What imaging modality is recommended to determine function in a pediatric patient with a double collecting system?

Imaging for Functional Assessment in Pediatric Double Collecting System

Order a Tc-99m MAG3 renal scan to determine differential renal function in a pediatric patient with a double collecting system. This is the gold standard functional imaging modality recommended by the American College of Radiology and American Academy of Pediatrics for assessing split renal function and drainage in complex urinary tract anatomy 1.

Primary Imaging Recommendation

Tc-99m MAG3 renal scintigraphy is the preferred modality for functional assessment in double collecting systems because 1:

• Provides split renal function (differential renal function) for each kidney and can differentiate function between moieties of a duplicated system 1
• Superior extraction fraction (40-50%) compared to DTPA (20%), resulting in better image quality with less background activity 1
• Optimal for impaired renal function due to primary tubular excretion, making it superior to DTPA in patients with suspected obstruction or compromised kidneys 1, 2
• Assesses drainage patterns with diuretic administration (furosemide 1.0 mg/kg, max 40 mg) to differentiate obstructed from non-obstructed dilated systems 1, 3

Technical Considerations for MAG3 Scan

The study requires specific preparation 1:

• Adequate hydration via oral or intravenous routes before the study 1
• Bladder catheterization to eliminate confounding effects from bladder distension and high-grade vesicoureteral reflux 1
• Timing: Delay until at least 2 months of age when possible due to lower neonatal glomerular filtration rates, though MAG3 performs better than DTPA in neonates 1
• Low radiation exposure: >95% of radiotracer clears within 3 hours in patients with normal function 1

Key Functional Parameters Measured

MAG3 scintigraphy provides two critical measurements 1:

1. Parenchymal uptake (split renal function): Reflects the relative contribution of each kidney (or moiety) to overall renal function 1
2. Drainage assessment: T1/2 washout time after diuretic administration differentiates true obstruction (T1/2 >20 minutes) from non-obstructive dilation 1, 2

Indications for Surgical Intervention Based on MAG3

Surgery is warranted when MAG3 demonstrates 1, 2, 4:

• Differential renal function <40% in the affected kidney or moiety 1, 2, 4
• Deteriorating function >5% change on consecutive scans 1, 2, 4
• T1/2 >20 minutes indicating true obstruction 1, 2, 4
• Worsening drainage on serial imaging 1, 2, 4

Alternative and Complementary Imaging

MR Urography (MRU)

MRU can be considered as an alternative or complementary study in select cases 1:

• Advantages: Provides superior anatomical detail of duplicated collecting systems, no radiation exposure, combines morphologic and functional information in one examination 1, 5
• Limitations: Underestimates split renal function in severely diminished kidneys (mean difference 6.6%) and severely hydronephrotic kidneys compared to MAG3 1, 5
• Requires sedation/anesthesia in young children, higher cost, and limited availability 1
• Best role: Presurgical planning for complex duplicated systems or when detailed anatomical information is needed beyond functional assessment 1, 5

Ultrasound

Ultrasound is essential for initial anatomical assessment but cannot determine function 1, 2:

• Identifies the presence of duplication and degree of dilation 1
• Cannot assess split renal function or differentiate obstruction from non-obstructive dilation 1
• Should be performed after 48 hours of life to avoid underestimation due to neonatal third spacing 1

VCUG Considerations

Voiding cystourethrography should be performed if vesicoureteral reflux is suspected 1:

• Duplicated collecting systems can have reflux into the lower pole moiety 1
• VUR occurs in 15% of children with prenatally diagnosed urinary tract dilation 1
• Helps determine if reflux is primary or secondary to duplication anomaly 1

Common Pitfalls to Avoid

• Do not rely on ultrasound alone for functional assessment—it cannot determine split renal function or obstruction 1, 2
• Avoid DTPA in favor of MAG3 in pediatric patients, especially with suspected obstruction or impaired function 1
• Do not perform MAG3 in first 48 hours of life when possible, as physiologic dehydration may affect results 1
• Ensure bladder catheterization during MAG3 to prevent false-positive obstruction from bladder distension 1
• Recognize MRU limitations in severely compromised kidneys where it systematically underestimates function compared to MAG3 1, 5