What type of CT (Computed Tomography) scan is recommended for diagnosing kidney stones?

CT Scan for Kidney Stones: Optimal Imaging Approach

Non-contrast CT of the abdomen and pelvis is the recommended imaging modality for diagnosing kidney stones, with low-dose protocols preferred to reduce radiation exposure while maintaining diagnostic accuracy. 1

Non-Contrast CT: The Gold Standard

• Non-contrast CT (NCCT) is considered the reference standard for kidney stone evaluation with a sensitivity as high as 97% and specificity of 95% 1
• Virtually all renal calculi are radiopaque on CT, allowing for accurate detection of even small stones without the need for IV contrast 1
• NCCT provides rapid acquisition with high spatial resolution and ability for multiplanar reformations 1
• Helical (spiral) NCCT allows for precise measurement of stone size and clear depiction of where in the ureter a stone has become lodged 1
• Secondary signs of urolithiasis and complications such as periureteral inflammation, perinephric inflammation, and ureteral dilatation can also be visualized 1

Low-Dose Protocol Considerations

• Low-dose CT protocols (<3 mSv) should be used in place of conventional dosing to reduce radiation exposure while maintaining excellent diagnostic performance 1
• Meta-analysis shows low-dose CT maintains a pooled sensitivity of 97% and specificity of 95% for detecting urolithiasis 1
• Low-dose CT yields equivalent stone measurements compared to standard-dose CT 1
• Ultra-low-dose CT has good detection rates for most stones larger than 2mm, though small uric acid fragments may require higher energy settings 2

CT Technique Optimization

• Stone location and size can be accurately depicted with NCCT, which is crucial for determining management as more proximal and larger stones have higher intervention needs 1
• CT techniques that improve accuracy of stone measurements include:
  • Use of coronal reformations
  • Viewing on bone window settings
  • 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 1

Alternative CT Approaches

• CT with IV contrast is usually not appropriate as a first-line test for kidney stones as the enhancing renal parenchyma may obscure stones within the collecting system 1
• If contrast-enhanced CT has already been performed, it can still detect larger stones (≥6 mm) with approximately 98% accuracy 1
• CT abdomen and pelvis without and with IV contrast offers no additional benefit over non-contrast CT alone for kidney stone evaluation 1

Special Considerations

• NCCT may underestimate stone size by approximately 12% compared to KUB (kidney-ureter-bladder) radiography, which could impact management decisions, particularly for stones around 5mm 3
• Stone composition can be specifically assessed through dual-energy CT techniques 4
• For pregnant patients, ultrasonography is the imaging tool of choice due to radiation concerns 1
• In patients with recurrent stone disease, limiting the CT scan to the area of interest or using ultra-low-dose protocols can help reduce cumulative radiation exposure 1, 2

Common Pitfalls to Avoid

• Relying on contrast-enhanced CT as first-line imaging for suspected kidney stones may lead to missed small stones 1
• Using standard-dose protocols unnecessarily increases radiation exposure, especially problematic for patients who may need multiple scans over their lifetime 2, 4
• The sensitivity for small stone detection decreases with increasing dose reduction, so ultra-low-dose protocols may miss stones <2mm in size 1, 5
• CT planning images (CTI) may underestimate the radiopacity of a stone that would be visible on a plain KUB radiograph 6