CT for Kidney Stones: Recommended Imaging Approach
Low-dose non-contrast CT of the abdomen and pelvis is the gold standard initial imaging test for suspected kidney stones, with sensitivity of 97% and specificity of 95%. 1, 2
Why Non-Contrast CT is First-Line
Non-contrast CT (NCCT) should be your default imaging choice for suspected kidney stones because:
- Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even small stones without IV contrast 1, 2
- Provides precise measurement of stone size and exact location within the ureter, which is crucial for determining whether conservative management or intervention is needed 1, 2
- Detects secondary signs including periureteral inflammation, perinephric inflammation, and ureteral dilatation that help confirm the diagnosis 1, 3
- Rapid acquisition with high spatial resolution and ability for multiplanar reformations 2
Critical Technical Considerations
Always use low-dose protocols (<3 mSv) rather than standard-dose CT to minimize radiation exposure while maintaining the same excellent diagnostic performance (pooled sensitivity 97%, specificity 95%) 1, 2, 3
For optimal stone detection and measurement accuracy:
- Use thin (1-1.5 mm) axial slice images rather than thick (5 mm) coronal maximum intensity projection images 2
- View images on bone window settings 2
- Use coronal reformations and magnified views 2
When to Use Alternative Imaging
For pregnant patients: Ultrasound of kidneys and bladder is the appropriate first-line test due to radiation concerns 1, 3
For pediatric patients and those with recurrent stone disease requiring frequent follow-up: Ultrasound is reasonable, though be aware sensitivity is only 24-57% compared to CT 1, 3
To improve ultrasound accuracy: Adding plain radiography (KUB) to ultrasound improves diagnostic accuracy to 79-90% sensitivity 1, 3
For recurrent stone formers: Limit the CT scan to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure, but recognize that ultra-low-dose protocols may miss stones <2mm in size 1, 2
What NOT to Do: Common Pitfalls
Never use contrast-enhanced CT as first-line imaging for suspected kidney stones—the enhancing renal parenchyma may obscure stones within the collecting system, particularly small ones 2, 3
Do not order CT abdomen and pelvis without and with IV contrast for kidney stone evaluation, as it offers no additional benefit over non-contrast CT alone and unnecessarily increases radiation exposure 2
Avoid relying solely on ultrasound for small stones (<5mm) as sensitivity decreases significantly 3
Role of Ultrasound in the Algorithm
While ultrasound has lower sensitivity for direct stone visualization (24-57%), it demonstrates up to 100% sensitivity and 90% specificity for diagnosing ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid) 3. In patients with moderate to severe hydronephrosis on ultrasound and moderate or high risk of ureteric calculi, this may provide sufficient diagnostic certainty without requiring CT 4. However, the absence of hydronephrosis on ultrasound does not rule out stones—negative predictive value is only 65% 4
Special Clinical Scenarios
When ultrasound shows moderate to severe hydronephrosis in a patient with high clinical suspicion for stones, you can consider reserving CT for cases where stone size and location are needed to plan surgical management, using low-dose protocols 4
In patients with flank pain and negative ultrasound, CT may identify important alternative diagnoses—one study found 15% of patients with negative urinalysis and no history of urolithiasis had diagnoses best characterized by contrast-enhanced CT 4