What is the best imaging test to diagnose a kidney stone?

Best Imaging for Kidney Stone Detection

Non-contrast CT (NCCT) of the abdomen and pelvis is the gold standard imaging test for diagnosing kidney stones, with sensitivity of 97% and specificity of 95%. 1, 2

Primary Recommendation: Low-Dose Non-Contrast CT

Use low-dose CT protocols (<3 mSv) as the first-line imaging modality for suspected kidney stones to minimize radiation exposure while maintaining excellent diagnostic accuracy (pooled sensitivity 97%, specificity 95%). 1, 2

Why Non-Contrast CT is Superior:

• Virtually all renal calculi are radiopaque on CT, allowing detection of even small stones without IV contrast 1
• Provides rapid acquisition with high spatial resolution and multiplanar reformation capabilities 1
• Accurately measures stone size and precisely depicts stone location within the ureter 1
• Detects secondary signs of complications including periureteral inflammation, perinephric inflammation, and ureteral dilatation 1, 2
• Helical (spiral) NCCT clearly shows where stones have become lodged in the ureter 1

Technical Optimization:

• Use thin (1-1.5 mm) axial slice images rather than thick (5 mm) coronal maximum intensity projections for optimal stone detection 1
• View images on bone window settings with magnified views and coronal reformations to improve measurement accuracy 1
• Stone location and size determination is crucial since more proximal and larger stones require higher intervention rates 1

Alternative First-Line Options

Ultrasound (US) of Kidneys and Bladder:

Consider ultrasound as the primary imaging modality in pregnant patients and children due to radiation concerns. 2, 3

• Sensitivity for stone detection is lower (24-57%) compared to CT, but sensitivity for detecting ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid) reaches up to 100% with specificity of 90% 2
• Adding color Doppler with twinkling artifact assessment improves sensitivity for small renal stones 2
• Combining ultrasound with plain radiography (KUB) improves diagnostic accuracy to 79-90% sensitivity 2
• In a randomized controlled trial, ultrasound and CT demonstrated equivalent diagnostic accuracy within the emergency department setting 4

What NOT to Use as First-Line Imaging

Contrast-Enhanced CT:

Avoid CT with IV contrast as first-line imaging because enhancing renal parenchyma may obscure stones within the collecting system. 1, 2 The American College of Radiology states unenhanced CT has higher sensitivity than contrast-enhanced CT for detecting small renal calculi. 5

Plain Radiography (KUB) Alone:

Limited sensitivity (29% overall, 72% for stones >5mm in proximal ureter) makes this inadequate as a standalone test. 2

MRI:

Has limited utility in stone detection and should only be considered when radiation must be avoided and ultrasound is inconclusive. 2

Intravenous Urography (IVU):

Lower sensitivity and specificity (87% and 94%) compared to non-contrast CT (96% and 100%). 2

Special Clinical Scenarios

Recurrent Stone Disease:

Limit the CT scan to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure. 1, 2

Pregnant Patients:

Ultrasonography is the imaging tool of choice due to radiation concerns. 1

Known Stone Disease Follow-up:

Plain KUB radiography is most helpful for evaluating interval stone growth in patients with known stone disease. 4

Critical Pitfalls to Avoid

• Do not rely solely on ultrasound for detecting small stones (<5mm) as sensitivity decreases significantly 2
• Do not use contrast-enhanced CT as first-line imaging as it may lead to missed small stones 1, 2
• Do not ignore hydronephrosis on ultrasound even when stones are not visualized, as this indicates possible obstruction 2
• Be aware that ultra-low-dose protocols may miss stones <2mm in size 1
• CT can occasionally be deceiving due to motion artifact causing warping distortion that makes stones appear larger than actual size; correlation with KUB and ultrasound is critical 6