Cobra head sign (ureter)

The cobra head sign, also known as the spring onion sign, refers to dilatation of the distal ureter, surrounded by a thin lucent line, which is seen in patients with an adult-type ureterocele. The cobra head appearance indicates an uncomplicated ureterocele.

During an excretory phase of an intravenous urography, the lucent "hood" of the cobra represents the combined thickness of the ureteral wall and prolapsed bladder mucosa, outlined by contrast material within the bladder lumen.

This lucent line should be thin and well defined. Any thickening, irregularity, or loss of definition of the cobra’s hood should raise concern for the presence of a pseudoureterocele.