Imaging for Kidney Stone
Low-dose non-contrast CT of the abdomen and pelvis is the gold standard initial imaging modality for suspected kidney stones, with sensitivity of 97% and specificity of 95%. 1
Primary Recommendation: Non-Contrast CT
Use low-dose non-contrast CT as your first-line imaging test for most patients with suspected nephrolithiasis. 1 This modality provides:
- Accurate detection of virtually all renal calculi, as they are radiopaque on CT regardless of composition 1, 2
- Precise measurement of stone size and exact location within the ureter, which is crucial for determining whether conservative management or intervention is needed 1, 2
- Visualization of secondary signs including periureteral inflammation, perinephric inflammation, and ureteral dilatation 1, 2
- Rapid acquisition with high spatial resolution and multiplanar reformation capabilities 2
Optimal CT Technique
- Request low-dose protocols (<3 mSv) rather than conventional dosing to minimize radiation exposure while maintaining diagnostic accuracy 2
- Ensure thin (1-1.5 mm) axial slice images are obtained rather than thick (5 mm) slices for optimal stone detection 2
- Review images using bone window settings and magnified views to improve accuracy of stone measurements 2
- Utilize coronal reformations to enhance stone visualization 2
Alternative First-Line Options for Special Populations
Pregnant Patients
Use ultrasound of kidneys and bladder as the first-line test due to radiation concerns 1. While ultrasound has limited sensitivity (24-57%) compared to CT, it avoids fetal radiation exposure 1.
Pediatric Patients and Recurrent Stone Formers
Consider ultrasound as initial imaging for children and patients requiring frequent follow-up imaging 1. To improve diagnostic accuracy:
- Add plain radiography (KUB) to ultrasound, which increases sensitivity to 79-90% 1
- For recurrent stone patients with known stone composition, limit the CT scan to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure 1, 2
What NOT to Do: Common Pitfalls
Avoid contrast-enhanced CT as first-line imaging for suspected kidney stones, as the enhancing renal parenchyma may obscure stones within the collecting system 2. CT with IV contrast is usually not appropriate for initial stone evaluation 2.
Do not order CT abdomen and pelvis without and with IV contrast, as this offers no additional benefit over non-contrast CT alone and unnecessarily increases radiation exposure 2.
Be aware that ultra-low-dose protocols may miss stones <2mm in size, so balance radiation reduction with diagnostic accuracy based on clinical context 1, 2. Small uric acid fragments specifically may require higher energy settings for detection 3.
Limited Role for Other Modalities
MRI
Reserve MRI for rare situations when radiation must be avoided and ultrasound is inconclusive 1. Noncontrast MRU can detect upper tract obstruction with sensitivity of 84% and specificity of 100% using secondary signs, but stones themselves are difficult to visualize directly 1.
Plain Radiography (KUB)
Use KUB primarily for evaluating interval stone growth in patients with known radiopaque stone disease, not for acute stone diagnosis 1.