Suspected upper urinary tract obstruction

Hydronephrosis seen on ultrasound or CT does not, by itself, distinguish a genuinely obstructed collecting system that will lose function without intervention from a dilated but freely draining one that needs no treatment. Diuretic renal scintigraphy answers that functional question: a tubular radiopharmaceutical, almost always Tc-99m-mercaptoacetyltriglycine (MAG3), is injected intravenously and its transit through the kidney and drainage into the bladder is followed with a gamma camera, with intravenous furosemide given partway through the study to washout tracer from a non-obstructed system.

The joint SNMMI/EANM procedure standard specifies a well-hydrated patient, an empty bladder at the start, and either a combined protocol (furosemide at a fixed time, typically 20 minutes, after tracer injection) or a two-stage protocol in which a first 20-30 minute baseline acquisition can exclude obstruction outright if drainage is already brisk, avoiding the need for furosemide at all. MAG3 is preferred over the older glomerular agent Tc-99m-DTPA because its extraction fraction (40-50%) is more than double DTPA's, giving better image quality and more reliable washout curves, especially in kidneys with reduced function. A postvoid image is mandatory whenever obstruction is suspected, since gravity-dependent pooling in an upright, empty-bladder position can otherwise be misread as true retention.

Interpretation combines the differential renal function (the relative contribution of each kidney to overall clearance, calculated from early parenchymal counts) with the washout half-time after furosemide: practical cut-offs commonly used are a half-time under about 10 minutes suggesting no obstruction and over about 20 minutes suggesting true obstruction, with the intermediate range considered equivocal and needing clinical correlation. The test is used both to confirm or exclude obstruction in a symptomatic patient and to establish a functional baseline before deciding on intervention, or before monitoring a conservatively managed kidney over time.