Will a CT (Computed Tomography) scan without contrast show a kidney stone?

Non-Contrast CT is Highly Effective for Detecting Kidney Stones

Yes, a CT scan without contrast will effectively show kidney stones with approximately 97% sensitivity and 95% specificity, making it the current gold standard for urolithiasis diagnosis. 1, 2

How Non-Contrast CT Detects Kidney Stones

• Virtually all renal calculi are radiopaque on CT, allowing for accurate detection even without contrast enhancement 1
• Non-contrast CT provides:
  • High spatial resolution with rapid acquisition
  • Ability for multiplanar reformations
  • Visualization of secondary signs of urolithiasis (periureteral/perinephric inflammation, ureteral dilatation)
  • Accurate stone size measurement, which guides management decisions 2

Advantages of Non-Contrast CT for Stone Detection

• Provides precise information about:
  • Stone size and location - critical factors that predict spontaneous passage rates
  • Stone density measurements - helps predict need for invasive management
  • Secondary signs of obstruction 1, 2
• CT techniques that improve accuracy of stone measurements include:
  • Coronal reformations
  • Viewing on bone window settings
  • Use of magnified views 1

Low-Dose CT Protocols

• Low-dose CT protocols (<3 mSv) maintain excellent diagnostic performance with 97% sensitivity and 95% specificity 1, 2
• Ultra-low-dose protocols can achieve radiation doses as low as 0.57 mSv (92% lower than standard low-dose CT) while maintaining good detection rates 2
• Important consideration for patients who may need multiple scans throughout their lifetime

Limitations and Special Considerations

• Detection sensitivity decreases with smaller stone size, especially with increased dose reduction 1
• Small uric acid stones (<2 mm) may be missed at ultra-low-dose settings 3
  • One study found that 1-mm uric acid fragments were not detected at 7 and 15 mAs settings 3
• Rare cases exist where certain stones may be missed even with standard CT techniques 4
• CT may sometimes overestimate stone size compared to actual dimensions 5
  • Motion artifacts can cause warping distortion effects that make stones appear larger than they actually are
• Conversely, some studies suggest CT underestimates stone size by approximately 12% compared to KUB radiography 6

When to Consider Alternative Imaging

• For pregnant patients, children, or radiation-sensitive situations, ultrasound with KUB radiography is recommended as the initial assessment 2
• MRI has limited ability to directly visualize stones but can detect secondary signs of obstruction 2

Non-contrast CT remains the most reliable imaging modality for detecting kidney stones, providing essential information for clinical decision-making while avoiding the potential complications and additional cost of contrast administration.