When to use ultrasound versus computed tomography urography (CTU) in patients suspected of having nephrolithiasis?

Choosing Between Ultrasound and CT for Suspected Nephrolithiasis

For suspected nephrolithiasis, non-contrast CT of the abdomen and pelvis should be used when precise stone characterization is needed, while ultrasound should be the first-line imaging modality for pregnant patients, children, and those requiring repeated imaging to reduce radiation exposure. 1

Initial Imaging Algorithm

First-line Imaging:

• Ultrasound should be used as first-line imaging in:

  • Pregnant patients (appropriateness rating of 8) 1
  • Children with suspected urolithiasis 2
  • Patients requiring repeated imaging for stone surveillance 1
  • Patients with low clinical suspicion of complicated stone disease

• Non-contrast CT should be used as first-line imaging in:

  • Patients with high clinical suspicion of stone disease requiring precise characterization
  • When alternative diagnoses must be ruled out
  • When complications such as obstruction need assessment 1
  • Patients with painful hematuria and negative ultrasound but high clinical suspicion 2

Diagnostic Performance Comparison

Ultrasound:

• Sensitivity: ~45%, Specificity: ~88% 1
• Advantages:
  • No radiation exposure
  • Lower cost
  • Readily available
  • Good for detecting hydronephrosis
• Limitations:
  • Decreased sensitivity for stones <3mm 3
  • Operator-dependent
  • Limited visualization of mid-ureteral stones (only 38% detection rate) 2

Non-contrast CT:

• Sensitivity: >95%, Specificity: >95% 1
• Gold standard for diagnosing kidney stones 1
• Advantages:
  • Precise information about stone size, location, and degree of obstruction
  • Can identify alternative diagnoses
  • Not operator-dependent
• Limitations:
  • Radiation exposure
  • Higher cost

Special Clinical Scenarios

Painful Hematuria:

• Start with ultrasound of kidneys and bladder
• If ultrasound is negative but clinical suspicion remains high, proceed to non-contrast CT 2
• CT is particularly useful when detection would impact treatment decisions 2

Pregnant Patients:

• Ultrasound is the first-line imaging modality 1
• MRI can be considered if ultrasound findings are inconclusive 4

Children:

• Ultrasound is the preferred initial imaging modality 2
• Non-contrast CT should be reserved for when ultrasound is non-diagnostic or when an alternative diagnosis is suspected 2

Follow-up Imaging:

• Ultrasound or KUB (kidney-ureter-bladder X-ray) is appropriate for follow-up of known stone disease 1, 4
• Periodic monitoring with ultrasound every 3-6 months is recommended to assess stability of hydronephrosis and calculi 1

Clinical Pearls and Pitfalls

• The STONE trial demonstrated that initial ultrasonography was associated with lower cumulative radiation exposure than initial CT, without significant differences in high-risk diagnoses with complications or adverse events 5

• When using ultrasound, consider these techniques to improve accuracy:

  • Adjust gain and depth settings
  • Use harmonic mode and flash angle imaging
  • Measure shadows for better size estimation 3

• For patients with moderate to severe hydronephrosis on ultrasound and suspected urolithiasis, consider low-dose CT rather than standard-dose CT, as these patients are at higher risk of stone passage failure 2

• The combination of X-ray and ultrasound may improve detection (combined sensitivity ~90%), although still inferior to CT 1

• Remember that CT with IV contrast may obscure stones within the renal collecting system during the portal venous or nephrographic phase, making non-contrast CT the preferred method for initial stone evaluation 2