Residency · Residency · Nuclear Medicine
Dynamic Renal Scintigraphy: Tc-99m MAG3 and Tc-99m DTPA
Overview
Dynamic renal scintigraphy evaluates renal perfusion, function, and drainage in a single study. Two primary agents with different renal handling are available: MAG3, which is cleared by tubular secretion, and DTPA, which is cleared by glomerular filtration. Key clinical applications include evaluation of obstructive uropathy, assessment of differential renal function, and transplant evaluation.
Radiopharmaceuticals
Tc-99m MAG3 (Mercaptoacetyltriglycine)
MAG3 is cleared by active tubular secretion in the proximal tubule, with an extraction efficiency of approximately 60% per pass. It is the preferred agent for most clinical applications, particularly in patients with impaired renal function, because its high first-pass extraction produces superior image quality with a high renal-to-background ratio. MAG3 is protein-bound and is not a pure GFR agent. Effective renal plasma flow can be estimated from MAG3 clearance.
Tc-99m DTPA (Diethylenetriaminepentaacetic acid)
DTPA is cleared exclusively by glomerular filtration, with no tubular secretion or reabsorption. Its extraction efficiency is approximately 20% per pass, which is lower than MAG3. DTPA can directly measure GFR from plasma clearance or camera-based methods. Image quality is inferior to MAG3, especially when renal function is impaired. DTPA is most useful when GFR measurement is specifically required.
<image>Comparison of dynamic renal scintigraphy images with Tc-99m MAG3 (superior renal-to-background ratio) and Tc-99m DTPA (more background activity) in a patient with moderate renal impairment</image>
Renogram Phases
Phase 1: Vascular (Perfusion) Phase (0-60 seconds)
Rapid sequential images at 1 to 3 second frames are acquired during bolus transit. The aortic peak is followed by a renal peak within 5 to 6 seconds. Delayed or diminished renal perfusion suggests renal artery stenosis, thrombosis, or severe parenchymal disease. Side-to-side comparison assesses symmetry.
Phase 2: Cortical (Parenchymal/Functional) Phase (1-5 minutes)
This phase shows progressive accumulation of tracer in the renal parenchyma. The time to peak activity, or Tmax, is normally 3 to 5 minutes for MAG3. It reflects tubular extraction with MAG3 or glomerular filtration with DTPA. A prolonged Tmax suggests impaired renal function or obstruction.
Phase 3: Excretory (Drainage) Phase (5-30+ minutes)
This phase demonstrates progressive washout of tracer into the collecting system, ureters, and bladder. Normal drainage produces a rapid decline after the peak, with a half-time below 10 to 15 minutes. Delayed washout with a half-time exceeding 20 minutes suggests obstruction or a dilated non-obstructed system. An intermediate half-time of 15 to 20 minutes is equivocal and may require a diuretic challenge.
Differential Renal Function
Differential renal function is calculated from counts over each kidney during the cortical phase at 1 to 3 minutes. Background subtraction using a perirenal background region of interest is essential. Normally, each kidney contributes approximately 45 to 55% of total function. Differential function below 40% is considered significantly reduced. This method is more reliable than GFR-based approaches for comparing the two kidneys.
Diuretic Renography
Purpose
Diuretic renography distinguishes true obstruction from dilated but non-obstructed collecting systems, such as those seen with vesicoureteral reflux or non-obstructive hydronephrosis.
Protocols
The F+20 protocol administers furosemide 20 minutes after tracer injection, after the collecting system has filled. This is the most commonly used and best-validated protocol. The F0 protocol gives furosemide at the time of tracer injection for a shorter study duration, but may produce false-positive obstruction if the diuretic does not act quickly enough. The F-15 protocol, also called the "well-tempered renogram," administers furosemide 15 minutes before tracer injection, establishing diuresis before tracer administration. This approach reduces false-positive results from a dilated but non-obstructed system and is increasingly preferred, especially in pediatric patients.
Furosemide Dose
The adult dose is 40 mg IV (0.5 mg/kg). The pediatric dose is 1 mg/kg IV with a maximum of 40 mg. Adequate hydration with 10 to 15 mL/kg of oral or IV fluids before the study is essential.
Interpretation of Diuretic Response
A half-time below 10 minutes indicates a non-obstructed system with good diuretic response. A half-time between 10 and 20 minutes is equivocal and may reflect obstruction or a dilated non-obstructed system. A half-time exceeding 20 minutes indicates obstruction with poor diuretic response. A rising curve after furosemide administration strongly suggests obstruction, while prompt washout excludes significant obstruction.
| Parameter | MAG3 | DTPA |
|---|---|---|
| Clearance mechanism | Tubular secretion | Glomerular filtration |
| Extraction efficiency | ~60% per pass | ~20% per pass |
| Image quality (impaired function) | Superior | Inferior |
| GFR measurement | No (measures ERPF) | Yes (direct) |
| Preferred agent | Most clinical indications | When GFR quantification needed |
| Diuretic T½ | Interpretation | Clinical Significance |
|---|---|---|
| <10 min | Non-obstructed | Good drainage response |
| 10–20 min | Equivocal | May be dilated non-obstructed or partially obstructed |
| >20 min | Obstructed | Poor drainage; likely significant obstruction |
| Rising curve | Strongly obstructed | High-grade obstruction |
<image>Diuretic renography (F+20 protocol) showing a dilated right collecting system with a T1/2 of 35 minutes (obstructed pattern) and a normal left kidney with prompt washout (T1/2 = 5 minutes)</image>
Renogram Curve Patterns
Normal Renogram
A normal renogram shows a rapid upstroke during the vascular phase, a peak at 3 to 5 minutes during the cortical phase, and a smooth descent during the excretory phase. Curves are symmetric bilaterally.
Obstructed Pattern
An obstructed pattern produces a rising or plateau curve that does not decline spontaneously and fails to wash out after furosemide, with a half-time exceeding 20 minutes. Causes include ureteropelvic junction obstruction, ureterovesical junction obstruction, ureteral stones, and extrinsic compression.
Dilated Non-Obstructed Pattern
A dilated non-obstructed pattern shows delayed drainage that improves significantly after furosemide, with a half-time below 10 minutes post-diuretic. The large capacitance system fills slowly but drains when challenged.
Cortical Retention (Parenchymal Dysfunction)
This pattern demonstrates slow uptake, prolonged Tmax, and poor excretion. The curve never achieves a sharp peak. When bilateral, it suggests medical renal disease. When unilateral, it suggests renal artery stenosis or chronic obstruction.
Clinical Applications
Ureteropelvic Junction (UPJ) Obstruction
UPJ obstruction is the most common indication for diuretic renography in children and young adults. The study differentiates obstructive from non-obstructive hydronephrosis. Serial studies monitor for worsening obstruction or improved drainage after pyeloplasty.
Post-Operative Assessment
After pyeloplasty, improvement in drainage is demonstrated by a decreasing half-time along with stable or improved differential function. After partial nephrectomy, the study assesses residual function in the operated kidney.
Ureteral Obstruction
Diuretic renography quantifies the degree of functional obstruction from stones, strictures, or extrinsic compression by tumor or retroperitoneal fibrosis.
<image>Serial diuretic renography in a child with UPJ obstruction showing pre-pyeloplasty obstructed pattern (T1/2 >30 minutes) and post-pyeloplasty improvement (T1/2 = 8 minutes) with preserved differential function</image>
Technical Pitfalls
Dehydration reduces urine flow and can mimic obstruction; adequate hydration must always be ensured. A full bladder creates back-pressure on the ureters that can delay drainage; the patient should void before imaging or a catheter should be placed. In neonates under 1 month, immature tubular function produces delayed drainage that may be normal; waiting until 4 to 6 weeks of age yields more reliable results. Poor renal function may produce insufficient tracer excretion to assess drainage, and furosemide may be ineffective if the GFR is severely reduced. Supine positioning is standard, though upright positioning may be used for equivocal supine results since gravity assists drainage. Incorrect placement of background regions of interest leads to inaccurate differential function calculations.
Clinical Pearls
MAG3 is preferred over DTPA for nearly all clinical indications due to its superior image quality.
Adequate hydration and an empty bladder must always be ensured before the study.
The F-15 protocol reduces false-positive obstructive results in patients with dilated collecting systems.
Differential renal function below 40% in a child with UPJ obstruction is often an indication for surgical repair.
Serial studies are more valuable than a single study for following patients with borderline obstruction.
In neonates, the renogram should be delayed until 4 to 6 weeks of age to avoid false-positive obstruction from immature kidneys.
A rising renogram curve after furosemide is the most specific pattern for true obstruction.
References
- SNMMI/EANM Practice Guideline for Diuretic Renography.
- Taylor, A. T. "Radionuclides in Nephrourology: Part 1." Journal of Nuclear Medicine, 2014.
- Gordon, I., et al. "Consensus Guidelines for Diuretic Renography in Children." European Journal of Nuclear Medicine, 2011.
- O'Reilly, P. H. "Diuresis Renography: Recent Advances and Recommended Protocols." BJU International, 2003.
