Imaging of the Urachus.

The urachus is a remnant fibrous cord of the allantois and the ventral aspect of the cloaca that connects the urinary bladder to the anterior abdominal wall at the level of the umbilicus. The authors explore the embryologic origins of the urachus and delineate its normal anatomy, followed by a discussion of urachal pathologic conditions seen with different imaging modalities-including US, CT, and MRI-using a case-based approach. Although it is a vestigial structure, the urachus can harbor significant disease, including congenital anomalies arising secondary to varying degrees of incomplete involution of the urachus and ranging from focal patency (urachal cyst, diverticulum, and sinus) to complete patency (patent urachus). The spectrum of congenital abnormalities can manifest with various clinical findings and is often discovered at imaging when infected. Understanding the embryogenesis of the urachus is therefore crucial for understanding the imaging manifestations of urachal abnormalities. Nonneoplastic urachal masses can be inflammatory and sometimes difficult to differentiate from malignancy. In women, the urachus can be involved by endometriosis. Neoplastic urachal entities can include mucinous cystadenoma, which can rupture with associated mucinous ascites. Adenocarcinoma is the most common urachal malignancy and frequently can extend to involve the urinary bladder. Other malignant urachal entities include urothelial carcinoma and metastasis. Mimics of urachal pathologic conditions can cause diagnostic misperception and include primary bladder malignancy, infections, and dropped gallstones or appendicoliths. This comprehensive overview aims to enhance radiologists' proficiency in recognizing and interpreting urachal abnormalities, thus contributing to improved patient outcomes. ^©RSNA, 2025 Supplemental material is available for this article.