What type of ultrasound should I order to evaluate for kidney stones?

Ultrasound for Kidney Stone Evaluation

For suspected kidney stones, you should NOT order ultrasound as your first-line imaging—instead, order a non-contrast CT scan of the abdomen and pelvis using a low-dose protocol, as this is the gold standard with 97% sensitivity and 95% specificity. 1, 2, 3

Why CT is Superior to Ultrasound

CT dramatically outperforms ultrasound for kidney stone detection. Non-contrast CT (NCCT) achieves 94-98% sensitivity for detecting renal stones, compared to only 19% sensitivity for ultrasonography. 4 This represents a clinically significant difference that directly impacts patient outcomes—missing stones can lead to delayed treatment, progression of obstruction, and potential kidney damage.

Key Advantages of Non-Contrast CT:

• Detects virtually all stone compositions regardless of radiopacity 1
• Accurately measures stone size and location, which is critical for determining whether intervention is needed 1, 3
• Identifies secondary signs of obstruction including hydronephrosis, periureteral inflammation, and perinephric fluid 1, 2
• Provides multiplanar reformations for precise anatomic localization 1

When Ultrasound IS Appropriate

Ultrasound should be reserved for specific clinical scenarios where radiation exposure must be avoided:

Primary Indications for Ultrasound:

• Pregnant patients: Ultrasound is the imaging tool of choice due to radiation concerns 1, 2, 3
• Children: To minimize cumulative radiation exposure 2, 3
• Follow-up imaging in known stone formers: When combined with plain radiography (KUB), sensitivity improves to 79-90% 2, 3

Critical Limitations of Ultrasound:

• Poor sensitivity for direct stone visualization: Only 24-57% compared to CT's 97% 2, 3
• Particularly poor for small stones: Sensitivity decreases significantly for stones <5mm 2, 3
• Operator-dependent: Image quality varies with technician experience and patient body habitus 4
• Limited by patient factors: Obesity, bowel gas, and narrow intercostal spaces can prevent adequate visualization 4

Optimal CT Protocol

When ordering CT for kidney stones, specify these technical parameters:

• Low-dose protocol (<3 mSv) to reduce radiation while maintaining 97% sensitivity and 95% specificity 1, 2, 3
• Non-contrast technique: Contrast obscures stones within the collecting system and should NOT be used as first-line imaging 1, 2, 3
• Thin slice acquisition (1-1.5mm) for optimal stone detection 1
• Helical (spiral) technique for precise measurements and multiplanar reformations 1

Common Pitfalls to Avoid

Do not order contrast-enhanced CT as your initial test—the enhancing renal parenchyma will obscure small stones within the collecting system, potentially leading to missed diagnoses. 1, 2, 3

Do not rely solely on ultrasound for acute stone evaluation unless the patient is pregnant or a child—the low sensitivity (19-57%) means you will miss the majority of stones, particularly smaller ones. 4, 2

Do not assume absence of hydronephrosis on ultrasound rules out a stone—many ureteral stones, especially small ones, do not cause hydronephrosis. 4 However, ultrasound does have excellent sensitivity (up to 100%) for detecting obstruction when present. 2, 3

Special Populations

For recurrent stone formers: Use ultra-low-dose CT protocols or limit scanning to the area of interest to reduce cumulative radiation exposure. 1, 2, 3

For pregnant patients: Start with renal ultrasound, and if inconclusive, consider MRI without contrast rather than CT. 2, 3

For emergency department evaluation: While one randomized trial suggested ultrasound and CT had equivalent diagnostic accuracy in the ED setting 5, this finding conflicts with the overwhelming evidence showing CT's superior sensitivity (94-98% vs 19-57%). The most recent AUA/SUFU guidelines (2025) continue to recommend CT urography as the preferred imaging modality. 4