast CT is inconclusive about whether a calcification is a ureteral stone versus a phlebolith or vascular calcification, contrast can help delineate the "soft tissue rim" sign and opacify iliac vessels 1
- To evaluate alternative diagnoses: In patients with negative urinalysis and no history of stones, contrast-enhanced CT may identify alternative causes of flank pain (found in 15% of such cases) 4
- To assess complications: In patients with moderate-to-severe hydronephrosis at risk of stone passage failure, contrast can evaluate for infection, abscess, or other complications 1, 3
However, contrast-enhanced CT changes clinical management in only 2-3% of cases overall 1
Technical Optimization for Accurate Stone Detection
To maximize accuracy of non-contrast CT stonograms:
- Use coronal reformations, bone window settings, and magnified views for precise stone measurements 1
- Include thin (1-1.5 mm) axial slices for detecting small stones 5
- Scan through the pelvis to detect distal ureteral and bladder stones 5
- Stone size and location on CT predict spontaneous passage rates—larger and more proximal stones require intervention 1, 2
Common Pitfalls to Avoid
- Do not order contrast CT as the initial study for suspected stones—it has inferior sensitivity and adds unnecessary cost, contrast risk, and radiation 2, 3
- Do not rely on ultrasound alone—it has only 24-57% sensitivity for detecting renal calculi and even worse performance for ureteral stones 5
- Bladder filling status is irrelevant—non-contrast CT directly visualizes calculi throughout the ureter independent of urinary tract opacification 5
- Be aware that sensitivity decreases for stones <3 mm, particularly with dose reduction protocols 1, 6