Initial imaging algorithm
Starting imaging requires a confirmed UTI: leukocyturia, positive nitrites, culture growth ($10^4$ via catheter or $10^6$ in midstream), and blood inflammatory markers.
The method of choice is ultrasound with amplitude color Doppler sonography (aCDS). Further steps depend on its results:
- Normal findings and clinical cystitis: imaging is discontinued, the child is monitored.
- Normal findings and upper tract UTI: acute DMSA scintigraphy or acute renal MRI is performed. If normal, further monitoring.
- Pyohydronephrosis: if there is no response to antibiotics, percutaneous nephrostomy is indicated.
- Acute pyelonephritis: follow-up ultrasound is ordered, reflux is investigated, and delayed scintigraphy is planned.
Alternative approach: if the anatomy of the tracts is initially known to be normal, acute-phase imaging can be skipped, prescribing only a delayed study to look for scars.
Diagnosis of vesicoureteral reflux (VUR)
Screening for VUR is mandatory in all infants, most children under 2 years of age, and children over 5 years of age with recurrent UTIs. Standard micturition cystourethrography is performed in boys, and contrast-enhanced voiding urosonography (ce-VUS) in girls. The ce-VUS method uses a low mechanical index, allowing exceptionally clear visualization of contrast in the collecting system.
VUR grades (per American Academy of Pediatrics):
- Grade I: no upper tract dilation.
- Grade II: mild dilation, minimal blunting of the fornices.
- Grade III: moderate dilation of the renal pelvis and ureter.
- Grade IV: tortuous ureter, blunted forniceal angles with preserved papillary impressions.
- Grade V: gross dilation and tortuosity; papillary impressions are not visualized.
Complicated and atypical forms
Hectic fever and poor response to therapy may indicate a renal abscess. Its intermediate stage, characterized by focal bacterial inflammation without purulent liquefaction yet, is called lobar nephronia (not to be confused with a tumor). In immunocompromised patients, abscesses are frequently caused by fungal flora (Candida), forming "fungal balls" on ultrasound.
An extremely severe form is xanthogranulomatous pyelonephritis (XPN). This is an abnormal chronic reaction to infection (usually Proteus or E. coli) in the presence of a calculus.
- Ultrasound: hypoechoic enlarged kidney with debris.
- MRI (preferred): hyperintense necroses on T2-weighted images.
- Scintigraphy: completely non-functioning kidney.
The outcome of the disease is nephrectomy (often preceded by embolization to reduce blood loss).
Cystic dysplasia and differential diagnosis
Congenital cystic anomalies can mimic the consequences of severe UTIs or serve as a background for them. An early sign of ciliopathies on antenatal ultrasound is often solely an inappropriately large kidney size.
Diseases are divided into genetic (autosomal dominant and recessive polycystic kidney disease, tuberous sclerosis, nephronophthisis) and non-hereditary (simple cysts, multicystic dysplastic kidney).
It is important to understand the term "dysplasia". Strictly speaking, it is a histological diagnosis meaning the presence of primitive embryonic tissues. However, in diagnostic imaging, clinicians use this term to refer to a small hyperechoic kidney with effaced corticomedullary differentiation. Such secondary dysplasia frequently develops secondary to obstruction (e.g., in posterior urethral valves) or high-grade VUR.
Briefly: answers to topical questions
What neoplasms and pathologies must xanthogranulomatous pyelonephritis be differentiated from in children?
Xanthogranulomatous pyelonephritis in children must be differentiated primarily from Wilms tumor (Wilms tumour): the specific imaging appearance of XPN allows avoiding an erroneous preoperative diagnosis of a Wilms tumor.
Tuberculosis and renal abscess are also considered within complicated urinary tract infections, but sources explicitly point to them as indications for the differential diagnosis of complicated UTI, rather than as specific alternatives to XPN. Complicated urolithiasis is an indication for unenhanced CT.
What direct ultrasound signs are characteristic of acute pyelonephritis in grayscale mode and with color Doppler sonography?
Direct ultrasound signs of acute pyelonephritis include changes in kidney size, echogenicity, and vascularization.
Grayscale mode reveals:
- renal enlargement: length greater than 15 cm or an increase in length of more than 1.5 cm compared to the healthy kidney;
- parenchymal hypoechogenicity in edema or hyperechogenicity in hemorrhage;
- loss of corticomedullary differentiation;
- blunting of renal sinus fat; increased echogenicity of the renal sinus is possible in children.
Power Doppler imaging reveals zones of parenchymal hypoperfusion associated with arteriolar spasm and interstitial edema.
What radiological signs of posterior urethral valves are detected during micturition cystourethrography?
During micturition cystourethrography, posterior urethral valves are characterized by changes in the urinary bladder, urethra, and upper urinary tract.
Main radiological signs:
- Change in urinary bladder volume — a very large or, conversely, small urinary bladder is visualized.
- Wall trabeculation — accompanied by the formation of urinary bladder diverticula.
- Vesicoureteral reflux (vesicoureteric reflux) — may be unilateral or bilateral, often with decreased renal function.
- Visualization of the valve itself — an abrupt caliber transition (acute-calibre transition) is identified in the posterior urethra.
What are the imaging findings of emphysematous pyelonephritis on plain urography and CT?
The imaging findings of emphysematous pyelonephritis in the provided sources are described solely based on computed tomography data.
Key CT signs include:
- Renal alterations — enlargement and destruction of the organ, as well as focal tissue necrosis, are visualized.
- Presence of gas — small punctate or linear accumulations of gas originating from the renal papillae, as well as "gas-fluid" levels, are identified.
- Loculated gas (loculated gas) — in type 1, it is detected without fluid accumulation; in type 2, it is combined with fluid accumulation in the kidney or perirenal space.
- Abscesses — the process may proceed with the formation of a renal or perirenal abscess.
Stages of VUR: I — ureter only, II — reaches the renal pelvis, III — moderately dilated, IV — blunted forniceal angles, V — erased papillary impressions (gross deformation).
Class and exam questions
What is the difference between lobar nephronia and renal abscess?
Lobar nephronia is a focal acute bacterial inflammation of the renal tissue without signs of purulent breakdown (liquefaction). It is an intermediate stage before the formation of a full-fledged abscess.
Why is diuretic MAG3 renography used in recurrent UTIs?
It is used when urinary tract obstruction, detected by ultrasound, is suspected. The study is performed 8 weeks after the episode of acute infection.
Why is it easy to misdiagnose xanthogranulomatous pyelonephritis?
The focal or diffuse form of XPN manifests as large mass lesions in the kidney, which requires careful differential diagnosis with Wilms tumor. A specific MRI appearance helps avoid an incorrect preoperative diagnosis.
What else to study on this topic
- Modified voiding cystourethrography and urodynamics in suspected bladder dysfunction
- Differential diagnosis of focal xanthogranulomatous pyelonephritis and Wilms tumor
- Genetic ciliopathies: autosomal dominant and autosomal recessive polycystic kidney disease
- Interpretation features of DMSA scintigraphy in renal dysplasia (pseudoscars)
- Classification and imaging semiotics of non-hereditary cystic kidney diseases
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