Beak sign (arterial dissection)
The beak sign of arterial dissection is the acute angle formed at the edge of the false lumen in aortic dissection in axial cross-section. It is formed by the borders of the outer aortic wall and the intimal flap, and may be partially thrombosed (blunted beak). It is a reliable characteristic that helps in distinguishing from the true lumen.
The acute "beak" angulation represents the transverse limit of the dissection, where false lumen propagation is occurring along with a tear in the media.
Terminology
The term bird's beak sign is used in a number of other contexts: see bird beak sign (disambiguation).