Rib Notching of 4–9th ribs with double bulging is seen in –
Radiological findings of COA
1. Abnormal Aortic arch : Commonest finding
Site of narrowing may be seen as an identation.
Classic '3 sign' or double bulge sign' on left border of aortic shadow is formed by (above downwards)

Prestenotic dilatation
Coarctation itself (indentation)


Poststenotic dilatation

2. Rib notching : is a frequent sign in adults. It is not seen until late childhood.
It is seen on the inferior margin of posterior halves of ribs from 3n1 or 4th ribs downwards. Rib notching is due to enlargement of intercostal arteries which act as collateral vessels. These collateral vessels produce indentation, on the ribs at the above sites.
Aortic aneurysm
Aortic dessection
Coarctation of aorta
Diaphragmatic hernia