Imaging for Kidney Stones
Low-dose non-contrast CT of the abdomen and pelvis is the gold standard initial imaging test for suspected kidney stones, with 97% sensitivity and 95% specificity. 1, 2
Primary Recommendation: Non-Contrast CT
Use low-dose non-contrast CT as your first-line imaging modality for most patients with suspected kidney stones. 1, 2 This approach provides:
- Detection of virtually all renal calculi regardless of composition, including even small stones without requiring IV contrast 1, 2, 3
- Precise stone size measurement and exact anatomical location within the ureter, which is crucial for determining whether observation versus intervention is appropriate 1, 2, 3
- Identification of secondary signs including periureteral inflammation, perinephric inflammation, and ureteral dilatation that confirm clinical significance 1, 2, 3
- Rapid acquisition with high spatial resolution and ability for multiplanar reformations 3
Technical Optimization
Use low-dose protocols (<3 mSv) rather than conventional dosing to reduce radiation exposure while maintaining excellent diagnostic performance 3. The low-dose approach maintains the same 97% sensitivity and 95% specificity as standard-dose CT 3. For optimal stone detection, use:
- Thin (1-1.5 mm) axial slice images rather than thick (5 mm) coronal maximum intensity projection images 3
- Coronal reformations, bone window settings, and magnified views to improve accuracy of stone measurements 3
Critical Pitfall to Avoid
Do not use contrast-enhanced CT as first-line imaging for suspected kidney stones — the enhancing renal parenchyma may obscure stones within the collecting system, leading to missed small stones 3. Unenhanced CT has higher sensitivity than contrast-enhanced CT for detection of small renal calculi 4.
Alternative First-Line Options for Special Populations
Pregnant Patients
Use ultrasound of kidneys and bladder as the first-line test for pregnant patients due to radiation concerns 1, 2. However, recognize that ultrasound alone has poor sensitivity (24-57%) for stone detection 1, 2. Adding plain radiography (KUB) to ultrasound improves diagnostic accuracy to 79-90% sensitivity, though this remains inferior to CT 1, 2.
Pediatric Patients and Recurrent Stone Formers
Ultrasound is reasonable for pediatric patients and those requiring frequent follow-up imaging for recurrent stone disease 1. For patients with recurrent stones, limit the CT scan to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure 1, 2. Be aware that ultra-low-dose protocols may miss stones <2mm in size 1, though research shows all stone types remain visible at ultra-low doses except 1-mm uric acid fragments 5.
When Radiation Must Be Avoided and Ultrasound Is Inconclusive
MRI with noncontrast MR urography can detect upper tract obstruction using secondary signs with 84% sensitivity, 100% specificity, and 86% accuracy 1, 2. However, MRI has limited utility for direct stone visualization and poor accuracy for detecting small urothelial calculi 4, 1.
Comparative Performance Data
CT and ultrasound have similar detection rates for hydronephrosis, but CT is vastly superior for stone detection 4. In a prospective study of 827 patients with acute pyelonephritis, the detection rate of urolithiasis was similar by CT and ultrasound, but CT detected renal abscess in 4.0% of cases versus only 1.1% by ultrasound 4. Both unenhanced and contrast-enhanced CT can detect urolithiasis, but unenhanced CT has higher sensitivity for small renal calculi 4.