Renal ultrasound is one of the most frequently requested imaging studies in children. Indications include urinary tract infection (UTI), renal dysfunction and to exclude structural abnormalities—for example, in association with other congenital malformations or syndromes. Normal kidneys can have a range of appearances, dependent on the patient’s age and the presence of certain anatomical variants which may or may not be clinically important. With the increasing use of PACS (picture archiving and communication system) for storage and viewing of radiological investigations, clinicians are able to access images and view them in clinical areas. Anatomical variants are often described in ultrasound reports and can cause confusion or anxiety if their clinical significance is not explained, especially when the results have to be conveyed to parents. The aim of this article is to demonstrate the age-related variation of normal renal ultrasound appearances, variations of renal anatomy and some congenital anomalies of the upper renal tract (kidney and ureter) that may be seen at all ages, perhaps discovered incidentally, with an explanation of their clinical significance.

RENAL EMBRYOLOGY

An appreciation of the embryological development of the urinary system gives insight into both normal anatomical variations and some of the congenital abnormalities that may arise. The human urinary system forms from intermediate mesoderm in a process that gives rise to three different, overlapping embryonic renal systems, beginning in the fourth week of gestation. The first system or pronephroi (singular  =  pronephros) is rudimentary and non-functioning, consisting of 5–10 paired nephrotomes that form in the embryonic cervical region. These are transient and disappear by day 24 or 25, during which time the next structures are forming.

The second system consists of the mesonephroi (singular  =  mesonephros) and the mesonephric (Wolffian) ducts. Mesonephroi arise as ovoid swellings on either side of the developing vertebral column in the thoracic …