Ultrasound examination (ultrasound)
Ultrasound is the optimal first-line method for detecting echogenic foci, assessing renal size, and identifying pelvicalyceal system (PCS) or ureteral dilatation.
Features of stone visualization:
- Acoustic shadowing: in pediatrics, this sign should not be relied upon. Stones may be too small (less than 4 mm) or contain insufficient calcium to form a clear shadow.
- Twinkling artifact (twinkling artefact): color Doppler mapping (CDMs) is critically important. This artifact significantly increases the physician's confidence in confirming the diagnosis.
- Localization: calculi in the pelviureteric and vesicoureteric junctions are clearly visible provided the patient is well-hydrated and has a full bladder. Stones in the middle third of the ureter are missed in 10–20% of cases, though their presence can be suspected due to proximal dilatation.
Important nuance: in acute urinary tract obstruction, PCS dilatation is significantly less pronounced than in long-standing chronic obstruction. It should also be remembered that stones can form within anomalous structures, such as ureteroceles or caliceal diverticula.
Imaging algorithm
According to the European Society of Paediatric Radiology guidelines, the baseline method is always B-mode ultrasound supplemented by Doppler evaluation. Further management depends on the findings:
- Ultrasound shows no pathology, clinical suspicion is low: diagnostic workup is stopped. Active surveillance (watchful waiting) is recommended.
- Ultrasound is positive: dynamic monitoring during treatment is scheduled. Plain radiography (KUB) may be required to prepare for interventions such as lithotripsy.
- Ultrasound shows only indirect signs (PCS dilatation) OR is negative with a high clinical probability of stones: proceed to CT urography.
Specifics of X-ray and CT imaging:
- Plain abdominal radiography (KUB): necessary for precise topographic localization prior to surgery. Most pediatric stones are radiopaque.
- Excretory urography (EU): performed as a limited series of views, rarely used, mainly when ultrasound results are equivocal and computed tomography is unavailable.
- CT urography: must be performed strictly using a low-dose protocol without intravenous contrast. Contrast enhancement or MR urography are reserved for complex differential diagnosis cases.
Nephrocalcinosis and hyperechogenic medullary pyramid syndrome
Increased echogenicity of the renal medulla in children is a multifactorial symptom requiring a thorough medical history. On ultrasound, it may present as concentric medullary nephrocalcinosis.
Main causes of hyperechogenicity:
- True nephrocalcinosis: associated with primary hyperoxaluria, xanthinuria, or cystinuria.
- Iatrogenic factors: treatment of hypophosphatemic rickets, as well as frequent use of furosemide in premature infants with bronchopulmonary dysplasia.
- Non-iatrogenic conditions: absorptive hypercalciuria, idiopathic hypercalcemia (e.g., in Williams syndrome).
- Protein and urate deposits: protein casts can cause transient hyperechogenicity in newborns; urate deposits are typical of metabolic disorders (Lesch-Nyhan syndrome).
- Other causes: vascular congestion in sickle cell anemia, congenital infections (CMV, candidiasis), tubulopathies, and autosomal recessive polycystic kidney disease.
Differential diagnosis: nephrogenic rests
Occasionally, small echogenic foci in the parenchyma may not be stones or calcifications, but nephrogenic rests (Nephrogenic rests, NR). These are abnormally persistent foci of embryonic renal tissue.
They occur in overgrowth syndromes (Beckwith-Wiedemann syndrome) and are considered precursors to Wilms tumor. On ultrasound, diffuse nephroblastomatosis may appear as a thickened hypoechoic rim around the kidney, while individual small foci mimic normal cortex. MRI is essential for accurate assessment of their extent, demonstrating low signal intensity and lack of contrast enhancement compared to healthy tissue.
Briefly: answers to topical questions
What congenital urinary tract anomalies predispose children to calculus formation?
Calculus formation in children is promoted by developmental anomalies associated with impaired urinary drainage. Such conditions include:
- Ureterocele — stones can form within it.
- Caliceal diverticula — contribute to urinary stasis and stone formation.
- Horseshoe kidney — a congenital anomaly that also impairs adequate drainage and increases the risk of urolithiasis.
What types of urinary stones by chemical composition are most common in pediatric practice?
Calcium-containing calculi are the most common in pediatric practice, accounting for about 90% of all stones in children. Overall, urinary stones are classified by chemical composition into the following types:
- Oxalates — composed of calcium oxalate.
- Phosphates — include calcium phosphate.
- Calcium carbonate stones.
- Urates — composed of uric acid and its salts, formed in acidic urine.
- Cystine stones — also form in an acidic environment.
- Xanthine.
- Struvite stones (triple phosphates) — form in alkaline urine and often have an infectious origin.
In children, an acoustic shadow from a stone is a welcome ultrasound bonus rather than a mandatory diagnostic criterion.
Class and exam questions
Why might ultrasound fail to show PCS dilatation in the presence of obvious renal colic?
In acute urinary tract obstruction, dilatation of the renal collecting system is significantly less pronounced and may be delayed compared to chronic obstruction.
In which part of the ureter are stones most frequently invisible on ultrasound?
Stones in the middle third of the ureter are missed in 10–20% of cases due to overlapping bowel loops, but their presence can be inferred from proximal obstruction.
Can nephrocalcinosis in a newborn be diagnosed based on hyperechoic pyramids?
The diagnosis requires confirmation, as in newborns, increased medullary echogenicity can be entirely transient due to the accumulation of proteinaceous deposits.
How is uro-CT performed in children with suspected urolithiasis?
The scan must be performed using an ultra-low-dose protocol without intravenous contrast administration.
What else to study on this topic
- Complications of obstructive uropathy: urinoma formation and renal rupture.
- Specifics of stone localization: ureterocele and caliceal diverticula.
- The link between nephrogenic rests, hemihypertrophy, and Wilms tumor.
- MR urography in the differential diagnosis of renal colic.
- The role of contrast-enhanced ultrasound (CEUS) in pediatric uronephrology.
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Comprehensive study guide on "Urolithiasis and Nephrocalcinosis in Children" and 2,199 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other Topics in "Pediatric Diagnostic Radiology"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.