Kidney–Bladder Ultrasound: What It Shows, and What It Doesn't

Ultrasound works with sound waves; it involves no radiation, needs no needle or catheter, and can be repeated within minutes. These features make ultrasound the first and most frequently used test in every child with a febrile urinary tract infection, every infant found to have kidney enlargement before birth, and the annual follow-up of a child with reflux.

A good kidney–bladder ultrasound doesn't just say ‘kidneys normal’ — it records several separate measurements and observations. Kidney length is compared with the normal range for age; a marked difference in length between the two kidneys can be a clue to a kidney that has failed to grow properly. The thickness and appearance of the kidney tissue (parenchyma) are examined for a thin or irregular cortex.

• Kidney length (for age) and the difference between the two kidneys
• Thickness and echogenicity of the kidney tissue, and cortical irregularity (suspected scarring)
• Pelvic anteroposterior diameter (mm) and calyceal dilation: degree of hydronephrosis
• Visibility and width of the ureter
• Duplex collecting system, ureterocele, stones
• Bladder wall thickness, post-void residual urine, rectal diameter

The most important limitation is this: ultrasound does not show reflux. Reflux is urine flowing back into the ureter while the bladder is filling or during voiding; it is a momentary event that can only be seen by filling the bladder with a contrast agent (VCUG or contrast-enhanced voiding urosonography, ceVUS). This is also why ultrasound cannot tell the grade of reflux.

Ultrasound is painless and usually takes 15–20 minutes. Image quality is better when infants are examined fed and calm; room-temperature gel is used, and the child lies on their back and side. There is no need to fast.

Seeing hydronephrosis, a dilated ureter, a duplex system, a thick bladder wall, or suspected scarring on ultrasound moves the decision toward VCUG and, if needed, DMSA. Even with a normal ultrasound, VCUG can still come up if there are risk factors such as age under 2, a recurrent or atypical infection, or a family history; these thresholds are detailed in VUR evaluation after febrile UTI.

• In a child with known reflux, ultrasound is the backbone of follow-up: kidney length and growth, new or increased dilation, and cortical thinning are monitored once a year. If a febrile infection occurs, the ultrasound is repeated earlier. This prevents the VCUG from being repeated unnecessarily; the follow-up VCUG decision is a separate topic.