What is the recommended imaging modality, CT (Computed Tomography) with or without contrast, to assess for kidney stones?

CT Without Contrast is the First-Line Imaging Modality for Kidney Stones

Non-contrast CT scan is the recommended first-line imaging modality for diagnosing kidney stones, with approximately 97% sensitivity and 95% specificity. 1

Rationale for Non-Contrast CT

Non-contrast CT offers several advantages for kidney stone assessment:

• Highest diagnostic accuracy (97% sensitivity, 95% specificity) 1
• Ability to detect stones of all compositions, including radiolucent stones that may be missed on plain radiography 2
• Provides information about stone size, location, and density which directly impacts clinical management decisions 1
• Can detect secondary signs of obstruction and alternative diagnoses

Radiation Considerations and Low-Dose Protocols

While radiation exposure is a concern with CT imaging, low-dose protocols have been developed:

• Low-dose CT protocols (<3 mSv) maintain excellent diagnostic performance (97% sensitivity, 95% specificity) 1
• Ultra-low-dose protocols can achieve radiation doses as low as 0.57 mSv (92% lower than standard low-dose CT) while maintaining excellent detection rates 1
• The American College of Radiology recommends these protocols to reduce radiation exposure while maintaining diagnostic accuracy 1

Why Contrast is Not Recommended

Contrast administration is generally not recommended for initial kidney stone assessment because:

• Contrast can obscure stones in the collecting system
• Contrast may alter the density measurements of stones 3
• Non-contrast CT provides sufficient information for diagnosis and management planning
• Patients with kidney stones may have impaired renal function, increasing the risk of contrast-induced nephropathy

Special Populations and Alternative Imaging

For radiation-sensitive patients, alternative imaging approaches may be considered:

• Ultrasound with KUB radiography is recommended for pregnant patients, children, and patients requiring frequent imaging 1
• Ultrasound has lower sensitivity (24-57%) compared to CT but can be up to 100% sensitive for diagnosing ureteral obstruction when associated signs are present 1
• Color Doppler and twinkling artifact assessment increases ultrasound sensitivity to 99% for stones <5 mm 1

Follow-Up Imaging Recommendations

For known kidney stones, follow-up imaging can be tailored:

• KUB or ultrasound for radiopaque stones to reduce cumulative radiation exposure 1
• Low-dose CT for uric acid or small stones that may be missed by other modalities 1
• Ultra-low-dose limited renal CT with volumetric analysis offers advantages for stone surveillance 1

Clinical Pitfalls to Avoid

• Relying solely on ultrasound for initial diagnosis in high-risk patients may miss small stones
• Using contrast-enhanced CT as initial imaging may obscure stones
• Failing to use low-dose protocols unnecessarily increases radiation exposure
• Missing small uric acid stones (<2 mm) that may be difficult to detect even with standard CT techniques 1

Non-contrast CT remains the gold standard for kidney stone assessment due to its superior diagnostic performance and ability to provide comprehensive information for clinical decision-making.