What imaging is done for a patient with lower abdominal pain suspected to have kidney stones?

Imaging for Suspected Kidney Stones in Lower Abdominal Pain

Non-contrast CT of the abdomen and pelvis is the gold standard imaging modality for suspected kidney stones, with 97% sensitivity and should be performed using low-dose protocols to minimize radiation exposure. 1, 2, 3

Primary Recommendation: Low-Dose Non-Contrast CT

Low-dose non-contrast CT (<3 mSv) should be the first-line imaging test for patients with lower abdominal pain suspected to have kidney stones. 1, 2, 3

Why Non-Contrast CT is Superior:

• Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even stones as small as 1 mm in diameter 1, 2
• Provides rapid acquisition with high spatial resolution and multiplanar reformation capabilities 1, 2
• Detects secondary signs of urolithiasis including periureteral inflammation, perinephric stranding, and ureteral dilatation 1, 3
• Accurately depicts stone size and location, which is critical for predicting spontaneous passage rates 2
• Low-dose protocols maintain diagnostic accuracy with confirmed 97% sensitivity and 95% specificity while minimizing radiation exposure 2, 3

Technical Optimization:

• Use thin (1-1.5 mm) axial slices for optimal stone detection 1
• Include coronal reformations and bone window settings for accurate measurements 2
• Ensure the scan includes the entire abdomen and pelvis, as stones can be located in the distal ureters or bladder 1

Why NOT to Use Contrast-Enhanced CT

IV contrast should be avoided for initial stone evaluation because it significantly reduces stone detection sensitivity. 1

• Contrast-enhanced CT has only 81% overall sensitivity for detecting all stones (versus 97% for non-contrast) 1
• Enhancing renal parenchyma during the nephrographic phase obscures stones within the renal collecting system 1
• Even for larger stones (≥3 mm), contrast-enhanced CT achieves only 95% sensitivity compared to 97% for non-contrast 1
• The only exception: If contrast CT has already been performed for another indication, stones ≥6 mm can still be detected with approximately 98% accuracy 1, 2

Alternative Imaging Options

Ultrasound:

Ultrasound has limited utility as a primary imaging modality due to poor sensitivity, but may be appropriate in specific populations. 3, 4

• Sensitivity is only 24-57% for stone detection compared to CT 3
• Should be reserved for pregnant patients where radiation must be avoided 3
• Reasonable for pediatric patients and those requiring frequent follow-up imaging for recurrent stone disease 3
• Adding plain radiography (KUB) to ultrasound improves diagnostic accuracy to 79-90% sensitivity 3
• In a randomized trial of 2,759 patients, ultrasound had similar 30-day outcomes to CT but with lower radiation exposure, though this was in an emergency department setting where close follow-up was ensured 4

Plain Radiography (KUB):

• Not recommended as a standalone initial test for acute stone evaluation 1
• More sensitive than CT scout images for detecting radiopaque stones (48% vs 17% sensitivity) 5
• Most useful for monitoring known stones over time rather than initial diagnosis 6

Clinical Algorithm

For a patient presenting with lower abdominal pain suspected to be kidney stones:

1. Order low-dose non-contrast CT abdomen and pelvis as the first-line test 1, 2, 3
2. If the patient is pregnant, use ultrasound of kidneys and bladder instead 3
3. If contrast CT was already performed for another indication, review it carefully—stones ≥6 mm can still be detected with high accuracy 1, 2
4. For patients with recurrent stones requiring frequent imaging, consider ultrasound with KUB to reduce cumulative radiation exposure 3

Common Pitfalls to Avoid

• Do not order contrast-enhanced CT as the initial test for suspected stones—it will miss smaller stones 1
• Do not rely on ultrasound alone in non-pregnant adults—it misses too many stones 3, 4
• Do not use standard-dose CT protocols—low-dose protocols have equivalent diagnostic accuracy with significantly less radiation 1, 2, 7
• Be aware that ultra-low-dose protocols may miss stones <2 mm, though these typically pass spontaneously 3