What is the recommended initial imaging test for patients suspected of having kidney stones?

Recommended Initial Imaging for Suspected Kidney Stones

Non-contrast CT of the abdomen and pelvis is the recommended initial imaging test for patients suspected of having kidney stones, with a sensitivity as high as 97% and specificity of 95%. 1, 2

Why Non-Contrast CT is Preferred

• Non-contrast CT (NCCT) is considered the reference standard for kidney stone evaluation due to its high accuracy in detecting even small stones 1, 2
• Virtually all renal calculi are radiopaque on CT, allowing for accurate detection without the need for IV contrast 1
• CT provides rapid acquisition with high spatial resolution and ability for multiplanar reformations 1
• Secondary signs of urolithiasis and complications such as periureteral inflammation, perinephric inflammation, and ureteral dilatation can also be visualized 2
• NCCT allows for precise measurement of stone size and location, which is crucial for determining management approach 2

Low-Dose CT Considerations

• Low-dose CT protocols (<3 mSv) should be used in place of conventional dosing to reduce radiation exposure while maintaining excellent diagnostic performance 2, 3
• Low-dose CT maintains a pooled sensitivity of 97% and specificity of 95% for detecting urolithiasis 2
• Ultra-low-dose protocols can reduce radiation exposure by up to 92% compared to standard low-dose CT while still accurately measuring stones 3

When to Consider Alternative Imaging

• Ultrasound should be used as the initial imaging modality in pregnant patients to avoid radiation exposure 2, 4
• 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 3
• For patients with known stone disease requiring follow-up, kidney-ureter-bladder (KUB) radiography may be helpful for evaluating interval stone growth 5

Contrast-Enhanced CT Limitations

• 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, 2
• 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

Common Pitfalls to Avoid

• Relying on contrast-enhanced CT as first-line imaging for suspected kidney stones may lead to missed small stones 1, 2
• The sensitivity for small stone detection decreases with increasing dose reduction, so ultra-low-dose protocols may miss stones <2mm in size 2
• Digital tomosynthesis (DT) is not widely available at many institutions, limiting its practical utility despite comparable accuracy to CT for intrarenal stones 1

Emerging Trends in Imaging Selection

• Recent multispecialty consensus guidelines suggest that ultrasound may be appropriate in specific clinical scenarios to reduce radiation exposure 6
• Automated stone volume analysis software can provide more reproducible measurements than manual linear measurements, with potential benefits for stone surveillance 3
• For known stone disease requiring follow-up, ultra-low-dose limited renal CT offers advantages over standard CT in terms of reduced radiation and cost 3