Renal Scintigraphy⁹⁹ᵐTc-MAG3 · ⁹⁹ᵐTc-DMSA · ⁹⁹ᵐTc-DTPA

Quick Answer

Renal scintigraphy measures function, not anatomy. Dynamic studies (Tc-99m-MAG3, tubular; or DTPA, filtered) show perfusion, function, and drainage and quantify split (differential) function; adding furosemide distinguishes obstructive from non-obstructive dilation (diuretic renography). Cortical imaging (Tc-99m-DMSA) maps functioning cortex for scarring and differential function. Captopril renography screens for renovascular hypertension. Getting the physiology right — hydration and bladder status — is what makes the numbers valid.

Definition & Role

Renal scintigraphy tracks a radiotracer through the kidney to answer functional questions cross-sectional imaging cannot: how much each kidney contributes, whether a dilated system is truly obstructed, where cortical scars are, and whether a stenosis is functionally significant. It complements — it does not replace — the anatomic detail of ultrasound, CT, and MRI.

Clinical Indications

• Differential (split) function before nephrectomy, donation, or pyeloplasty.
• Suspected obstruction in a dilated collecting system (diuretic renography).
• Cortical scarring / acute pyelonephritis (DMSA), especially in children with UTI/reflux.
• Renovascular hypertension screening (captopril renography).
• Transplant dysfunction work-up; measured GFR (DTPA/EDTA).

Agents and What They Show

The Dynamic Renogram — Phases & Curve

A dynamic (MAG3) study yields a time–activity (renogram) curve with three phases:

1. Vascular/perfusion (first ~30 s) — bolus arrival, compared between kidneys.
2. Cortical/functional (uptake) — parenchymal accumulation; peaks at Tmax (~3–5 min normally); the basis of split function (measured in the 1–2.5 min uptake window).
3. Excretory/drainage — washout; drainage half-time (T½) and response to furosemide.

Split function = each kidney's share of parenchymal uptake (normal ~50:50). Curve patterns (normal, obstructive-rising, non-obstructive dilated, poorly-functioning-flat) integrate with the numbers.

Interactive: Renogram Explorer — drag either kidney's curve to see the phases, washout T½, output efficiency, NORA, split function, diuretic response, and O'Reilly pattern decoded live.

Diuretic Renography (the Obstruction Question)

Furosemide tests whether a dilated collecting system washes out (non-obstructed) or retains tracer (obstructed). Validity depends on:

• Adequate hydration — dehydration mimics obstruction.
• Bladder emptying / catheterization — a full bladder produces false retention (and vesicoureteral reflux/back-pressure).
• Stated protocol — F+20 (furosemide 20 min after tracer), F−15 (15 min before), or F0 (with tracer); timing changes interpretation.
• A very poorly functioning kidney may not respond to diuretic — report as indeterminate, not obstructed.

Cortical Imaging (DMSA)

DMSA binds functioning proximal-tubule cortex, mapping parenchyma with high resolution:

• Acute pyelonephritis: focal cortical defect(s) without volume loss.
• Established scar: cortical defect with volume loss/contour deformity.
• Pseudotumor (prominent column of Bertin, dromedary hump): normal cortical uptake confirms it is normal tissue.
• Provides differential function when drainage is not the question.

Special Studies

• Captopril renography: ACE inhibition unmasks the angiotensin-II-dependent GFR of a hemodynamically significant renal-artery stenosis (post-captopril deterioration vs baseline); best with preserved renal function.
• Transplant renography (MAG3): sorts ATN (preserved perfusion, poor excretion, early/self-limited), rejection (reduced perfusion + function), obstruction, and urine leak (see that page).

Reporting Checklist

• Report split function, curve pattern, Tmax, and drainage T½, with the protocol/agent used and hydration/bladder status.
• State whether a dilated system is obstructed, non-obstructed, or indeterminate (poor function).
• For DMSA, distinguish acute pyelonephritis (no volume loss) from scar (volume loss) and confirm pseudotumors.
• Note interfering factors (dehydration, full bladder, medications).

Common Pitfalls

• Calling obstruction without controlling hydration and bladder status (the two commonest causes of a falsely obstructive study).
• Misreading a poorly functioning kidney's flat diuretic response as obstruction (it is indeterminate).
• Using DTPA in poor renal function, where MAG3's higher extraction gives far better images.
• Ignoring the stated diuretic protocol when interpreting washout.

Board Pearls

Renal scintigraphy answers functional questions — split function (which kidney and how much), obstruction (diuretic washout), and cortical scarring (DMSA). Match the agent to the question: MAG3 (tubular) for dynamic function/drainage, DTPA (filtered) for GFR, DMSA (cortical) for scarring/differential function.

A diuretic renogram distinguishes an obstructed system (retains tracer despite furosemide) from a dilated non-obstructed one (washes out) — but only if hydration and bladder emptying are controlled, since dehydration or a full bladder mimic obstruction; a poorly functioning kidney that does not respond is indeterminate.

Captopril renography screens for functionally significant renovascular hypertension (best with preserved function). DMSA separates acute pyelonephritis (defect, no volume loss) from scar (defect + volume loss) and confirms a column-of-Bertin pseudotumor as normal cortex. Always report split function, curve pattern, and drainage half-time together with the protocol used.

Related Pages

• Tracer: MAG3; calculator: renal split-function tool.
• Related: Captopril renography, renal transplant evaluation, GFR measurement, DMSA cortical, radionuclide cystography.

Figure / Diagram Suggestions

• A three-phase renogram curve annotated (vascular / uptake / excretory; Tmax, T½).
• Obstructed vs non-obstructed vs indeterminate diuretic washout curves.
• A DMSA cortical plate (pyelonephritis vs scar vs pseudotumor).

Self-Check (Board-Style)