CT Imaging for Kidney Stones and Constipation
Order a CT abdomen and pelvis WITHOUT contrast for kidney stone evaluation. 1, 2
For Kidney Stones: Non-Contrast CT is the Gold Standard
Non-contrast CT (NCCT) is the reference standard imaging modality for suspected urolithiasis, with 97% sensitivity and 95% specificity. 1, 2
Why Non-Contrast CT is Superior:
Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even small stones (as small as 1 mm) without IV contrast 1, 2
NCCT provides rapid acquisition with high spatial resolution and multiplanar reformation capabilities that precisely measure stone size and location 2, 3
Secondary signs of urolithiasis are clearly visible, including periureteral inflammation, perinephric inflammation, and ureteral dilatation 1, 3
Stone size and location predict spontaneous passage rates and need for intervention—larger and more proximal stones require more aggressive management 1, 2
Contrast CT is Inferior for Stone Detection:
Contrast-enhanced CT has only 81% overall sensitivity for detecting all stones (1 mm+), compared to 97% for non-contrast CT 1
Even for larger stones (≥3 mm), contrast CT sensitivity is only 95%, still inferior to non-contrast CT 2
The enhancing renal parenchyma on contrast studies may obscure stones within the collecting system 3
There is no documented additional benefit of adding contrast phases to non-contrast CT for urolithiasis evaluation 1
Optimize Your CT Order:
Request low-dose CT protocol (<3 mSv) to minimize radiation exposure while maintaining diagnostic accuracy (97% sensitivity, 95% specificity). 1, 2, 3
Specify thin (1-1.5 mm) axial slices rather than thick (5 mm) slices for optimal stone detection 1, 3
Request coronal reformations, bone window settings, and magnified views for accurate stone measurements 1, 3
Low-dose protocols yield equivalent stone measurements to standard-dose CT 1, 3
For Constipation: Contrast Depends on Clinical Concern
If evaluating for constipation alone (fecal impaction, bowel obstruction), non-contrast CT is typically sufficient. 4
However, if you suspect complications requiring vascular assessment (ischemia, perforation, inflammatory processes), order CT WITH IV contrast. 5
Key Decision Points:
Non-contrast CT adequately visualizes bowel distention, fecal loading, and mechanical obstruction 4
IV contrast is essential when you need to assess bowel wall enhancement, mesenteric vasculature, or inflammatory/infectious complications 5
Administering IV contrast on the first CT increases detection of urgent pathology compared to non-contrast CT (p = 0.004) and avoids the need for repeat scanning within 72 hours 5
Common Pitfalls to Avoid:
Do not order contrast CT as first-line for kidney stones—you will miss small stones and expose patients to unnecessary contrast and radiation 2, 3
Do not assume you need contrast to see stones—this is a common misconception; virtually all stones are visible without contrast 1
If a patient already had contrast CT performed for another indication, stones ≥6 mm can still be detected with approximately 98% accuracy, so repeat non-contrast CT may not be necessary 1, 2
For pregnant patients with suspected stones, ultrasound is preferred despite lower sensitivity (45% for renal calculi) to avoid radiation exposure 2, 3
Practical Ordering Algorithm:
Primary concern is kidney stones: Order CT abdomen/pelvis WITHOUT contrast (low-dose protocol)
Primary concern is constipation with suspected complications (ischemia, perforation, abscess): Order CT abdomen/pelvis WITH IV contrast
Evaluating both kidney stones AND potential abdominal complications: Order non-contrast CT first; if additional pathology is suspected after reviewing images, contrast can be added selectively 5
Known contraindication to contrast (renal insufficiency, allergy): Non-contrast CT is appropriate for both indications 4