For initial evaluation of suspected renal stones, which imaging modality is commonly used due to high sensitivity?

• A. Ultrasound
• B. CT
• C. X-ray
• D. MRI

Answer: (B)

The key idea is choosing an imaging test that detects stones reliably across different types and locations to promptly confirm the diagnosis and assess obstruction. Noncontrast CT of the abdomen and pelvis has the highest sensitivity and specificity for renal stones, capable of detecting stones of all compositions (calcium-containing, uric acid, etc.), determining exact size and location, and revealing signs of obstruction or hydronephrosis. This makes it the most reliable initial test in suspected renal stones and it also helps rule out other causes of acute flank pain.

Ultrasound is valuable when radiation should be avoided (such as in pregnancy or in children) and can show hydronephrosis, but its sensitivity for detecting stones—especially in the distal ureter—is lower. A plain X-ray can miss radiolucent stones and small stones, and MRI does not visualize calcifications well, so it isn’t used for acute stone evaluation.