Kidney Stones Appear Larger on Ultrasound Than on X-ray
Ultrasound tends to overestimate kidney stone size compared to both X-ray and CT, particularly for smaller stones ≤5 mm, where US measurements can be 1.9 mm larger on average. 1
Size Measurement Accuracy by Modality
Ultrasound Overestimation
- US consistently overestimates stone size compared to CT (the reference standard), with mean measurements 1.8 mm larger overall (9.2 mm on US vs 7.4 mm on CT). 1
- For stones ≤5 mm specifically, US measurements exceed CT by a mean of 1.9 ± 1.2 mm, with 60% of these small stones showing discordant measurements between the two modalities. 1
- The overestimation is influenced by skin-to-stone distance rather than body mass index or stone location. 1
- Stone size estimation at US is particularly limited for smaller (≤5 mm) stones, with a consistent tendency to overestimate. 2
X-ray Limitations
- Plain radiography (KUB) has poor overall sensitivity for stone detection (29% for stones of any size in any location), though it reaches 72-78% sensitivity for larger stones >5 mm. 2, 3
- X-ray does not inherently magnify or minimize stone size when stones are visible, but many stones simply cannot be detected at all due to composition, location, or overlying structures. 2
- Only approximately 90% of calcium oxalate stones are radio-opaque and detectable on plain film. 3
Clinical Implications
Why This Matters
- The overestimation by US can lead to inappropriate treatment decisions, as stones measured at >5-7 mm may prompt intervention when they might actually pass spontaneously. 4
- CT remains superior for accurate stone size determination and should be used when precise measurements are needed for treatment planning. 4
- The combination of US and KUB improves detection rates (sensitivity of 79-90%) but does not resolve the size measurement inaccuracy of US. 2
Practical Approach
- Use US for initial screening and detecting hydronephrosis (100% sensitive for obstruction), but recognize that stone size measurements will likely be overestimated. 2, 5
- When stone size is critical for management decisions (e.g., distinguishing 5 mm from 7 mm stones), obtain CT imaging for accurate measurement. 2
- For follow-up of known radio-opaque stones, KUB can be used but provides no size magnification advantage over US. 3
Common Pitfalls
- Do not rely on US measurements alone for treatment planning, especially when deciding between conservative management and intervention for borderline-sized stones. 1
- The posterior acoustic shadow on US may provide more accurate sizing than the stone itself (within 0.8 mm of CT vs 1.6 mm for direct stone measurement). 6
- Within the first 2 hours of presentation, secondary signs of obstruction may not yet be visible on US, reducing diagnostic sensitivity. 2, 5