Imaging description

In early fetal life, the urachus connects the cloaca (which gives rise to the bladder) to the yolk sac (which is outside the fetal abdomen) [1]. As the yolk sac regresses, the urachus closes but persists into postnatal life as a fibrous band connecting the dome of the bladder to the umbilicus. This vestigial structure is known as the median umbilical ligament, and is sometimes seen incidentally at CT when outlined by fluid or air in patients with ascites or pneumoperitoneum (Figure 85.1). A normal urachal remnant may also be seen as a hypoechoic area at the dome of the bladder in up to 60% of children [2]. Occasionally, the urachus fails to regress and remains patent either partially or entirely. Depending on which part remains patent, such urachal remnant disorders are classified as patent urachus (fistulous track connecting the bladder and umbilicus), urachal umbilical sinus (blind ending track connecting to the umbilicus), vesicourachal diverticulum (upward tubular extension from the bladder), or urachal cyst (non-communicating cyst along the course of the urachus). At imaging, urachal remnants are seen as fluid-filled structures along the course of the urachus, i.e., in the midline anteriorly between the bladder and umbilicus (Figures 85.2–85.4). Eggshell mural calcification is sometimes seen in urachal cysts [3]. Urachal remnants may become infected or develop malignancy (Figures 85.5 and 85.6), and such complications should be suspected if a urachal remnant is thick-walled, echogenic, or contains solid elements.