Imaging for Suspected Kidney Stones
Non-contrast CT of the abdomen and pelvis is the gold standard imaging study for suspected kidney stones, with a sensitivity of up to 97% and specificity of 95%. 1
Rationale for Non-contrast CT
Non-contrast CT offers several advantages for kidney stone diagnosis:
- Virtually all renal calculi are radiopaque on CT, allowing for accurate detection of even small stones
- Provides high spatial resolution with ability for multiplanar reformations
- Visualizes secondary signs of urolithiasis (periureteral inflammation, perinephric inflammation, ureteral dilatation)
- Allows accurate assessment of stone size, location, and density, which helps predict spontaneous passage rates
- Rapid acquisition time
Low-dose non-contrast CT protocols (using <3 mSv) maintain excellent diagnostic performance with pooled sensitivity of 97% and specificity of 95%, making them particularly suitable for younger patients where radiation exposure is a concern. 1
Clinical Implications of CT Findings
CT findings directly impact clinical management decisions:
- Stone size and location help predict spontaneous passage rates
- Larger and more proximal stones have higher need for intervention
- Stone density measurements help predict need for invasive management
- Accurate stone measurements guide urologic treatment planning
Alternative Imaging Options
When CT is not available or contraindicated:
Ultrasound
- Radiation-free alternative
- Less sensitive than CT but can detect hydronephrosis and larger stones
- Enhanced with color Doppler to detect "twinkling artifact" behind stones
- Best used in pregnant patients, children, or for follow-up of known stones
KUB (Kidney, Ureter, Bladder) Radiography
- Limited sensitivity (29-72% depending on stone size and location)
- Only 8% detection rate for stones <5 mm, but 78% for stones >5 mm 1
- Most useful for follow-up of known radiopaque stones rather than initial diagnosis
- Cannot reliably distinguish ureteral stones from phleboliths
MR Urography (MRU)
- Radiation-free alternative
- Better at detecting secondary signs of obstruction than direct stone visualization
- In recent studies, direct stone detection sensitivity only 50% compared to 91% with CT
- When combined with secondary signs (perinephric fluid and ureteral dilation), sensitivity improves to 84% 1
- Reserved for cases where radiation must be avoided and ultrasound is inconclusive
Clinical Pitfalls to Avoid
Contrast-enhanced CT limitation: The presence of IV contrast may obscure stones within the renal collecting system, making non-contrast CT the preferred initial study.
Stone size measurement: CT tends to overestimate stone size in the craniocaudal dimension by approximately 0.8 mm compared to plain radiography. 2 Use coronal reformations, bone window settings, and magnified views for more accurate measurements. 1
Small stone detection: Sensitivity decreases with smaller stone size, especially with increasing dose reduction in low-dose protocols. 1
Radiation exposure: While CT is the most accurate test, radiation exposure remains a concern, particularly in young patients or those requiring multiple studies. Consider low-dose protocols.
Recommended Imaging Algorithm
First-line: Non-contrast CT abdomen and pelvis (preferably low-dose protocol in younger patients)
- Provides definitive diagnosis with highest sensitivity and specificity
- Allows assessment of stone characteristics that guide management
For pregnant patients, children, or radiation-sensitive situations: Ultrasound with KUB radiography
- Radiation-free initial assessment
- Less sensitive but can detect hydronephrosis and larger stones
For follow-up of known stones: KUB radiography or ultrasound
- Reduces cumulative radiation exposure
- Adequate for monitoring known stone disease
When both CT and ultrasound are contraindicated: Consider MRU
- Provides information on obstruction and secondary signs
- Limited direct stone visualization