Tc-99m-MAG3⁹⁹ᵐTc-MAG3

Quick Answer

Tc-99m-MAG3 (mercaptoacetyltriglycine) is cleared predominantly by renal tubular secretion with high single-pass extraction, giving excellent dynamic images of renal perfusion, function, and drainage even when function is reduced. It is the workhorse for split (differential) renal function and for diuretic (Lasix) renography to separate obstructive from non-obstructive dilation. Two facts govern its use: MAG3 beats DTPA in poor function (tubular extraction > glomerular filtration for target-to-background), and a diuretic study is only valid with adequate hydration and an empty bladder.

Mechanism

MAG3 is protein-bound in plasma and cleared almost entirely by proximal-tubular secretion (via the organic-anion transporter), with minimal glomerular filtration. Its high extraction fraction (~50–60%, though its absolute clearance is lower than the true tubular agent OIH) means a large fraction is removed on each pass, producing high renal-to-background contrast and reliable time–activity (renogram) curves — the decisive advantage over the glomerularly-filtered DTPA when renal function is poor. Rapid cortical transit into the collecting system generates the three renogram phases: perfusion, uptake/function (peak ~3–5 min), and excretion/drainage.

Biodistribution

Kidneys and collecting system, then bladder; low soft-tissue background owing to efficient extraction and plasma protein binding. Hepatobiliary activity is minor and increases with impaired renal function or free pertechnetate/impurity.

Physics

Clinical Indications

• Split (differential) renal function — % contribution of each kidney (before nephrectomy, donor work-up, etc.).
• Diuretic renography for suspected obstruction (UPJ obstruction, hydronephrosis).
• Renal transplant evaluation (perfusion, function, urine leak/obstruction).
• Captopril (ACE-inhibitor) renography for renovascular hypertension (MAG3 physiology reused).

Preparation & Protocol

• Ensure good hydration (oral or IV) — the single most important validity factor for diuretic studies.
• Dynamic acquisition immediately post-injection (posterior, supine; transplant imaged anteriorly).
• Furosemide (~0.5 mg/kg) timing defines the protocol: F+20 (20 min after tracer), F−15 (15 min before), or F0 (simultaneous).
• Empty/catheterize the bladder where a full bladder could back-pressure the system (especially children).
• Record background/depth correction method for split-function accuracy.

Interpretation Highlights

• Renogram curve phases: perfusion → uptake (peak, reflecting function) → excretion (drainage).
• Split function from relative uptake in the 1–2.5 min window (background-corrected).
• Post-diuretic washout half-time (T½): < 10 min = non-obstructed, > 20 min = obstructed, 10–20 min = indeterminate (values are guides, not absolutes).
• A very poorly functioning kidney may not respond to diuretic — read as indeterminate, not obstructed.

Reporting Checklist

• Report perfusion, split (differential) function (%), the renogram curve pattern, and the drainage half-time after diuretic — with the protocol (F+20/F−15/F0) stated.
• Note hydration/bladder status and any catheterization.
• Flag a non-responding poorly functioning kidney as indeterminate.

Common Pitfalls

• Dehydration or a full bladder producing falsely "obstructive" washout curves.
• Poor renal function blunting the diuretic response → indeterminate, not obstructive.
• Furosemide timing (F+20 vs F−15 vs F0) changing the washout — always state the protocol.
• Reservoir effect of a capacious collecting system slowing washout despite no true obstruction.

Board Pearls

MAG3 is cleared by renal tubular secretion with high single-pass extraction, so it gives good images and reliable curves even when renal function is reduced — where glomerularly-filtered DTPA yields poor target-to-background. That is why MAG3 is the default dynamic-renography agent for split function, obstruction, transplant, and captopril studies.

A diuretic (furosemide) renogram is only interpretable with adequate hydration and an empty bladder — both dehydration and a full bladder mimic obstruction. Washout T½ < 10 min = non-obstructed, > 20 min = obstructed, 10–20 min = indeterminate; a very poorly functioning kidney may not respond and should be read as indeterminate, not obstructed.

Always state the protocol (F+20 / F−15 / F0) because furosemide timing changes the washout curve. Split function is measured in the early 1–2.5 min uptake window (background/depth-corrected); a capacious system can show a reservoir effect (delayed washout without true obstruction). The same tubular physiology underlies the transplant and captopril-renography applications.

Related Pages

• Protocol: Renal transplant imaging, Captopril (ACE-inhibitor) renography, GFR measurement.
• Contrast agent: DTPA (glomerular; GFR).

Figure / Diagram Suggestions

• A three-phase renogram curve (perfusion / uptake / excretion) with obstructed vs non-obstructed washout overlay.
• A tubular-secretion vs glomerular-filtration schematic (MAG3 vs DTPA) explaining poor-function superiority.
• A diuretic-timing diagram (F+20 / F−15 / F0).

Self-Check