Can renal stones be detected on computed tomography (CT) without contrast?

Detection of Renal Stones on CT Without Contrast

Yes, non-contrast CT is the reference standard for detecting kidney stones with a sensitivity as high as 97% and specificity of 95%, making it the optimal imaging modality for urolithiasis evaluation. 1, 2

Why Non-Contrast CT Excels at Stone Detection

• Virtually all renal calculi are radiopaque on CT, allowing for accurate detection of even small stones without the need for intravenous contrast 1, 2
• Non-contrast CT provides rapid acquisition with high spatial resolution and ability for multiplanar reformations 1, 2
• Secondary signs of urolithiasis and complications such as periureteral inflammation, perinephric inflammation, and ureteral dilatation can also be visualized without contrast 1, 2
• With a reported sensitivity as high as 97%, non-contrast CT is currently considered the reference standard for urolithiasis evaluation 1, 2

Advantages of Non-Contrast Over Contrast-Enhanced CT

• Contrast-enhanced CT 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, 2
• Unenhanced CT has higher sensitivity than contrast-enhanced CT for detection of small renal calculi 1, 2
• There is no relevant literature documenting additional benefit of non-excretory phase post-contrast CT in addition to non-contrast CT in urolithiasis evaluation 1

Optimization Techniques for Stone Detection

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

Low-Dose CT Protocols

• Low-dose CT protocols (<3 mSv) should be used to reduce radiation exposure while maintaining excellent diagnostic performance 1, 2
• A meta-analysis of 7 studies assessing low-dose CT for urolithiasis found a pooled sensitivity of 97% and specificity of 95% 1, 2
• Low-dose CT yields equivalent stone measurements compared to standard-dose CT 1, 2
• The sensitivity for stone detection decreases with smaller stone size and increasing dose reduction 1, 2

Limitations and Special Considerations

• Despite high accuracy, non-contrast CT may miss certain types of stones:
  • Small uric acid fragments (1mm) may require higher radiation settings to be detected 3
  • Rare non-radiopaque stones like those made of n-butyl α-cyanoacrylate (surgical adhesive) may not be detectable on CT 4
  • Some lucent renal stones may be missed even by spiral CT 5
• Motion artifact in CT images can cause warping distortion that makes stones appear larger than they actually are 6
• For pregnant patients, ultrasonography is preferred due to radiation concerns 2

When Contrast CT May Still Provide Value

• If contrast-enhanced CT has already been performed, it can still detect larger stones (≥6 mm) with approximately 98% accuracy 1
• Contrast-enhanced CT in the portal venous phase has been shown to be 81% sensitive overall for detection of all (1 mm) renal stones when compared with non-contrast CT, with improved performance for larger stones (95% sensitivity for stones ≥3 mm) 1
• The phase of enhancement (corticomedullary versus nephrographic) has not been shown to affect stone detection rate 1

Non-contrast CT remains the gold standard for kidney stone detection, providing excellent sensitivity and specificity while allowing for accurate assessment of stone size, location, and associated complications that guide clinical management decisions.