The exchange centers on a Matters Arising letter authored by Tjaš Žvar, Peter Slak, and Domen Plut, radiologists affiliated with both the Department of Radiology at Ljubljana University Medical Centre and the Faculty of Medicine at the University of Ljubljana. Their reply, published on 8 September 2026, responds to a commentary by Surve and Shitole concerning the optimization of imaging strategies after a first pediatric urinary tract infection. The tone of the reply is notably collegial. The Slovenian team thanks their colleagues for their insightful comments and frames the critique as a contribution that advances the discussion, but beneath the courteous surface lies a substantive disagreement about what the evidence currently supports and where pediatric imaging practice should go next.

At the heart of the discussion is a category of conditions known as congenital anomalies of the kidney and urinary tract, abbreviated CAKUT. These anomalies, which range from structural malformations of the kidney to obstructions and abnormal reflux of urine from the bladder back up the ureters, are among the most common abnormalities detected in children, and clinically significant forms can predispose a child to recurrent infections, hypertension, and progressive renal scarring. The clinical stakes are real. The goal of imaging after a first infection is to identify the subset of children whose anatomy puts them at risk of future kidney damage, so that surveillance, prophylaxis, or surgical correction can be considered before irreversible harm occurs.

The problem, as the Ljubljana authors acknowledge candidly, is that the current approach to identifying children with clinically significant CAKUT is not ideal. Routine ultrasonography after a first urinary tract infection has a low diagnostic yield, meaning that the vast majority of scans performed reveal nothing that changes management. This inefficiency carries costs that extend beyond the price of the examination itself. Every ultrasound appointment represents a burden on families, an addition to already stretched radiology departments, and, in some health systems, a delay that pushes other patients further down the waiting list. It was precisely this inefficiency, the authors write, that served as one of the principal motivations for their original study.

Crucially, the Slovenian team is careful to clarify the intent behind their work. Their aim, they explain, was not simply to reduce imaging, but to explore whether it could be more selectively targeted while maintaining patient safety. This distinction matters. Proposals to scale back imaging after a first urinary tract infection have sometimes been met with concern that children with hidden, clinically significant anomalies would be missed, with potentially serious consequences for their kidneys. By emphasizing that safety was a guiding constraint rather than an afterthought, the authors position their work as an attempt at refinement rather than retrenchment. They believe their findings point in the right direction, though they are equally forthright that further optimization is needed, because even within the higher-risk groups their study proposed, the diagnostic yield remains low.

This admission is one of the most scientifically interesting elements of the reply. It reflects a mature understanding of the limits of risk stratification in this field. The traditional predictors used to decide which children warrant imaging after a first infection have been relatively blunt instruments, principally age and sex, with atypical or recurrent infections triggering more aggressive workups. The authors concede that these variables alone do not capture the full spectrum of risk, and that even a risk-informed pathway built on them will leave a low yield within its targeted groups. In other words, refining the filter helps, but the filter itself may need to be rebuilt from richer material.

The reply to Surve and Shitole also engages with evidence from outside the immediate debate, drawing on a study by Doğan and colleagues that examined children diagnosed with vesicoureteral reflux, or VUR, only after they had already suffered recurrent infections. That study reported that children diagnosed with VUR following recurrent urinary tract infections had a higher prevalence of high-grade reflux and renal scarring. The implication is sobering: children whose significant urinary tract anatomy goes undetected after their first infection may return later with recurrent disease and already-established kidney damage. For the Ljubljana authors, this underscores the importance of identifying these children earlier, and it explains why they resist any interpretation of their work as a simple call for less imaging. The cost of missing a child with high-grade reflux is measured in permanent renal scarring, a price that no efficiency gain can justify.

So what would a better approach look like? The authors offer a concrete research agenda rather than a finished protocol. They argue that future studies should specifically examine children in whom clinically significant CAKUT or VUR was diagnosed only after the initial urinary tract infection, that is, the very patients whose anomalies were missed or not yet manifest at first presentation. By retrospectively analyzing these children’s initial clinical, microbiological, laboratory, and ultrasound findings together with their subsequent clinical course, researchers may uncover predictors that are not captured by age and sex alone. Such predictors could include specific patterns of organism virulence, inflammatory marker profiles, or subtle ultrasound findings that currently escape routine interpretation. The proposal effectively treats the missed cases as a natural experiment, a cohort whose histories hold the clues to earlier detection.

This methodology has a certain elegance, but it also illustrates the challenges inherent in the field. Retrospective analyses depend on the quality and completeness of the original records, and ultrasound findings in particular are operator-dependent and variably documented. Moreover, the number of children who experience clinically significant missed or delayed diagnoses after a first infection is small relative to the total population of children presenting with febrile urinary tract infections, which means that multi-center collaboration and careful pooling of data are likely to be essential. The authors’ call is therefore best understood as an invitation to the international pediatric radiology and urology communities to build the evidence base collaboratively, rather than as a claim that the answer is already at hand.

What emerges from this exchange is a picture of a specialty genuinely wrestling with its own practices in public. The original study, the critique by Surve and Shitole, and the reply from Žvar, Slak, and Plut together form a small but instructive case study in how scientific self-correction is supposed to work: findings are published, challenged, defended, and refined, with each participant acknowledging the limits of the current evidence. For clinicians, the practical message of the moment is one of continuity with caution. Risk-informed imaging pathways are, as the authors put it, a logical direction for future development, but the diagnostic yield remains low even within proposed higher-risk groups, and the consequences of delayed diagnosis of vesicoureteral reflux and renal scarring are serious enough that no wholesale abandonment of post-infection imaging is currently warranted. For families, the takeaway is that the ultrasound after a child’s first urinary tract infection remains a decision grounded in an active, evolving scientific debate, one in which researchers on all sides share the same goal: finding the children who need help, as early as possible, while sparing everyone else an unnecessary test.

Subject of Research: Risk-informed ultrasound imaging after a first pediatric urinary tract infection to detect congenital anomalies of the kidney and urinary tract

Article Title: Reply to Surve K

Article References: Žvar, T., Slak, P., & Plut, D. (2026). Reply to Surve K. Pediatric Radiology. https://doi.org/10.1007/s00247-026-06776-y

Image Credits: AI Generated

DOI: 10.1007/s00247-026-06776-y

Keywords: pediatric radiology, urinary tract infection, CAKUT, vesicoureteral reflux, ultrasound, renal scarring, risk stratification, diagnostic yield, children, imaging guidelines, nephrology, urology

CAKUT is far from rare. The anomalies, which arise when the ureteric bud fails to develop normally from the Wolffian duct or when it interacts defectively with the metanephric blastema during organogenesis, affect more than one percent of live births and account for thirty to fifty percent of all malformations detected before birth. They represent a leading cause of chronic kidney disease in children, and a substantial proportion of affected patients progress toward renal failure if the underlying problems are not identified and managed early. Because some anomalies remain silent while others quietly destroy nephrons, accurate and timely imaging is the linchpin of pediatric nephro-urology, guiding decisions about when to operate, when to monitor, and when to leave well enough alone.

Until now, the diagnostic backbone has been a pairing of ultrasound and dynamic renal scintigraphy. Ultrasound, the universal first-line tool, is safe and widely available but operator-dependent and often imprecise in complex anatomical variants. Dynamic renal scintigraphy using the radiotracer 99mTc-MAG3 has long been considered the gold standard for quantifying split renal function and urinary drainage, but it delivers a dose of ionizing radiation to patients who, in many cases, will need serial follow-up throughout childhood. Vesicoureteral reflux, meanwhile, is assessed separately with voiding cystourethrography or contrast-enhanced voiding urosonography. The result is a piecemeal diagnostic pathway that the Genoa team describes as ripe for consolidation.

Functional MRU collapses that pathway into a so-called one-stop-shop. In a single session, dynamic contrast-enhanced sequences combined with dedicated post-processing software deliver detailed anatomical imaging alongside quantitative measurements of differential renal function, urinary drainage, excretory patterns and urinary flow. Comparative studies cited in the review show that MRU-derived volumetric split renal function estimates, calculated with both area-under-the-curve and Rutland-Patlak kinetic methods, agree well with scintigraphy while demonstrating excellent intra- and inter-reader repeatability. Drainage curve classifications also correspond closely, although the authors note that differences in diuretic timing protocols between the two modalities explain some borderline discrepancies. Crucially, the technique achieves this without a single photon of ionizing radiation, an advantage that compounds over the many follow-up scans a child with CAKUT may require.

The practical protocol described in the review unfolds in three phases: preparation, acquisition and post-processing. Children younger than five are generally studied under sedation, although neonates needing only morphological assessment can be imaged awake using the feed-and-wrap swaddling technique, in which a sleeping, recently fed infant is gently secured in a folded sheet. Cooperative older children may be accompanied by a screened parent. Preparation includes peripheral venous access, intravenous hydration with ten to twenty milliliters per kilogram of saline, and bladder management, catheterization in sedated patients and simple voiding in cooperative ones. Because gadolinium-based contrast agents are cleared exclusively by glomerular filtration, renal function must be verified beforehand through serum creatinine and an estimated glomerular filtration rate calculated with the Schwartz formula.

Acquisition begins with morphology: axial and coronal T2-weighted turbo spin echo sequences with and without fat suppression map the parenchyma, the collecting system and anatomical variants such as duplex systems, while diffusion-weighted imaging flags areas of parenchymal inflammation and three-dimensional heavily T2-weighted sequences hunt for ectopic ureteral insertions. The functional phase then follows a tightly choreographed script. Furosemide at 0.5 to 1 milligram per kilogram, capped at 20 milligrams, is injected immediately before dynamic T1-weighted gradient echo acquisition using the F0 technique. The dynamic sequence runs continuously for twelve to fifteen minutes at high temporal resolution of three to five seconds during the first five minutes, then every thirty seconds, with a 0.1 millimole per kilogram gadolinium bolus delivered after the first three cycles. A three-dimensional angiographic breath-hold study, using a cumulative contrast dose of up to 0.3 millimole per kilogram, follows, and delayed urographic images are captured at five-minute intervals up to twenty minutes to characterize the renal pelvis, ureters and bladder.

The quantitative magic happens in post-processing. The review highlights a new analysis pipeline in which artificial intelligence models automatically segment the kidneys, renal pelvis and abdominal aorta across both morphological and functional volumes, with the operator retaining the ability to review and correct each mask. From these segmentations the software computes absolute and relative renal volumes, percentage-enhancement curves normalized to the pre-contrast baseline, and a battery of functional indices. Drainage curves are classified as normal, borderline or accumulating depending on whether signal intensity falls steadily after its peak, plateaus, or keeps rising, a distinction that directly informs whether hydronephrosis reflects true obstruction requiring surgery or benign stasis. Differential renal function is quantified twice, through the area under the filtration-phase curve and through Rutland-Patlak kinetic modeling referenced to a suprarenal aortic input function, with both measures weighted by parenchymal volume. Two transit-time biomarkers, calyceal transit time and renal transit time, complete the panel, with prolonged values serving as sensitive early indicators of urodynamic impairment.

The clinical payoffs are illustrated condition by condition. In ureteropelvic junction obstruction, the most common cause of upper urinary tract blockage in children, MRU distinguishes intrinsic stenosis from extrinsic compression by a crossing accessory renal artery, a distinction that determines whether surgeons perform a standard Anderson-Hynes dismembered pyeloplasty or a Hellström vascular hitch procedure. A quantitative scoring system proposed in 2022 standardizes MRU-based diagnosis and management of this condition, and postoperative scans can document functional recovery through improving transit times and filtration parameters. In megaureter and ureterovesical junction obstruction, where many cases resolve spontaneously by age two to five, MRU supports conservative surveillance and identifies the minority of children whose declining differential renal function warrants ureteral reimplantation, increasingly performed with robotic assistance.

The technique also excels where scintigraphy struggles. In dysplastic and hypoplastic kidneys, including multicystic dysplastic kidney, precise MR segmentation separates enhancing parenchyma from cysts and collecting systems, and the degree of dysplasia correlates negatively with residual function. In duplex systems, which follow the Weigert-Meyer rule with an obstructed upper moiety and a reflux-prone lower moiety, MRU characterizes each moiety’s separate functional contribution while defining complex anatomy that ultrasound and reflux-only studies cannot. For ectopic, malrotated and fused kidneys such as the horseshoe kidney, dynamic sequences separate harmless urinary stasis from genuine obstruction and reveal aberrant vessels that may compress the ureter. The authors close with a proposal for a structured reporting model intended to standardize MRU interpretation across centers. With sedation requirements low, no adverse events reported, and new AI-driven software interfacing directly with hospital PACS systems, the review positions functional MRU as a cornerstone of personalized, radiation-free care for the most common group of fetal anomalies encountered in modern medicine.

Subject of Research: Functional magnetic resonance urography for evaluating congenital anomalies of the kidney and urinary tract in children

Article Title: How we do it: functional magnetic resonance urography in congenital anomalies of the kidney and urinary tract

Article References: Anfigeno, L., Basso, L., Gnocchi, G., Barbieri, L., Minetti, E., Olivi, B., Verrina, E., Mattioli, G., & Damasio, M. B. (2026). How we do it: functional magnetic resonance urography in congenital anomalies of the kidney and urinary tract. Pediatric Radiology. https://doi.org/10.1007/s00247-026-06745-5

Image Credits: AI Generated

DOI: 10.1007/s00247-026-06745-5

Keywords: functional MR urography, CAKUT, pediatric radiology, kidney anomalies, differential renal function, renal scintigraphy, hydronephrosis, gadolinium contrast, AI segmentation, ureteropelvic junction obstruction, duplex kidney, chronic kidney disease