Diagnosing Kidney Stones with Imaging

First-Line Imaging

Non-contrast CT scan of the abdomen and pelvis is the gold standard for diagnosing kidney stones, with 97% sensitivity and 95% specificity, and should be the first-line imaging test 1, 2
The American College of Radiology recommends using low-dose CT protocols (<3 mSv) to minimize radiation exposure while maintaining excellent diagnostic accuracy (pooled sensitivity 97%, specificity 95%) 1, 2
Non-contrast CT detects virtually all kidney stones regardless of composition, accurately measures stone size, and precisely localizes stones within the ureter 2
CT also identifies secondary signs of obstruction including hydronephrosis, periureteral inflammation, perinephric fluid, and ureteral dilation 1, 2

The American College of Radiology recommends using ultrasound as first-line imaging in pregnant patients and children to avoid radiation exposure 1
Ultrasound has lower sensitivity for direct stone visualization (24-57%) but excellent sensitivity (up to 100%) for detecting ureteral obstruction through hydronephrosis and ureterectasis 1
Combining ultrasound with plain radiography (KUB) improves diagnostic accuracy to 79-90% sensitivity when CT is not appropriate 1

If initial non-contrast CT is inconclusive, obtain CT with IV contrast to visualize the "soft tissue rim" sign around ureteral stones, confirm iliac arterial calcifications, and detect delayed nephrogram from obstruction 3
The Journal of the American College of Radiology recommends against using contrast-enhanced CT as first-line imaging because enhancing renal parenchyma obscures stones within the collecting system, particularly small stones 1, 2
The American College of Radiology recommends against relying solely on ultrasound for detecting small stones (<5 mm) as sensitivity decreases significantly for smaller calculi 1

In pregnant patients, start with ultrasound of kidneys and bladder as first-line imaging, and consider MRU without contrast if ultrasound is inconclusive 3, 1
In recurrent stone formers, use ultra-low-dose CT protocols or limit scanning to the area of interest to reduce cumulative radiation exposure 1, 2