If contrast administration is contraindicated, which CT protocol is preferred to evaluate urinary stones?

• A. Non-contrast CT
• B. Ultrasound
• C. MRI with contrast
• D. CT urography

Answer: (A)

When evaluating suspected urinary stones, you want an imaging method that shows calcifications clearly without introducing anything that could mask or complicate interpretation. Stones appear as dense, bright structures on CT, and non-contrast images maximize their visibility while also allowing quick assessment of any resulting obstruction or hydronephrosis. Injecting contrast can blur or obscure stones and is unnecessary for detecting stones, so if contrast is contraindicated you skip contrast-enhanced protocols.

CT urography relies on IV contrast to evaluate the entire urinary tract, which is exactly what you want when contrast can be used; but in this scenario it’s not, so this approach isn’t suitable. Ultrasound is useful for detecting hydronephrosis and can sometimes reveal stones, but it’s less sensitive for stones, especially in the distal ureter and in obese patients, and it’s operator dependent. MRI with contrast isn’t advantageous for stone detection and is impractical for rapid stone evaluation.

Therefore, the best option when contrast administration is not possible is non-contrast CT, because it offers the highest accuracy for identifying stones and assessing obstruction without the risks or limitations of contrast.