Residency · Residency · Nuclear Medicine

Pediatric Renal Scintigraphy: VUR and UTI Evaluation

Introduction

Renal scintigraphy plays a central role in evaluating urinary tract infections and vesicoureteral reflux (VUR) in children. Tc-99m DMSA cortical scintigraphy is the gold standard for detecting renal parenchymal damage, while direct radionuclide cystography (DRC) provides a sensitive, low-radiation method for detecting and monitoring VUR. These studies guide clinical decisions regarding antibiotic prophylaxis, surgical intervention, and long-term follow-up.

Clinical Context

Urinary Tract Infections in Children

UTIs affect approximately 8% of girls and 2% of boys by age 7. A febrile UTI raises concern for acute pyelonephritis and potential renal scarring. Risk factors include VUR, posterior urethral valves, and dysfunctional voiding. AAP guidelines recommend renal and bladder ultrasound after the first febrile UTI in children aged 2 to 24 months.

Vesicoureteral Reflux

Primary VUR results from congenital incompetence of the ureterovesical junction. The International Classification grades VUR from Grade I through Grade V based on the degree of ureteral and pelvicalyceal dilation. Spontaneous resolution occurs in many low-grade cases. Higher grades are associated with increased risk of renal scarring.

Tc-99m DMSA Cortical Scintigraphy

Mechanism and Protocol

Tc-99m dimercaptosuccinic acid (DMSA) binds to proximal tubular cells, providing a map of functioning renal cortex. Imaging is performed 2 to 4 hours post-injection during the cortical phase. Planar posterior and posterior oblique views are obtained, and SPECT improves sensitivity for detecting small cortical defects. The administered activity is 1.85 MBq/kg with a minimum of 18.5 MBq per Image Gently guidelines.

Acute Pyelonephritis

Acute pyelonephritis appears as photopenic areas without volume loss, which may be focal or diffuse. The sensitivity for acute pyelonephritis is 85 to 95%, which is superior to ultrasound. Cortical swelling may be present in severe cases. The study is best performed within the first week of a febrile UTI for acute assessment.

Renal Scarring

Renal scarring manifests as cortical defects with volume loss and contour irregularity on delayed imaging performed more than 6 months after the infection. Differential renal function is calculated from posterior images, with normal values ranging from 45 to 55% for each kidney. Scarring correlates with the risk of hypertension and chronic kidney disease over the long term.

Direct Radionuclide Cystography

Technique

Bladder catheterization is performed, and Tc-99m pertechnetate or Tc-99m sulfur colloid mixed in saline is instilled into the bladder. Continuous imaging is performed during both bladder filling and voiding phases. The total administered activity is approximately 37 MBq (1 mCi) in the instilled saline. Dynamic acquisition is obtained at 1 to 5 seconds per frame.

Advantages Over Fluoroscopic VCUG

Direct radionuclide cystography provides continuous monitoring, which yields higher sensitivity for detecting intermittent reflux. The radiation dose is 10 to 50 times lower than fluoroscopic voiding cystourethrography (VCUG). Sensitivity for VUR detection is approximately 95%. It is the preferred modality for follow-up studies and screening of siblings of affected patients.

Limitations

Direct radionuclide cystography cannot grade reflux as precisely as fluoroscopic VCUG and does not evaluate urethral anatomy, which is important for boys with suspected posterior urethral valves. For these reasons, fluoroscopic VCUG remains the initial study of choice in boys with a first UTI. The grading system for radionuclide cystography uses mild, moderate, and severe categories rather than the I through V scale used for VCUG.

Indirect Radionuclide Cystography

Indirect radionuclide cystography is performed at the end of a Tc-99m MAG3 diuretic renogram. The patient voids during imaging, and reflux is detected as activity returning to the ureters or kidneys. The advantage is that no catheterization is required, making it more acceptable for older, toilet-trained children. However, it has lower sensitivity than the direct method and requires patient cooperation for voiding on command.

Diuretic Renography in Pediatric Patients

Tc-99m MAG3 Protocol

Tc-99m mercaptoacetyltriglycine (MAG3) is preferred over DTPA in neonates because MAG3 is cleared by tubular secretion, which is more mature at birth than glomerular filtration. The well-tempered renogram protocol requires adequate hydration and bladder catheterization in infants. Furosemide is administered using either an F+20 protocol (furosemide given 20 minutes after the tracer) or an F-15 protocol (furosemide given 15 minutes before the tracer), depending on institutional preference. Drainage is assessed by the half-time (T1/2): less than 10 minutes is normal, 10 to 20 minutes is indeterminate, and greater than 20 minutes suggests obstruction.

Clinical Applications

Diuretic renography differentiates obstructive from non-obstructive hydronephrosis. It provides assessment of differential renal function. It is used for post-surgical evaluation after procedures such as pyeloplasty. It also aids in the evaluation of duplex kidneys and ectopic ureters.

Clinical Decision Pathways

For a first febrile UTI in a child aged 2 to 24 months, renal ultrasound is performed, with VCUG added if the ultrasound is abnormal. DMSA scanning is obtained for recurrent UTIs or to assess for scarring 4 to 6 months after infection. Direct radionuclide cystography is used for VUR follow-up and sibling screening. Serial DMSA scans monitor renal function and scarring progression over time.

Clinical Pearls

Tc-99m DMSA is the most sensitive imaging modality for detecting both acute pyelonephritis and renal scarring in children, outperforming ultrasound and CT.

Direct radionuclide cystography offers continuous monitoring with substantially lower radiation dose than fluoroscopic VCUG, making it ideal for follow-up and screening studies.

Fluoroscopic VCUG remains the initial study of choice in boys to evaluate urethral anatomy and exclude posterior urethral valves.

A differential renal function below 45% on DMSA warrants close monitoring and may influence decisions regarding surgical intervention for VUR.

References

1. Piepsz A, et al. "Guidelines on Tc-99m-DMSA Scintigraphy in Children." Eur J Nucl Med. 2001;28(3):BP37-41.
2. Fettich J, et al. "EANM Guidelines for Direct Radionuclide Cystography in Children." Eur J Nucl Med Mol Imaging. 2003;30(5):B39-44.
3. Mattoo TK, et al. "Renal Scarring in the RIVUR Trial." Pediatrics. 2016;137(5):e20154183.
4. Subcommittee on UTI, AAP. "Reaffirmation of AAP Clinical Practice Guideline: Diagnosis and Management of Initial UTI in Febrile Infants and Young Children 2-24 Months." Pediatrics. 2016;138(6):e20163026.

For the intern

At 3am, if a child spikes a fever within days of a urinary tract infection (UTI), draw cultures and start antibiotics—don't wait for imaging. Tc-99m dimercaptosuccinic acid (DMSA) scintigraphy for acute pyelonephritis is best performed within the first week but can wait until morning. For vesicoureteral reflux (VUR) follow-up, direct radionuclide cystography is scheduled electively. If a neonate has severe hydronephrosis and declining urine output, call urology urgently; diuretic renography helps differentiate obstruction from physiologic dilation but is never an emergency. Renal scarring assessment with DMSA is performed 4 to 6 months post-infection and always scheduled during office hours.

Common pitfalls

• Ordering fluoroscopic voiding cystourethrography (VCUG) for every follow-up study: Direct radionuclide cystography delivers 10 to 50 times less radiation and is more sensitive for detecting intermittent reflux; reserve VCUG for initial evaluation, especially in boys where urethral anatomy matters.
• Scanning too early for scarring: Acute inflammatory changes on DMSA mimic scars; wait at least 4 to 6 months after infection to accurately assess permanent parenchymal damage.
• Using Tc-99m diethylenetriaminepentaacetic acid (DTPA) in neonates: Choose Tc-99m mercaptoacetyltriglycine (MAG3) instead—tubular secretion is more mature than glomerular filtration at birth, giving better image quality.
• Forgetting hydration and bladder catheterization: The "well-tempered renogram" protocol requires intravenous hydration and catheterization in infants to ensure accurate drainage assessment and prevent false-positive obstruction.

<image>Simple flowchart showing pediatric UTI/VUR evaluation algorithm. Three main decision branches: (1) "First febrile UTI, age 2-24 months" leads to "Renal ultrasound" then "Abnormal?" splits to "Yes→VCUG" and "No→Clinical follow-up"; (2) "Acute pyelonephritis suspected" leads to "DMSA within 1 week"; (3) "VUR follow-up or sibling screening" leads to "Direct radionuclide cystography". Separate box: "Assess scarring: DMSA at 4-6 months post-infection". Use color-coding: acute pathway (red), follow-up pathway (blue), screening pathway (green).</image>

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