Carotid aneurysm. Coloured magnetic resonance imaging (MRI) scan of a section through the head and neck of a 22-year-old patient with a large aneurysm (round, centre-left) affecting the right internal carotid artery at the level of the carotid bifurcation. An aneurysm occurs when an artery wall weakens and swells like a balloon. If it bursts, an aneurysm can cause severe internal haemorrhage. here, it was the result of a traumatic injury.