Which imaging modality best demonstrates hydronephrosis due to obstruction?

• A. Ultrasound
• B. X-ray of the abdomen
• C. MRI without contrast
• D. CT without contrast

Answer: (A)

Hydronephrosis from obstruction causes dilation of the kidney’s collecting system, seen as an expanded renal pelvis and calyces on imaging. Ultrasound is the best initial modality for this because it directly visualizes the kidneys and collecting system without radiation. It can detect dilation clearly, differentiate hydronephrosis from other cystic processes, and is quick and safe for most patients, including pregnant people and children. It’s also easily repeatable to monitor changes.

Other options have limitations for this purpose. An abdomen X-ray isn’t sensitive enough to pick up hydronephrosis reliably. CT without contrast can show hydronephrosis and help identify the obstructing cause (like stones) but involves radiation and is usually reserved if ultrasound findings are inconclusive or if a more detailed assessment is needed. MRI without contrast isn’t typically employed first-line due to longer exam times, higher cost, and less accessibility.