What is the recommended initial imaging modality for a patient complaining of flank pain to view the kidneys?

Non-Contrast CT is the Recommended Initial Imaging for Flank Pain to View Kidneys

For patients with acute flank pain, non-contrast CT abdomen and pelvis is the recommended first-line imaging modality to evaluate the kidneys due to its superior sensitivity and specificity for detecting urolithiasis and other causes of flank pain. 1

Rationale for Non-Contrast CT as First Choice

• Non-contrast CT is considered the reference standard for evaluation of urolithiasis with a reported sensitivity as high as 97% and specificity of 95-98% 1
• Virtually all renal calculi are radiopaque on CT, allowing for accurate detection of even small stones without the use of IV contrast 1
• Non-contrast CT provides high spatial resolution and ability for multiplanar reformations, allowing for comprehensive evaluation of the kidneys and urinary tract 1
• Secondary signs of urolithiasis and complications such as periureteral and perinephric inflammation and ureteral dilatation can be visualized with non-contrast CT 1

Advantages of Non-Contrast CT Over Other Modalities

• Non-contrast CT allows for accurate assessment of stone size, which is important in planning urologic management 1
• CT techniques shown to improve accuracy of stone measurements include use of coronal reformations, viewing on bone window, and use of magnified views 1
• Low-dose non-contrast CT protocols have been developed to address radiation concerns while maintaining diagnostic accuracy with a pooled sensitivity of 97% and specificity of 95% 1
• Non-contrast CT can identify alternative diagnoses in patients with flank pain who do not have urolithiasis 2

Limitations of Alternative Imaging Modalities

• CT with IV contrast: The presence of enhancing renal parenchyma may obscure stones within the renal collecting system, making it usually not appropriate as a first-line test 1
• Ultrasound: While useful in some settings, US has variable performance with overall sensitivity of 24-57% for stone detection compared to CT, with decreased sensitivity for smaller stones 1
• Radiography (KUB): Limited sensitivity (29-72%) for stone detection compared to non-contrast CT, with many stones not visible on plain radiographs 1
• Intravenous urography (IVU): Lower sensitivity (75-87%) and specificity (92-94%) compared to non-contrast CT (96-100% sensitivity, 98-100% specificity) 1

Special Considerations

• In pregnant patients or children where radiation exposure is a greater concern, ultrasound may be considered as the initial imaging modality 1
• For patients with recurrent symptoms and known stone disease, low-dose CT protocols should be considered to minimize radiation exposure 1
• If there is concern for other pathology beyond urolithiasis, contrast-enhanced CT may be considered, though studies show it changes management in only 2-3% of cases 3

Pitfalls to Avoid

• Do not rely solely on the presence or absence of hydronephrosis on ultrasound to rule in or rule out urolithiasis, as this finding does not accurately predict the presence of a ureteral stone in up to 25% of patients 1
• Be aware that the sensitivity of low-dose CT for detecting stones decreases with smaller stone size (<3mm) 1
• Contrast-enhanced CT should not be used as the initial study for suspected urolithiasis as it may obscure small stones, though it can detect stones >6mm with 98% sensitivity 1