What are the clinical utilities of a CT (Computed Tomography) stonogram?

CT Stonogram (Non-Contrast CT for Kidney Stones): Clinical Utilities

Non-contrast CT of the abdomen and pelvis (CT stonogram) is the gold standard imaging modality for suspected urolithiasis, with sensitivity as high as 97% and specificity of 95%, providing precise stone detection, size measurement, and anatomic localization essential for treatment planning. 1, 2

Primary Clinical Utilities

Stone Detection and Characterization

• Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even small stones without IV contrast 2
• Provides precise measurement of stone size and clear depiction of where in the ureter a stone has become lodged, which is crucial for determining management strategy 1, 2
• Stone location and size directly determine intervention needs: smaller, more proximal stones are likely to pass spontaneously, while larger or more distal stones may require intervention 1
• Sensitivity ranges from 89.5% to 94.7% even with low-dose protocols 3

Assessment of Complications and Secondary Signs

• Detects secondary signs of obstruction including hydronephrosis, ureterectasis, perinephric fluid, and periureteral inflammation 1, 2
• Identifies complications such as perinephric abscess, urinoma, or emphysematous pyelonephritis 1
• Can evaluate for pyonephrosis, though collecting system content density may be helpful in distinguishing from simple hydronephrosis 1

Alternative Diagnosis Detection

• Identifies alternative causes of flank pain in 5-18% of cases, including cholecystitis, appendicitis, liver abscess, and other acute abdominal conditions 1
• Particularly valuable when symptoms persist or worsen after 72 hours of treatment 1
• In patients with negative urinalysis and no history of urolithiasis, 15% may have diagnoses best characterized by CT 1

Optimal Imaging Protocol

Low-Dose CT: The Preferred Approach

• Low-dose CT protocols (<3 mSv) should be used in place of conventional dosing when evaluating for renal or ureteral stones 1, 2
• Maintains pooled sensitivity of 97% and specificity of 95% for detecting urolithiasis while reducing radiation exposure by over 85% 2, 4
• Mean effective dose of 1.2 mSv in men and 1.9 mSv in women at 30 mAs settings 3
• Yields equivalent stone measurements compared to standard-dose CT 2

Technical Optimization

• Thin (1-1.5 mm) axial slice images are preferred over thick (5 mm) coronal maximum intensity projection images for optimal stone detection 2
• Use coronal reformations, bone window settings, and magnified views to improve accuracy of stone measurements 2
• Rapid acquisition with high spatial resolution and multiplanar reformation capabilities 2

Clinical Context-Specific Applications

Acute Flank Pain with Suspected Stone Disease

• Non-contrast CT is the reference standard for initial evaluation, with ACR appropriateness rating of 8 (usually appropriate) 1
• More comprehensive than ultrasound, which has sensitivity of only 24-57% for stone detection overall 1
• Superior to ultrasound for detecting ureteral calculi (CT sensitivity 97% vs US sensitivity up to 61%) 1

Recurrent Stone Disease

• Non-contrast CT (preferably at reduced doses) is usually appropriate for evaluation of recurrent symptoms, with ACR rating of 7 1
• Can limit scanning to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure 2
• Reviewing previous images helps avoid problems from repetitive CT use 1

Complicated Pyelonephritis

• CT abdomen and pelvis is highly sensitive for detection of stones and hydronephrosis in patients with acute pyelonephritis 1
• Detection rate of renal abscess is 4.0% by CT compared to only 1.1% by ultrasound 1
• Both unenhanced and contrast-enhanced CT can detect urolithiasis, perinephric fluid, renal swelling, and hydronephrosis 1

Hydronephrosis of Unknown Cause

• More comprehensive evaluation can be achieved with CT compared to ultrasound or MRI for determining etiology of hydronephrosis 1
• Useful in patients with renal colic and moderate to severe hydronephrosis, as these patients are at higher risk of stone passage failure 1

Important Caveats and Pitfalls

When NOT to Use Contrast

• CT with IV contrast is usually not appropriate as first-line test for kidney stones, as enhancing renal parenchyma may obscure stones within the collecting system 2
• Unenhanced CT has higher sensitivity than contrast-enhanced CT for detection of small renal calculi 1
• CT without and with IV contrast offers no additional benefit over non-contrast CT alone for kidney stone evaluation 2

Limitations in Early Presentation

• Secondary signs of obstruction may not have developed within the first 2 hours of presentation, reducing sensitivity 1
• Hydronephrosis on ultrasound does not accurately predict presence or absence of ureteral stone on CT in up to 25% of patients 1

Special Populations

• In pregnant patients, ultrasonography is the imaging tool of choice due to radiation concerns, with ACR rating of 8 (usually appropriate) 1, 2
• Low-dose non-contrast CT may be appropriate in pregnancy (rating of 6) when ultrasound is non-diagnostic 1

Risk of Overimaging

• In patients with known stone disease where the provider does not expect CT to change management, it often does not 1
• Consider point-of-care ultrasound combined with clinical prediction tools (like STONE score) to identify patients who may not need CT 5

Comparison with Alternative Modalities

Versus Ultrasound

• CT detection rate of acute pyelonephritis is 84.4% compared to only 40% by ultrasound 1
• CT and ultrasound have similar accuracy for detecting hydronephrosis, but CT is superior for identifying etiology 1
• Combining ultrasound with radiography has sensitivity of 79-90% for clinically significant stones, providing an acceptable alternative to low-dose CT in select patients 1

Versus Plain Radiography

• Abdominal radiography has ACR appropriateness rating of only 3 (usually not appropriate) for suspected stone disease 1
• Most helpful for evaluating interval stone growth in patients with known stone disease, less useful for acute stones 6

Versus MRI

• MRI is less accurate for identifying suspected stones but highly dependable for depicting hydronephrosis and perinephric edema, with ACR rating of 4 (may be appropriate) 1