A 30-year-old woman with bilateral hydronephrosis undergoes a Tc-99m MAG3 diuretic renogram. Tracer washout from the left pelvis is slow at baseline, but after intravenous furosemide there is rapid washout of more than half the retained activity. This pattern indicates:

• A Non-obstructive dilatation of the collecting system ✓
• B Fixed mechanical obstruction at the pelviureteric junction
• C Non-functioning left kidney
• D Acute tubular necrosis

Explanation

The diuretic renogram separates true obstruction from simple dilatation. In unobstructed dilated systems, furosemide-induced high urine flow washes retained tracer out rapidly, conventionally defined as excretion of more than half the pelvic activity within about 20 minutes. In mechanical obstruction, increased flow cannot clear the pelvis and washout remains poor. Non-functioning kidney would show minimal tracer uptake at all, before the diuretic phase matters.

Reference: Grainger & Allison's Diagnostic Radiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.