Intimal calcification in aortic knuckle – X-ray chest PA
Calcification of the intima of aortic knuckle (arrow) is a common finding in the elderly. Usually it is of no significance. But a separation of the intimal calcification from the edge of the aortic knuckle shadow indicates a separation of the intima from the media as in aortic dissection. This has been called the “calcium sign” in aortic dissection. A separation of more than 1 cm is considered significant. A recent case report by Lin SH et al  demonstrates the calcium sign in aortic dissection.
Arrow points to intimal calcification. LPA: left pulmonary artery; RPA: right pulmonary artery
Aortic knuckle, also known as aortic knob is the distal part of aortic arch which curves posteriorly and continues as the descending aorta. It appears like a lateral projection at the upper most part of the left border of cardiac silhouette. This description applies to the posteroanterior view of chest X-ray. Aortic knuckle is considered as the first mogulwhile describing the third mogul signof enlargement of left atrial appendage. Main pulmonary artery being the second mogul.
A prominent aortic knuckle can occur due to unfolding of arch of aorta. Rarely it could be due to aortic arch aneurysm as well. Dilatation of the pre-stenotic segment in coarctation of aorta can also cause a prominent aortic knuckle on X-ray chest PA view .
Singh S, Hakim FA, Sharma A, Roy RR, Panse PM, Chandrasekaran K, Alegria JR, Mookadam F. Hypoplasia, pseudocoarctation and coarctation of the aorta – a systematic review. Heart Lung Circ. 2015 Feb;24(2):110-8.