What is the recommended imaging scan for suspected kidney stones?

Optimal Imaging for Kidney Stones

Non-contrast CT (NCCT) is the gold standard imaging modality for suspected kidney stones, with a sensitivity of up to 97% and specificity of 95%. 1

First-Line Imaging Options

• Non-contrast CT (NCCT) is the reference standard for kidney stone evaluation due to its high accuracy in detecting virtually all renal calculi without requiring IV contrast 2, 1
• NCCT provides rapid acquisition with high spatial resolution and multiplanar reformations, allowing for accurate assessment of stone size and location 2
• Low-dose CT protocols (<3 mSv) should be used to reduce radiation exposure while maintaining excellent diagnostic performance (97% sensitivity, 95% specificity) 2, 1
• NCCT allows visualization of secondary signs of urolithiasis such as periureteral inflammation, perinephric inflammation, and ureteral dilatation 1

CT Technique Optimization

• Stone location and size can be accurately depicted with NCCT, which is crucial for determining management as proximal and larger stones have higher intervention needs 2
• Techniques to improve accuracy of stone measurements include:
  • Use of coronal reformations 1
  • Viewing on bone window settings 1
  • Use of magnified views 1
• Thin (1-1.5 mm) axial slice images are preferred over thick (5 mm) coronal maximum intensity projection images for optimal stone detection 2

Alternative Imaging Approaches

• Ultrasound (US) is an alternative first-line imaging option, particularly useful for:
  • Pregnant patients due to radiation concerns 1
  • Children 3
  • Patients with recurrent stone disease to reduce cumulative radiation exposure 1, 4
• Ultrasound has lower sensitivity (24-57%) compared to NCCT but can still detect clinically significant stones and hydronephrosis 2, 3
• Kidney, Ureter, Bladder (KUB) plain film radiography has limited utility as initial imaging but is helpful for monitoring known stone disease 2, 5
  • KUB detects only 8% of stones ≤5 mm but has 78% detection rate for stones >5 mm 2
• MRI can evaluate the urinary system without radiation but has limited sensitivity (50%) for stone detection compared to NCCT (91%) 2

Special Considerations

• For contrast-enhanced CT that has already been performed, stones ≥6 mm can still be detected with approximately 98% accuracy 2
• CT abdomen and pelvis without and with IV contrast offers no additional benefit over non-contrast CT alone for kidney stone evaluation 2
• Ultra-low-dose CT protocols maintain good detection rates for most stones, though small uric acid stones (<1 mm) may require higher energy settings 6

Common Pitfalls to Avoid

• Relying on contrast-enhanced CT as first-line imaging may lead to missed small stones 1
• The sensitivity for small stone detection decreases with increasing dose reduction, so ultra-low-dose protocols may miss stones <2mm 1
• Overreliance on KUB alone may miss non-radiopaque stones or misidentify phleboliths as ureteral stones 2

Imaging Algorithm Based on Clinical Scenario

• First presentation with suspected kidney stone: Non-contrast low-dose CT is recommended 2, 1
• Pregnant patients or children: Ultrasound is the imaging modality of choice 1, 3
• Known stone disease follow-up: Consider ultrasound or KUB to limit radiation exposure 5, 3
• Complex cases with suspected complications: Non-contrast CT provides the most comprehensive evaluation 2, 1