On CTA of the aorta, 'intimal flap' separating true from false lumen is the defining feature of aortic dissection. The false lumen can be distinguished from the true lumen by which CT sign?

• A 'Beak sign' — the false lumen has a wedge-shaped cross-section with an acute angle at its lateral edge ✓
• B Earlier opacification with intravenous contrast
• C Smaller cross-sectional area
• D Absence of any calcification on the wall

Explanation

The beak sign on CTA distinguishes the false from the true lumen of aortic dissection: the false lumen has a crescent-shaped or wedge-shaped profile with an acute 'beak' angle where the dissection flap meets the aortic wall, due to haematoma dissecting along curved tissue planes. Additional false-lumen features include: larger cross-sectional area (usually), lower enhancement (slower flow), presence of 'cobwebs' (residual strands of medial tissue), and displacement of intimal calcifications inward. The true lumen is usually smaller, enhances first, and is outlined by the native intima.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.