An adult with a urinary tract infection is rarely imaged afterwards. A child often is, because the infection may be the first visible sign of a urinary tract that was malformed before birth.
Imaging after a first febrile infection answers three separate questions, and each modality answers only one of them. Is something obstructing the tract or malforming it? Is there vesicoureteral reflux, a backflow of urine from the bladder towards the kidney? And is there already renal damage? The first two questions are about the route the urine takes. The third is about the kidney tissue the infection reached. Each question is taken in turn.
Is there a blockage or a malformation?
The first question is about structure, so the first study is structural. Ultrasound of the kidneys and urinary tract is the first-line study after a first febrile UTI in a young child. It is non-invasive and uses no radiation, and it addresses the structural question directly.
The sonographer looks for several things:
- Renal hypoplasia or dysplasia: a small or disorganised kidney, with abnormal parenchymal thickness.
- Dilatation of the renal pelvis (hydronephrosis) or of the ureter (megaureter), which suggests obstruction.
- Bladder wall thickening and post-void residual urine, which suggest bladder outlet obstruction or incomplete emptying.
- A duplex collecting system.
These findings need care in reading. Hydronephrosis on ultrasound means obstruction, reflux, megaureter, or transient dilatation. A megaureter is a dilated ureter from obstruction, reflux or both. In a duplex collecting system there are two collecting systems, each draining one part, or moiety, of the kidney. The upper moiety classically obstructs and the lower moiety classically refluxes. A duplex system is an anatomical variant that often coexists with reflux, and vesicoureteral reflux is the most common anomaly associated with duplex systems.
A normal ultrasound substantially reduces the probability of an obstructive malformation.
Why can ultrasound not answer the reflux question?
A normal ultrasound still leaves the second question open, and that question is about urine moving during voiding. Reflux is recorded by its phase, during bladder filling or during voiding, so it is asked about with the bladder in motion. Reflux is a dynamic event, and a static image of the kidney cannot catch it. A normal ultrasound therefore does not exclude reflux. It makes the highest grades of reflux very unlikely, but it cannot rule out the lower grades. The ladder starts with ultrasound for structure, and ultrasound cannot exclude reflux.
How does cystography show reflux?
Only the cystogram shows the reflux itself. Voiding cystourethrography (VCUG) is the only study that directly demonstrates reflux, and it remains the gold standard for diagnosing and grading it, because it shows the reflux during filling and voiding. Contrast is instilled into the bladder through a catheter and followed on fluoroscopy as the child voids. As the foundations note set out, cystography is an X-ray that follows contrast as the child voids. If the contrast climbs into the ureter and renal pelvis, reflux is present and can be graded. On cystography, reflux is graded I to V. It is retrograde flow of urine from the bladder to the kidney, and it is either idiopathic or secondary.
The study is also a catheter and radiation study. Catheterisation is unpleasant, and as the urine sampling note set out, a catheter passed into the bladder is an invasive way to collect urine. VCUG is a dynamic fluoroscopic study that involves ionising radiation. Cystography is therefore used selectively, for the children whose findings call for it.
When is cystography justified?
Because it is invasive and uses radiation, the cystogram is ordered when the findings call for it. The pediatric teaching decision rule rests on three findings, and any one of them justifies cystography. Cystography is selective: abnormal ultrasound, a non-E. coli organism, or recurrent febrile UTI.
Which findings count?
The first finding is an abnormal ultrasound: hypoplasia, abnormal echogenicity, pelvic or ureteral dilatation, or a bladder abnormality. Echogenicity means the brightness of kidney tissue on ultrasound.
The second is a non-E. coli organism, namely Klebsiella, Pseudomonas or Proteus. As the acute treatment note set out, a non-E. coli organism raises the probability of a structural abnormality.
The third is recurrent febrile UTI, meaning a second or third febrile infection. It counts even after a normal ultrasound and a first E. coli infection.
The combination of findings matters more than any single item.
What do the two examples show?
A normal ultrasound with a first E. coli infection does not by itself call for a cystogram. The same normal ultrasound with a Pseudomonas infection does call for one. The ultrasound is identical, so it is the organism that changes the decision.
Are the thresholds rigid?
Not entirely. Current European urology guidance holds that the criteria for selecting children for reflux detection are supported only weakly. Renal ultrasound and clinical risk factors (age, sepsis, white cell count, abnormal ultrasound) can be combined into predictive scores rather than rigid thresholds.
What do posterior urethral valves look like?
One finding that a cystogram may show is severe enough to name by itself. Posterior urethral valves are congenital folds in the posterior urethra of boys. The folds obstruct the outflow of urine. This produces characteristic changes in the bladder and the upper tracts. In a male infant the valves appear as a dilated posterior urethra, a thickened bladder wall and dilated upper tracts. On cystography they mean urethral obstruction with high-pressure secondary reflux. Posterior urethral valves are one of the most damaging structural abnormalities. VCUG is the diagnostic study for them, so cystography is how they are diagnosed.
Has the kidney already been damaged?
The third question is about the kidney tissue itself, so it needs a scan of the tissue.
Dimercaptosuccinic acid (DMSA) scintigraphy is a nuclear scan using a radioactive tracer, and it answers the third question. It reads the kidney tissue, and a patch of reduced uptake may be scarring. The tracer is taken up by functioning tubular cells, and working tissue takes it up evenly. A scar therefore appears as a focal photopenic defect, a patch where no tracer is taken up.
Reduced uptake also comes from infection, so the timing matters. The decisive DMSA scan is done 4 to 6 months after the infection, once the acute inflammation has resolved and the defect can be called permanent. A focal photopenic defect at that point means permanent scarring. About 15% of children have such a defect after a first UTI, and the proportion is higher after recurrent febrile episodes.
A scan taken during the acute illness shows parenchymal involvement. It cannot distinguish a lesion that will heal from one that will scar, because the reduced uptake may be infection, which can resolve. As the foundations note set out, an acute kidney lesion does not automatically become a scar. So the acute scan shows involvement that may or may not resolve, and the scan at 4 to 6 months shows permanent scarring.
DMSA localises the damage but does not explain it. That is the role of cystography.
Does urine drain away from the kidney?
DMSA shows where the damage is but not why, and the drainage question is answered by another scan. Mercaptoacetyltriglycine (MAG-3) scintigraphy is a dynamic study, used when the question is drainage rather than cortical scarring. The scan follows drainage rather than the cortex.
It is used in two situations:
- Suspected ureteropelvic junction obstruction, a blockage where the renal pelvis meets the ureter.
- A dilated system, where it matters whether the urine empties.
MAG-3 also reports the differential function of each kidney as a percentage, called split function. Split function is the share of total kidney function that each side contributes.
DMSA and MAG-3 can both give split function. The practical difference is that MAG-3 reads drainage, and DMSA reads focal cortical defects in the kidney tissue. So MAG-3 answers drainage and split function.
How do the studies fit together?
Each study has its own question, so the order of studies follows which finding has turned up. Across these studies a small set of findings recurs, and each points towards a different underlying problem. The ladder escalates only when a finding or the clinical course justifies it.
Step by step
The first step is a renal and urinary tract ultrasound after a first febrile UTI in a young child.
If the ultrasound is abnormal, the next step is cystography plus a delayed DMSA, to establish a baseline for scarring.
If the ultrasound is normal but the organism is not E. coli, or the febrile UTI recurs, the step is again cystography plus a delayed DMSA.
If the ultrasound is normal, there is a single E. coli infection and no recurrence, the delayed DMSA is the study that matters. Cystography is usually deferred.
Two strategies
Where the ladder is applied differs between guidelines. The bottom-up strategy is the one just described: ultrasound, then cystography if indicated, then DMSA. It starts with the least invasive study and escalates. The top-down strategy starts with an acute DMSA to confirm parenchymal involvement, and proceeds to cystography only if the kidney is affected. It has become more common in European practice because it exposes fewer children to catheterisation and radiation.
Both strategies are in use, and the choice belongs to local protocol.
What if the infection keeps coming back?
Recurrence is one reason the ladder reaches the cystogram, and it appears on the ladder as a reason for cystography. In a toilet-trained child with returning infections, the explanation often lies less in anatomy than in how the bladder and bowel behave between infections. So what are the bladder and the bowel doing between infections?