Kidney Stone Imaging: Diagnostic Approach
Non-contrast helical (spiral) CT scan is the first-line imaging modality for diagnosing kidney stones, with low-dose protocols recommended to reduce radiation exposure while maintaining diagnostic accuracy. 1, 2
Primary Imaging Modalities
Non-Contrast CT
- First-line imaging test with 95-97% sensitivity and specificity 1, 2
- Provides precise measurement of stone size and location, which are critical for treatment decisions
- Low-dose CT protocol (<3 mSv) should be used instead of conventional dosing to reduce radiation exposure 1, 2
- Advantages:
- Accurately identifies stone size and location
- Detects ureteral stones that may be missed by other modalities
- Helps predict spontaneous passage based on stone characteristics
- Can identify alternative diagnoses if pain is not due to stones
Ultrasound
- Imaging of choice for pregnant patients 1
- Variable sensitivity (24-57%) compared to CT, but can reach up to 100% sensitivity for detecting ureteral obstruction when secondary signs are present 2
- Addition of color Doppler and twinkling artifact assessment increases sensitivity to 99% for stones <5mm 2
- Advantages:
- No radiation exposure
- Can be used for follow-up imaging
- Readily available in most settings
- Safe for pregnant patients
Plain Radiography (KUB)
- Limited utility as initial diagnostic tool (appropriateness rating of 3 - "usually not appropriate") 1
- Better suited for follow-up of known radiopaque stones 2
- Sensitivity is significantly lower than CT (47.6% compared to CT) 3
- Combining KUB with ultrasound improves detection (sensitivity 79-90%) 2
Special Populations and Situations
Pregnant Patients
- Ultrasound is the imaging modality of choice 1
- MRI can be considered if ultrasound is inconclusive and radiation must be avoided 2
Follow-up Imaging
- For known radiopaque stones: KUB or ultrasound recommended to reduce cumulative radiation exposure 2
- For uric acid or small stones: low-dose CT recommended 2
- Non-contrast CT is significantly more sensitive than KUB for detecting residual stones after procedures (79.2% vs 37.7%) 3
Clinical Considerations
Stone Size Impact
- Stone size significantly influences treatment decisions:
- Smaller stones (average 3.3mm) are more likely to pass spontaneously
- Larger stones (average 7mm) more frequently require intervention 4
- Ultra-low-dose CT protocols maintain excellent diagnostic performance for most stones but may miss very small uric acid stones (<2mm) 2
Enhanced CT Techniques
- If low-dose CT sensitivity is insufficient, consider:
- Intravenous contrast media
- Evaluation of secondary signs (ureteral dilatation, perinephric stranding)
- Dual-energy CT 1
Common Pitfalls and Caveats
Radiation exposure concerns: Multiple CT scans can lead to significant cumulative radiation exposure. Use low-dose protocols whenever possible.
Overreliance on KUB: Plain radiography misses many stones and should not be used as the sole diagnostic test.
Ultrasound limitations: Tends to overestimate stone size and has lower sensitivity for smaller stones.
Stone size measurement: CT techniques that improve accuracy include coronal reformations, viewing on bone window settings, and use of magnified views 2.
Small uric acid stones: May be missed on ultra-low-dose CT protocols 2.